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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("将 Result<T, E> 设置成函数返回值，这样就将错误传递给了调用者，若文件 foo 存在的话，最终便可以输出文件中的内容。")]),t._v(" "),s("p",[t._v("Rust 中还提供了 "),s("code",[t._v("?")]),t._v(" 运算符，用其来简化传播错误的操作。")]),t._v(" "),s("p",[t._v("如果 Result 是 Ok：Ok 中的值就是表达式的结果，然后继续执行程序；\n如果 Result 是 Err：Err 就是 "),s("strong",[t._v("整个函数")]),t._v(" 的返回值，就像使用了 return。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("read_text_from_file_easy")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token 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function"}},[t._v("open")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"foo"')]),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 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("new")]),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 f"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("read_to_string")]),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("mut")]),t._v(" s"),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 punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Ok")]),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("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("上面这段简化后的代码的含义就是，若 "),s("code",[t._v("?")]),t._v(" 前 Result 类型的值是 Ok，那么 Ok 里的值就会作为表达式的返回值进行返回，若类型是 Err，那么 Err 就当做整个函数的返回值进行返回。而 "),s("code",[t._v("f.read_to_string(&mut s)?;")]),t._v(" 中，若 Result 类型是 Ok，实际上里面值为空，没有用到，因此当表达式返回 Ok 后，返回一个 Ok(s) 作为函数的返回值，若类型为 Err，则将其作为函数返回值进行返回。")]),t._v(" "),s("p",[t._v("上面这个例子还可以继续进行优化，使用链式调用：")]),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("read_text_from_file_easist")]),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("Result")]),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(" 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punctuation"}},[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 punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[s("strong",[t._v("值得注意的是，要使用 ? 运算符，必须保证函数返回类型为 Result<T, E>")]),t._v("，倘若我们尝试一下函数返回类型不是 Result，将会得到这样一条报错信息："),s("code",[t._v("error[E0277]: the '?' operator can only be used in a function that returns 'Result' or 'Option' (or another type that implements 'FromResidual')")])]),t._v(" "),s("p",[t._v("因此，? 运算符只能用于返回类型为 Result 或 Option 的函数")]),t._v(" "),s("p",[s("strong",[t._v("? 运算符与 main 函数")])]),t._v(" "),s("ul",[s("li",[t._v("main 函数返回类型是:()")]),t._v(" "),s("li",[t._v("main 函数的返回类型也可以是：Result<T，E>")])]),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 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string"}},[t._v('"foo"')]),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 punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Ok")]),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("p",[s("code",[t._v("Box<dyn Error>")]),t._v(" 是 trait 对象，可以简单理解为“任何可能的错误类型”。")]),t._v(" "),s("p",[t._v("这样就可以在 main 函数中使用 "),s("code",[t._v("?")]),t._v(" 运算符了。")]),t._v(" "),s("p",[t._v("（开始玄学 o_o）")]),t._v(" "),s("p",[s("strong",[t._v("? 与 from 函数")])]),t._v(" "),s("ul",[s("li",[s("code",[t._v("Trait std:convert::From")]),t._v(" 上的 from 函数\n"),s("ul",[s("li",[t._v("用于错误之间的转换")])])]),t._v(" "),s("li",[t._v("被 ? 所应用的错误，会隐式的被 from 函数处理")]),t._v(" "),s("li",[t._v("当 ? 调用 from 函数时\n"),s("ul",[s("li",[t._v("它所接收的错误类型会被转化为当前函数返回类型所定义的错误类型")])])]),t._v(" "),s("li",[t._v("用于：针对不同错误原因，返回同一种错误类型\n"),s("ul",[s("li",[t._v("只要每个错误类型实现了转换为所返回的错误类型的 from 函数")])])])]),t._v(" "),s("h3",{attrs:{id:"_2-4-什么时候应该使用-panic"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_2-4-什么时候应该使用-panic"}},[t._v("#")]),t._v(" 2.4 什么时候应该使用 panic!")]),t._v(" "),s("p",[s("strong",[t._v("总体原则")])]),t._v(" "),s("p",[t._v("在定义一个可能失败的函数时，优先考虑返回 Result，若你觉得这个错误一定无法恢复，那就可以代替调用者调用 panic!")]),t._v(" "),s("p",[s("strong",[t._v("编写示例、原型代码、测试")])]),t._v(" "),s("p",[t._v("可以使用panic!")]),t._v(" "),s("ul",[s("li",[t._v("演示某些概念: unwrap")]),t._v(" "),s("li",[t._v("原型代码: unwrap、expect")]),t._v(" "),s("li",[t._v("测试: unwrap、expect\n"),s("ul",[s("li",[t._v("测试的失败是用 panic! 进行标记的")])])])]),t._v(" "),s("p",[s("strong",[t._v("有时你比编译器掌握更多的信息")])]),t._v(" "),s("p",[t._v("你可以确定 Result 就是 Ok，那么可以使用 unwrap，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("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("net"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("::")])]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IpAddr")]),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("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 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"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token comment"}},[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("这样一个功能就是判断输入的数是否符合 i32 类型，若符合的话表达式返回 num，然后再判断是否在 1~100 之间，如果不满足则继续循环。如果有多个函数中都需要类似这样的判断，则代码便会显得冗余，我们可以自定义一个验证逻辑：")]),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 keyword"}},[t._v("struct")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token type-definition class-name"}},[t._v("Guess")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" 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punctuation"}},[t._v("(")]),t._v("guess"),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("如果能够成功创建 Guess 实例的话，那么就说明值通过了验证，而不需要将验证功能写在函数里了。")]),t._v(" "),s("p",[t._v("上述 value 方法是获得 Guess 结构体中的 value 字段值，因为结构体中的字段是私有的，外部无法直接对字段赋值。")])])}),[],!1,null,null,null);s.default=r.exports}}]);
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