| 1 |
- (window.webpackJsonp=window.webpackJsonp||[]).push([[25],{306:function(t,s,a){"use strict";a.r(s);var n=a(14),r=Object(n.a)({},(function(){var t=this,s=t._self._c;return s("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[s("h1",{attrs:{id:"rust入门教程-七-生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#rust入门教程-七-生命周期"}},[t._v("#")]),t._v(" Rust入门教程（七）：生命周期")]),t._v(" "),s("blockquote",[s("p",[t._v("Rust 生命周期机制是与所有权机制同等重要的资源管理机制。生命周期，简而言之就是引用的有效作用域，之所以引入这个概念主要是应对复杂类型系统中资源管理的问题。引用是对待复杂类型时必不可少的机制，毕竟复杂类型的数据不能被处理器轻易地复制和计算，但引用往往导致极其复杂的资源管理问题。")])]),t._v(" "),s("h2",{attrs:{id:"生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#生命周期"}},[t._v("#")]),t._v(" 生命周期")]),t._v(" "),s("ul",[s("li",[t._v("Rust 的每个引用都有自己的生命周期")]),t._v(" "),s("li",[t._v("生命周期：引用保持有效的作用域")]),t._v(" "),s("li",[t._v("大多数情况：生命周期是隐式的、可被推断的")]),t._v(" "),s("li",[t._v("当引用的生命周期可能以不同的方式互相关联时：手动标注生命周期")])]),t._v(" "),s("p",[t._v("生命周期的主要目标：避免悬垂引用（dangling reference）")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test01")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" r"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("3")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n r "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"{}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" r"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("上面这段代码会在 "),s("code",[t._v("r = &x;")]),t._v(" 处报错，因为当打印 "),s("code",[t._v("r")]),t._v(" 的值的时候，"),s("code",[t._v("x")]),t._v(" 已经离开了他的作用域，这时 "),s("code",[t._v("r")]),t._v(" 指向的 "),s("code",[t._v("x")]),t._v(" 的内存已经被释放，因此会报错。")]),t._v(" "),s("p",[t._v("Rust 实际上是通过"),s("strong",[t._v("借用检查器")]),t._v("来检查一些变量的生命周期。")]),t._v(" "),s("h3",{attrs:{id:"借用检查器"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#借用检查器"}},[t._v("#")]),t._v(" 借用检查器")]),t._v(" "),s("p",[t._v("Rust 编译器的借用检查器（borrow checker），用来比较作用域来判断所有的借用是否合法")]),t._v(" "),s("p",[t._v("在上例中，借用检查器检测到 "),s("code",[t._v("r")]),t._v(" 的生命周期大于 "),s("code",[t._v("x")]),t._v("，即被引用者的生命周期小于引用者的生命周期，因此编译会报错。")]),t._v(" "),s("h3",{attrs:{id:"函数中的泛型生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#函数中的泛型生命周期"}},[t._v("#")]),t._v(" 函数中的泛型生命周期")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test02")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string1 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Congratulations"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"fantastic"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" result "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"The longest string is {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n y\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("编译报错：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[t._v("➜ ~"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Code")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("rust"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("life_cycle git"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("master"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" ✗ cargo run \n "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Compiling")]),t._v(" life_cycle v0"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v(".1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("home"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("cherry"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Code")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("rust"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("life_cycle"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\nerror"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token constant"}},[t._v("E0106")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" missing lifetime specifier\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" src"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("main"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("rs"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("24")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("33")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("24")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),t._v(" expected named lifetime parameter\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" help"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" this function"),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'s")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("type")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token type-definition class-name"}},[t._v("contains")]),t._v(" a borrowed value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" but the signature does not say whether it is borrowed from `x` or `y`\nhelp"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" consider introducing a named lifetime parameter\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("24")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("For")]),t._v(" more information about this error"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("try")]),t._v(" `rustc "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v("explain "),s("span",{pre:!0,attrs:{class:"token constant"}},[t._v("E0106")]),t._v("`"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("\n")])])]),s("p",[t._v("我们发现编译器会提示"),s("strong",[t._v("缺少一个命名的生命周期参数")]),t._v("，这个函数返回一个借用的值，但是没有声明这个借用的值是来自 "),s("code",[t._v("x")]),t._v(" 还是来自 "),s("code",[t._v("y")]),t._v("。值得说明的是，这个返回值的借用跟函数体的逻辑没有关系，要从函数签名就要看出返回值借用的值来自哪一个参数。")]),t._v(" "),s("p",[t._v("根据编译器提示，我们声明一个泛型生命周期 "),s("code",[t._v("'a")]),t._v("，代码修改如下：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n y\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("h3",{attrs:{id:"生命周期标注语法"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#生命周期标注语法"}},[t._v("#")]),t._v(" 生命周期标注语法")]),t._v(" "),s("ul",[s("li",[t._v("生命周期的标注不会改变引用的生命周期长度")]),t._v(" "),s("li",[t._v("当指定了泛型生命周期参数，函数可以接收带有任何生命周期的引用")]),t._v(" "),s("li",[t._v("生命周期的标注：描述了多个引用的生命周期间的关系，但不影响生命周期")])]),t._v(" "),s("p",[t._v("生命周期参数名语法如下：")]),t._v(" "),s("ul",[s("li",[t._v("以 "),s("code",[t._v("'")]),t._v(" 开头")]),t._v(" "),s("li",[t._v("通常全小写且非常短")]),t._v(" "),s("li",[t._v("习惯以 "),s("code",[t._v("'a")]),t._v(" 表示")])]),t._v(" "),s("p",[t._v("生命周期标注的位置：")]),t._v(" "),s("ul",[s("li",[t._v("在引用符号 "),s("code",[t._v("&")]),t._v(" 后面标注")]),t._v(" "),s("li",[t._v("使用空格将标注和引用类型区分开")])]),t._v(" "),s("p",[t._v("生命周期标注的例子：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("i32")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 一个引用")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("i32")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 带有显式生命周期的引用")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("i32")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 带有显式生命周期的可变引用")]),t._v("\n")])])]),s("p",[t._v("值得注意的是，单个生命周期标注本身没有意义，我们再看上面的 "),s("code",[t._v("longest")]),t._v(" 函数：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n y\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("泛型的生命周期参数声明在函数名和参数列表之间的 "),s("code",[t._v("<>")]),t._v(" 中。")]),t._v(" "),s("p",[t._v("我们仔细分析这个函数：")]),t._v(" "),s("p",[s("code",[t._v("longest")]),t._v(" 函数的两个参数都声明了生命周期，就要求这两个引用必须和泛型的生命周期存活相同的时间，而且函数所返回的字符串切片的存活时长也不能小于 "),s("code",[t._v("'a")]),t._v(" 这个生命周期。为引用指明生命周期，是要确保当引用失去了所有权后而被移出内存。当在函数参数中指明生命周期时，我们并没有改变参数和返回值的生命周期，只是向调用检查器指出了一些可用于检查非法调用的约束。而 "),s("code",[t._v("longest")]),t._v(" 函数本身并不需要知道参数 "),s("code",[t._v("x")]),t._v(" 和 "),s("code",[t._v("y")]),t._v(" 具体的存活时长，只需要某个可以代替 "),s("code",[t._v("'a")]),t._v(" 的作用域，同时满足函数的签名约束。实际上，若函数引用其外部的代码或者被外部代码引用，只靠 rust 本身确定参数和返回值的生命周期时不可能的，这样的话，函数所使用的生命周期在每次调用中都会发生变化，正因为如此，我们才需要手动对生命周期进行标注。")]),t._v(" "),s("p",[t._v("当我们将两个引用传入函数时，"),s("code",[t._v("x")]),t._v(" 和 "),s("code",[t._v("y")]),t._v(" 作用域重叠的部分将用来代替 "),s("code",[t._v("'a")]),t._v(" 这个生命周期的作用域，换句话说，这个泛型生命周期得到的具体的生命周期就是 "),s("code",[t._v("x")]),t._v(" 和 "),s("code",[t._v("y")]),t._v(" 两者生命周期较短的那个，因为返回值也标注了相同的生命周期，因此返回值的引用在两者比较短的生命周期内都是有效的。")]),t._v(" "),s("p",[t._v("那么生命周期标注是如何对 "),s("code",[t._v("longest")]),t._v(" 函数进行限制的？我么修改一下代码：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test02")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string1 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Congratulations"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"fantastic"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" result "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"The longest string is {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n y\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("将 string1 下面三行代码放到一个单独的作用域里，string2 是一个字符串字面值（字符串切片），他的生命周期相当于是一个静态的生命周期，在整个程序运行期间都存活，而 result 引用也会在 Line 7 大括号结束之前保持有效，因此代码不会报错。")]),t._v(" "),s("p",[t._v("【注】："),s("code",[t._v("&str")]),t._v(" 是直接在可执行文件中加载的，即这块内存直接放到可执行文件里面的，所以整个程序运行期间，这块内存比较特殊，不会由于所有权而消失，所以指这块内存的引用，一定会一直指向一个合法内存，所以其引用的生命周期是 "),s("code",[t._v("'static")]),t._v("，也就是全局静态，也不可能出现什么悬垂引用。")]),t._v(" "),s("p",[t._v("再改一下代码，将 result 声明放到外面，然后将 print 也放到外面，将 string2 改成 String 类型：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test02")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string1 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Congratulations"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"fantastic"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n result "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"The longest string is {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("我们发现 Line 6 报错了。string1 的生命周期为 Line 2~9，string2 的生命周期为 Line 5~7，所以 "),s("code",[t._v("'a")]),t._v(" 所表示的生命周期为 Line 5~7，而 result 的生命周期为 Line 3~9，不在 "),s("code",[t._v("'a")]),t._v(" 的范围内，因此编译报错，我们来看一下编译具体的错误：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[t._v("➜ ~"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Code")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("rust"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("life_cycle git"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("master"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" ✗ cargo run\n "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Compiling")]),t._v(" life_cycle v0"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v(".1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("home"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("cherry"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Code")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("rust"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("life_cycle"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\nerror"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token constant"}},[t._v("E0597")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" `string2` does not live long enough\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" src"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("main"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("rs"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("30")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("44")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("30")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" result "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("^")]),t._v(" borrowed value does not live long enough\n"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("31")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v(" `string2` dropped here "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("while")]),t._v(" still borrowed\n"),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("32")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"The longest string is {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("|")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v(" borrow later used here\n\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("For")]),t._v(" more information about this error"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("try")]),t._v(" `rustc "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),t._v("explain "),s("span",{pre:!0,attrs:{class:"token constant"}},[t._v("E0597")]),t._v("`"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("\nerror"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" could not compile `life_cycle` due to previous error\n")])])]),s("p",[t._v("这个报错的含义是，为了让 result 这个变量在打印时是有效的，那么 string2 必须在外部作用域结束之前一直保持有效，因为在函数声明中参数和返回值都使用了相同的生命周期。")]),t._v(" "),s("p",[t._v("在上例中，尽管 string1 的长度大于 string2 的长度，函数返回的是 string1 的引用，但是编译器并不知道这一点，编译器只知道 "),s("code",[t._v("longest")]),t._v(" 函数返回引用的生命周期是 "),s("code",[t._v("x")]),t._v(" 和 "),s("code",[t._v("y")]),t._v(" 生命周期比较短的那个。")]),t._v(" "),s("h2",{attrs:{id:"深入理解生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#深入理解生命周期"}},[t._v("#")]),t._v(" 深入理解生命周期")]),t._v(" "),s("ul",[s("li",[t._v("指定生命周期参数的方式依赖于函数所做的事情，在上面的例子中，若 "),s("code",[t._v("longest")]),t._v(" 函数改为：")])]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("这个时候，函数只返回变量 "),s("code",[t._v("x")]),t._v("，而与 "),s("code",[t._v("y")]),t._v(" 无关，因此无需为 "),s("code",[t._v("y")]),t._v(" 指定生命周期。")]),t._v(" "),s("ul",[s("li",[t._v("从函数返回引用时，返回类型的生命周期参数需要与其中一个参数的生命周期匹配")]),t._v(" "),s("li",[t._v("如果返回的引用没有指向任何参数，那么他只能引用函数内创建的值\n"),s("ul",[s("li",[t._v("这就是"),s("strong",[t._v("悬垂引用")]),t._v("，该值在函数结束时就走出了作用域，见下面的例子")])])])]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test02")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string1 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Congratulations"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" string2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"fantastic"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" result "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"The longest string is {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" result"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" res "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"abc"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n res"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("上面的代码中，"),s("code",[t._v("longest")]),t._v(" 函数中返回了局部变量 "),s("code",[t._v("res")]),t._v("，当函数执行完毕时，局部变量 "),s("code",[t._v("res")]),t._v(" 所指向的内存已经被释放掉，因此 "),s("code",[t._v("test02")]),t._v(" 中的 "),s("code",[t._v("result")]),t._v(" 变量指向的 "),s("code",[t._v("res")]),t._v(" 内存已经被清理，这就造成了"),s("strong",[t._v("悬垂引用")]),t._v("，非常类似于 "),s("code",[t._v("C/C++")]),t._v(" 的野指针。")]),t._v(" "),s("p",[t._v("那么我就是想返回函数中的局部变量，应该怎么办呢？解决办法也很简单，就是直接返回这个值而不是返回引用，这样就将变量的所有权移交出去了，如下所示：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" res "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"abc"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n res\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[s("strong",[t._v("因此从根本上讲，生命周期这种语法规则，是用来关联函数的不同参数及返回值之间的生命周期，一旦他们取得了某种联系，rust 就会获得足够的信息来支持保证内存安全的操作，并且阻止那些可能会导致悬垂指针或者其他违反内存安全的行为。")])]),t._v(" "),s("h2",{attrs:{id:"struct-定义中的生命周期标注"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#struct-定义中的生命周期标注"}},[t._v("#")]),t._v(" Struct 定义中的生命周期标注")]),t._v(" "),s("p",[t._v("struct 里可以包括：")]),t._v(" "),s("ul",[s("li",[t._v("自持有类型（类似于 i32 等）")]),t._v(" "),s("li",[t._v("引用：需要在每个引用上添加生命周期标注")])]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("struct")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token type-definition class-name"}},[t._v("ImportantExcerpt")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n part"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test04")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" novel "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("from")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Today is Tuesday. And I will take part in a meeting."')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" first_sentence "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" novel"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("split")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"."')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("next")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("expect")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("\"Can't find a '.'\"")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ImportantExcerpt")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n part"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" first_sentence"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("h2",{attrs:{id:"生命周期的省略"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#生命周期的省略"}},[t._v("#")]),t._v(" 生命周期的省略")]),t._v(" "),s("p",[t._v("每个引用都有生命周期，需要为使用生命周期的函数或 struct 指定生命周期参数")]),t._v(" "),s("p",[t._v("但是下面这个例子，没有任何生命周期的标注，仍然可以通过编译：")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("first_word")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" byte "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("as_bytes")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),t._v("item"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" byte"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("iter")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("enumerate")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" item "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("==")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token char"}},[t._v("b' '")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),t._v("s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("..")]),t._v("i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),t._v("s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("..")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("按照原来的 rust 规范，函数声明、参数和返回类型前都是要加上生命周期标注的，但是 rust 团队发现程序员总是一遍又一遍地标注同样的生命周期，而且这些场景是可以预测的，有着明确的模式，因此 rust 团队就将这些模式写入了编译器，使得借用检查器可以自动对这些模式进行推导而无需显式标注。")]),t._v(" "),s("p",[s("strong",[t._v("生命周期省略规则")])]),t._v(" "),s("ul",[s("li",[t._v("在 Rust 引用分析中所编入的模式称为"),s("strong",[t._v("生命周期省略规则")]),t._v(" "),s("ul",[s("li",[t._v("这些规则无需开发者来遵守")]),t._v(" "),s("li",[t._v("它们是一些特殊情况，由编译器来考虑")]),t._v(" "),s("li",[t._v("如果你的代码符合这些情况，那么就无需显式标注生命周期")])])]),t._v(" "),s("li",[t._v("生命周期省略规则不会提供完整的推断：\n"),s("ul",[s("li",[t._v("如果应用规则后，引用的生命周期仍然模糊不清→编译错误")]),t._v(" "),s("li",[t._v("解决办法：添加生命周期标注，表明引用间的相互关系")])])])]),t._v(" "),s("h3",{attrs:{id:"输入、输出生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#输入、输出生命周期"}},[t._v("#")]),t._v(" 输入、输出生命周期")]),t._v(" "),s("p",[t._v("生命周期在：")]),t._v(" "),s("ul",[s("li",[t._v("函数/方法的参数中，叫做输入生命周期")]),t._v(" "),s("li",[t._v("函数/方法的返回值中，叫输出生命周期")])]),t._v(" "),s("h3",{attrs:{id:"生命周期省略的三个规则"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#生命周期省略的三个规则"}},[t._v("#")]),t._v(" 生命周期省略的三个规则")]),t._v(" "),s("p",[t._v("编译器使用三个规则在没有显式标注生命周期的情况下，来确定引用的生命周期")]),t._v(" "),s("ul",[s("li",[t._v("规则 1 应用于输入生命周期")]),t._v(" "),s("li",[t._v("规则 2、3 应用于输出生命周期")]),t._v(" "),s("li",[t._v("如果编译器应用完三个规则后，仍然无法确定有效的生命周期，则报错")]),t._v(" "),s("li",[t._v("这些规则适用于 fn 和 impl 块")])]),t._v(" "),s("p",[s("strong",[t._v("规则 1：")]),t._v(" 每个引用类型都有自己的生命周期\n"),s("strong",[t._v("规则 2：")]),t._v(" 如果只有 1 个输入生命周期参数，那么该生命周期被赋给所有输出生命周期参数\n"),s("strong",[t._v("规则 3：")]),t._v(" 如果有多个输入生命周期参数，但其中一个是 "),s("code",[t._v("&self")]),t._v(" 或 "),s("code",[t._v("&mut self")]),t._v("，那么 "),s("code",[t._v("self")]),t._v(" 的生命周期会被赋给所有的输出生命周期参数")]),t._v(" "),s("p",[s("strong",[t._v("生命周期省略的三个规则-例子")])]),t._v(" "),s("p",[t._v("假设我们是编译器:")]),t._v(" "),s("p",[s("code",[t._v("fn first_word(s: &str) -> &str {}")]),t._v(" "),s("code",[t._v("fn first_word<'a>(s: &'a str) -> &str {}")]),t._v(" "),s("code",[t._v("fn first_word<'a>(s: &'a str) -> &'a str {}")])]),t._v(" "),s("p",[s("code",[t._v("fn longest(x: &str, y: &str) -> &str {}")]),t._v(" "),s("code",[t._v("fn longest<'a, 'b>(x: &'a str, y: &'b str) -> &str {}")])]),t._v(" "),s("h3",{attrs:{id:"方法定义中的生命周期标注"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#方法定义中的生命周期标注"}},[t._v("#")]),t._v(" 方法定义中的生命周期标注")]),t._v(" "),s("ul",[s("li",[t._v("在 struct 上使用生命周期实现方法，语法和泛型参数的语法一样")]),t._v(" "),s("li",[t._v("在哪声明和使用生命周期参数，依赖于：\n"),s("ul",[s("li",[t._v("生命周期参数是否和字段、方法的参数或返回值有关")])])]),t._v(" "),s("li",[t._v("struct 字段的生命周期名：\n"),s("ul",[s("li",[t._v("在 impl 后声明")]),t._v(" "),s("li",[t._v("在 struct 名后使用")]),t._v(" "),s("li",[t._v("这些生命周期是 struct 类型的一部分")])])]),t._v(" "),s("li",[t._v("impl 块内的方法签名中\n"),s("ul",[s("li",[t._v("引用必须绑定于 struct 字段引用的生命周期，或者引用是独立的也可以")]),t._v(" "),s("li",[t._v("生命周期省略规则经常使得方法中的生命周期标注不是必须的")])])])]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("struct")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token type-definition class-name"}},[t._v("ImportantExcerpt")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n part"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("impl")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ImportantExcerpt")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("level")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("self")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("u32")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("3")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("printSome")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("self")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" words"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"There are some words: {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("self")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("part"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("self")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("part\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("h2",{attrs:{id:"静态生命周期"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#静态生命周期"}},[t._v("#")]),t._v(" 静态生命周期")]),t._v(" "),s("ul",[s("li",[s("code",[t._v("'static")]),t._v(" 是一个特殊的生命周期：整个程序的持续时间\n"),s("ul",[s("li",[t._v("例如：所有的字符串字面值都拥有 "),s("code",[t._v("'static")]),t._v(" 生命周期\n"),s("ul",[s("li",[s("code",[t._v('let s: &\'static str = "I have a static lifetime.";')])]),t._v(" "),s("li",[t._v("字符串字面值是存在二进制程序中，总是可用")])])])])]),t._v(" "),s("li",[t._v("为引用指定 "),s("code",[t._v("'static")]),t._v(" 之前要三思\n"),s("ul",[s("li",[t._v("是否需要引用在整个生命周期内都存活")])])])]),t._v(" "),s("h3",{attrs:{id:"一个泛型参数类型-trait-bound-和生命周期的综合例子"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#一个泛型参数类型-trait-bound-和生命周期的综合例子"}},[t._v("#")]),t._v(" 一个泛型参数类型，Trait Bound 和生命周期的综合例子")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("use")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token namespace"}},[t._v("std"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),t._v("fmt"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")])]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Display")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("longest_with_an")]),t._v(" announcement"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" ann"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token lifetime-annotation symbol"}},[t._v("'a")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("where")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Display")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token macro property"}},[t._v("println!")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Announcement! {}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" ann"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" y"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n x\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n y\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[s("strong",[t._v("要注意的是，生命周期也是泛型的一种。")])])])}),[],!1,null,null,null);s.default=r.exports}}]);
|
