rust_camp_tutorial/assets/js/31.e1aa8cbc.js

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"),s("h3",{attrs:{id:"_2-3-实现-trait-的约束"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_2-3-实现-trait-的约束"}},[t._v("#")]),t._v(" 2.3 实现 trait 的约束")]),t._v(" "),s("ul",[s("li",[t._v("可以在某个类型上实现某个 trait 的前提条件是\n"),s("ul",[s("li",[t._v("这个类型或这个 trait 是在本地 crate 里定义的")])])]),t._v(" "),s("li",[t._v("无法为外部类型来实现外部的trait\n"),s("ul",[s("li",[t._v("这个限制是程序属性的一部分（也就是一致性）")]),t._v(" "),s("li",[t._v("更具体地说是"),s("strong",[t._v("孤儿规则")]),t._v("：之所以这样命名是因为父类型不存在")]),t._v(" "),s("li",[t._v("此规则确保其他人的代码不能破坏您的代码，反之亦然")]),t._v(" "),s("li",[t._v("如果没有这个规则，两个 crate 可以为同一类型实现同一个 trait，Rust 就不知道应该使用哪个实现了")])])])]),t._v(" "),s("p",[s("strong",[t._v("默认实现")])]),t._v(" "),s("p",[t._v("默认实现的方法可以调用 trait 中的其他方法，即使这些方法没有默认实现，但是注意，无法从方法的重写实现中调用默认实现。")]),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("pub")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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"),s("p",[t._v("我们再来看一下"),s("a",{attrs:{href:"#12-%E5%9C%A8%E5%87%BD%E6%95%B0%E5%AE%9A%E4%B9%89%E4%B8%AD%E4%BD%BF%E7%94%A8%E6%B3%9B%E5%9E%8B"}},[t._v("之前的代码")]),t._v("。解决如下：")]),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("largest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[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 class-name"}},[t._v("PartialOrd")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("list"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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errors\n")])])]),s("p",[t._v("报错原因是：无法从 list 中移除 T，因为没有实现 Copy trait，建议采用借用")]),t._v(" "),s("p",[t._v("因为上面两个 vector 中的元素分别为整型和字符型，这两种类型有确定的大小并且都是存储在栈中，因此都实现了 Copy trait，于是在 T 的 trait 约束中再加上 Copy 即可：")]),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("largest")]),s("span",{pre:!0,attrs:{class:"token operator"}},[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 class-name"}},[t._v("PartialOrd")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Copy")]),s("span",{pre:!0,attrs:{class:"token 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`"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),t._v("`"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" which does not implement the `"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Copy")]),t._v("` "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("trait")]),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(" consider borrowing here"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" `"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("&")]),t._v("list"),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("`\n")])])]),s("p",[t._v("是因为这里 list[0] 是字符串切片，是一个借用，没有所有权，因此一个借用给一个变量赋值，这个借用对应的类型必须要实现 Copy trait。因此在 list 前面加上引用，并且将 item 也设为引用，最后返回 &T。")]),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("get_max_ele")]),s("span",{pre:!0,attrs:{class:"token operator"}},[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 class-name"}},[t._v("PartialOrd")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Clone")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("list"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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