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的函数调用不按照声明顺序执行，即在后面声明的函数也可以在前面调用（这点与 C 语言不同）")])]),t._v(" "),s("p",[s("strong",[t._v("函数的参数")])]),t._v(" "),s("p",[t._v("parameter（定义函数的参数），arguments（调用函数的参数）")]),t._v(" "),s("p",[t._v("必须声明每个参数的类型")]),t._v(" "),s("p",[s("strong",[t._v("函数体中的语句和表达式")])]),t._v(" "),s("ul",[s("li",[t._v("语句 "),s("code",[t._v("statement")]),t._v(" 和表达式 "),s("code",[t._v("expression")])]),t._v(" "),s("li",[t._v("函数体由一系列语句组成，可选的由一个表达式结束")]),t._v(" "),s("li",[t._v("Rust 是一个基于表达式的语言")]),t._v(" "),s("li",[t._v("语句是执行一些动作的指令")]),t._v(" "),s("li",[t._v("表达式会计算产生一个值")]),t._v(" "),s("li",[t._v("函数的定义也是语句")]),t._v(" "),s("li",[t._v("语句不返回值，因此不能用 let 将一个语句赋值个一个变量")])]),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("let")]),t._v(" y "),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 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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 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(" s1"),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("^")]),t._v(" value borrowed here after "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("move")]),t._v("\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(" note"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" this error originates "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" the "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("macro")]),t._v(" `"),s("span",{pre:!0,attrs:{class:"token variable"}},[t._v("$crate")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")]),t._v("format_args_nl` "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Nightly")]),t._v(" builds"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" run with "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("-")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Z")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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"),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"owner"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" generated "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),t._v(" warning\nerror"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" could not compile `owner` due to previous error"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),t._v(" warning emitted\n")])])]),s("p",[t._v("也许这跟浅拷贝（shadow copy）比较类似，但是 Rust 同时还让 s1 失效了，因此用一个新的术语 "),s("code",[t._v("move")]),t._v(" 来形容。同时 Rust 也隐含了一个设计原则：即 Rust 不会自动创建数据的深拷贝，通俗的说就是一块内存只能有一个变量进行操作。就运行时性能而言，任何自动赋值操作都是廉价的。")]),t._v(" "),s("p",[s("strong",[t._v("变量与数据交互的方式：Clone")])]),t._v(" "),s("p",[t._v("要想对 heap 上面的数据进行深拷贝，可以使用 "),s("code",[t._v("clone")]),t._v(" 方法，"),s("code",[t._v("clone")]),t._v(" 是 "),s("code",[t._v("copy")]),t._v(" 子集。例子如下：")]),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("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(" s1 "),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('"Owner of Rust#"')]),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(" s2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" s1"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("clone"),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(" s1"),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("Stack上的数据:复制")]),t._v(" "),s("ul",[s("li",[t._v("Copy trait，可以用于像整数这样完全存放在stack上面的类型")]),t._v(" "),s("li",[t._v("如果一个类型实现了 Copy 这个 trait，那么旧的变量在赋值后仍然可用")]),t._v(" "),s("li",[t._v("如果一个类型或者该类型的一部分实现了 Drop trait，那么 Rust 不允许让它再去实现 Copy trait 了")])]),t._v(" "),s("p",[t._v("标准库文档里有说，std::ops::Drop 这个 trait 与 Copy_trait 无法共存于一个类型，因为在 Move 时，若发生 Copy 行为，Copy 行为是隐式的，因为是隐式的，编译器很难预测什么时候调用 Drop 函数，而实现了 Clone_trait 的，因为 clone 是显式的，需要 a.clone() 这样，那么编译器就能通过这种显式的 clone，确定被 clone 的变量的位置，决定何时调用 drop 函数。")]),t._v(" "),s("p",[s("strong",[t._v("一些拥有 Copy trait 的类型")])]),t._v(" "),s("ul",[s("li",[t._v("任何简单标量的组合类型都可以是 Copy 的")]),t._v(" "),s("li",[t._v("任何需要分配内存或某种资源的都不是 Copy 的")]),t._v(" "),s("li",[t._v("一些拥有 Copy trait 的类型:–所有的整数类型\n"),s("ul",[s("li",[t._v("例如 u32-bool")]),t._v(" "),s("li",[t._v("char")]),t._v(" "),s("li",[t._v("所有的浮点类型，例如 f64")]),t._v(" "),s("li",[t._v("Tuple(元组），如果其所有的字段都是 Copy 的\n"),s("ul",[s("li",[t._v("(i32, i32) 是")]),t._v(" "),s("li",[t._v("(i32, String) 不是")])])])])])]),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(" s1 "),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('"Owner of Rust#"')]),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 function"}},[t._v("take_ownership")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("s1"),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('"{}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" s1"),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 comment"}},[t._v("//报错，因为 s1 被 take_ownership 调用过后就会释放掉")]),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("20")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("makes_copy")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),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('"{}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" x"),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("take_ownership")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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(")")]),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('"{}"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" string"),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("makes_copy")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("num"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("u32")]),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 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(" num"),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",[s("strong",[t._v("返回值与作用域")])]),t._v(" "),s("p",[t._v("函数在返回值的过程中也会发生所有权的转移，下面的例子可以很好的帮助理解所有权这一概念：")]),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("test03")]),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(" s1 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("give_ownership")]),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\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" s2 "),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('"Rust"')]),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(" s3 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("take_and_give_ownership")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("s2"),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("give_ownership")]),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(" "),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(" string "),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('"$Rust$"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n string\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("take_and_give_ownership")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("string"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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(")")]),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 string\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("其中 s2 在函数 "),s("code",[t._v("take_and_give_ownership")]),t._v(" 调用后，所有权转移到了函数中，随着函数执行完，s2 的所有权也没有了。实际上函数的作用就是获得 s2 的所有权，然后这个所有权又返回给了 s3.")]),t._v(" "),s("p",[t._v("一个变量的所有权总是遵循同样的模式:")]),t._v(" "),s("ul",[s("li",[t._v("把一个值赋给其它变量时就会发生移动")]),t._v(" "),s("li",[t._v("当一个包含 heap 数据的变量离开作用域时，它的值就会被drop 函数清理，除非数据的所有权移动到另一个变量上")])]),t._v(" "),s("p",[t._v("那么如何让函数使用某个值，而不获得其所有权？例子如下：")]),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("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(" s1 "),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('"Welcome!"')]),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(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("s2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" len"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("calc_len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("s1"),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 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 string {}\'s length is {}."')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" s2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),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("\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("calc_len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 class-name"}},[t._v("String")]),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 punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("usize")]),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(" len "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("str")]),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(")")]),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 keyword"}},[t._v("str")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" len"),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("我们将 s1 作为参数传递进去，返回一个包含 String 和 usize 类型的元组，这样就将 s1 的所有权转移给了 s2。")]),t._v(" "),s("p",[t._v("那么如果不要传递参数能做到吗？下一节进行介绍。")]),t._v(" "),s("h3",{attrs:{id:"引用与借用"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#引用与借用"}},[t._v("#")]),t._v(" 引用与借用")]),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("test05")]),s("span",{pre:!0,attrs:{class:"token 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function"}},[t._v("calc_len_2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 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 string {}\'s length is {}."')]),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(" len"),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("calc_len_2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),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("usize")]),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("str")]),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("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("ul",[s("li",[t._v("参数类型是 "),s("code",[t._v("&String")]),t._v(" 而不是 "),s("code",[t._v("String")])]),t._v(" "),s("li",[s("code",[t._v("&")]),t._v("就表示引用，允许引用某些值而不得到其所有权")])]),t._v(" "),s("p",[s("img",{attrs:{src:"https://raw.githubusercontent.com/CherryYang05/PicGo-image/master/images/20220512190841.png",alt:"引用"}})]),t._v(" "),s("p",[t._v("注：Rust 中解引用的符号和 C/C++ 中是一样的，都是 "),s("code",[t._v("*")]),t._v(".")]),t._v(" "),s("ul",[s("li",[t._v("把引用作为函数参数的行为就叫"),s("strong",[t._v("借用")])]),t._v(" "),s("li",[t._v("和变量一样，引用也是默认不能被修改的")]),t._v(" "),s("li",[t._v("若要使其能够修改，需要加上 "),s("code",[t._v("mut")]),t._v(" 关键字")])]),t._v(" "),s("p",[t._v("例子如下：")]),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 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len"),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("calc_len_2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 operator"}},[t._v("&")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),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 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punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("若修改了一个引用对象，则会报这样的错误：")]),t._v(" "),s("p",[s("code",[t._v("cannot borrow *str as mutable, as it is behind a & reference")])]),t._v(" "),s("p",[s("strong",[t._v("可变引用")])]),t._v(" "),s("p",[t._v("可变引用有一个重要的限制:在特定作用域内，对某一块数据，只能有一个可变的引用。")]),t._v(" "),s("ul",[s("li",[t._v("这样做的好处是可在编译时防止数据竞争")])]),t._v(" "),s("p",[t._v("以下三种行为下会发生数据竞争:")]),t._v(" "),s("ul",[s("li",[t._v("两个或多个指针同时访问同一个数据一至少有一个指针用于写入数据")]),t._v(" "),s("li",[t._v("没有使用任何机制来同步对数据的访问")]),t._v(" "),s("li",[t._v("可以通过创建新的作用域，来允许非同时的创建多个可变引用")])]),t._v(" "),s("p",[t._v("例：")]),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("test06")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("这里 s1 和 s2 同时对可变变量 s 进行了引用，就会报这样的错误："),s("code",[t._v("cannot borrow s as mutable more than once at a time")])]),t._v(" "),s("p",[t._v("通过创建新的作用域，可以允许非同时的创建多个可变引用")]),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("test06")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),s("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" s2 "),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 keyword"}},[t._v("mut")]),t._v(" s"),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("另一个限制")])]),t._v(" "),s("ul",[s("li",[t._v("不可以同时拥有一个可变引用和一个不变的引用（因为不可变引用的作用就是不让值改变，被可变引用改变后，不可变引用就失去了其作用）")]),t._v(" "),s("li",[t._v("多个不变的引用是可以的")])]),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("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("test06")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),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('"Hello"')]),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(" s2 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),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(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" s3 "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),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(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("let")]),t._v(" s4 "),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 keyword"}},[t._v("mut")]),t._v(" s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//报错")]),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(" s2"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" s3"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" s4"),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("这样便会报错："),s("code",[t._v("cannot borrow s as mutable because it is also borrowed as immutable")])]),t._v(" "),s("p",[s("strong",[t._v("悬空引用 Dangling References")])]),t._v(" "),s("p",[t._v("悬空指针（Dangling Pointer): 一个指针引用了内存中的某个地址，而这块内存可能已经释放并分配给其它人使用了。")]),t._v(" "),s("ul",[s("li",[t._v("在Rust里，编译器可保证引用永远都不是悬空引用")]),t._v(" "),s("li",[t._v("如果你引用了某些数据，编译器将保证在引用离开作用域之前数据不会离开作用域")])]),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("test07")]),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(" r "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("dangle")]),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\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("dangle")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[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(" s "),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('"Dangling reference"')]),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\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("程序在 "),s("code",[t._v("dangle")]),t._v(" 函数中声明了一个字符串，期望返回其的引用，但是函数结束后 s 便离开了他的作用域，即被销毁，因此返回的引用为空。这和 C 语言中返回局部变量的地址如出一辙，但是 Rust 在编译时就将避免这样的问题发生。")]),t._v(" "),s("p",[t._v("报错："),s("code",[t._v("missing lifetime specifier")])]),t._v(" "),s("p",[s("strong",[t._v("引用的规则")])]),t._v(" "),s("p",[t._v("在任何给定的时刻，只能满足下列条件之一:一个可变的引用，或者任意数量不可变的引用，而且引用必须一直有效。")]),t._v(" "),s("h2",{attrs:{id:"六、切片"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#六、切片"}},[t._v("#")]),t._v(" 六、切片")]),t._v(" "),s("p",[t._v("Rust 的另一种不持有所有权的数据类型：切片（Slice）")]),t._v(" "),s("p",[t._v("下面编写这样一个函数进行示范：")]),t._v(" "),s("ul",[s("li",[t._v("它接收字符串作为参数")]),t._v(" "),s("li",[t._v("返回它在这个字符串里找到的第一个单词")]),t._v(" "),s("li",[t._v("如果函数没有找到任何空格，则返回整个字符串")])]),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("main")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),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('"Hello World"')]),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(" space_index "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("first_word")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 punctuation"}},[t._v(";")]),t._v("\n s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("clear")]),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 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 first blank\'s position is in {}."')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" space_index"),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("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 class-name"}},[t._v("String")]),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("usize")]),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(" bytes "),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 comment"}},[t._v("/*\n byte 的 iter 方法会为数组 byte 创建一个迭代器，这个方法会依次返回集合中的每个元素\n enumerate 方法会将 iter 返回的结果进行包装，并把每个结果作为一个元组的一部分进行返回\n 元组的第一个元素就是遍历的索引，第二个元素就是数组中的元素（是一个引用），这里实际用到的是模式匹配\n 声明了两个变量对这个元组进行解构\n */")]),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(" bytes"),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(" i"),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"),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("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("实际上这个函数的设计有一个缺陷，这个函数是将字符串空格的索引位置返回，而一旦这个结果脱离了这个字符串的上下文，这个返回值便没有了意义。换句话说，这个索引位置的结果是独立于字符串而存在的，在函数返回以后，我们就再也无法保证其有效性。举个例子，若函数获取 "),s("code",[t._v("Hello World")]),t._v(" 这个字符串的空格位置，获取到函数返回值为 "),s("code",[t._v("5")]),t._v(" 后，将该字符串清空 "),s("code",[t._v("s.clear()")]),t._v("，但是此时函数返回值 "),s("code",[t._v("space_index")]),t._v(" 的值仍然是 "),s("code",[t._v("5")]),t._v("，这跟现在的字符串便没有了任何关联，因此这个返回值便没有了意义了。这样的 API 需要关注两者之间的同步性，但是往往都会比较繁琐。")]),t._v(" "),s("p",[t._v("Rust 提供了切片类型用来解决这一问题。")]),t._v(" "),s("p",[s("strong",[t._v("字符串切片")])]),t._v(" "),s("p",[t._v("字符串切片是指向字符串中一部分内容的引用")]),t._v(" "),s("p",[t._v("形式：[开始索引..结束索引]，前闭后开")]),t._v(" "),s("p",[s("img",{attrs:{src:"https://raw.githubusercontent.com/CherryYang05/PicGo-image/master/images/20220513022342.png",alt:"字符串切片"}})]),t._v(" "),s("p",[t._v("切片是放在 stack 上，右边的数组是放在 heap 上的。")]),t._v(" "),s("p",[s("em",[t._v("【更正】：s 切片的长度和容量应该为 11.")])]),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("main")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),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('"Hello World"')]),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(" hello "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),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("..")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),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(" world "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),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("6")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("..")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("11")]),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("这里切片有三个语法糖，若切片的开始位置为 0，则可以省略写，若切片的末尾时字符串最后一个位置，即等于字符串长度，那么也可以省略不写，下面的例子和上面是等价的：")]),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("fn")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function-definition function"}},[t._v("main")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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whole"),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 comment"}},[t._v("//输出为 Hello World")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[s("strong",[t._v("注意：")])]),t._v(" "),s("ul",[s("li",[s("p",[t._v("字符串切片的范围索引必须发生在有效的 "),s("code",[t._v("UTF-8")]),t._v(" 字符边界内。")])]),t._v(" "),s("li",[s("p",[t._v("如果尝试从一个多字节的字符中创建字符串切片，程序会报错并退出")])])]),t._v(" "),s("p",[t._v("下面用切片重写上面的函数：")]),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("main")]),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(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("mut")]),t._v(" s "),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('"Hello World"')]),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(" space_index_slice "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("first_word_slice")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[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 punctuation"}},[t._v(";")]),t._v("\n s"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("clear")]),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(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//报错")]),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 first world is {}."')]),s("span",{pre:!0,attrs:{class:"token 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"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" bytes"),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 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