箭头操作符（ - >） [英] arrow operator (->) in function heading

问题描述

我发现这样的代码：

template <typename T, typename T1> auto compose(T a, T1 b) -> decltype(a + b) {
   return a+b;
}

我想到了所有的细节，
告诉我，在哪里可以看到，箭头运算符（ - > ）是什么意思在函数标题？
我认为纯逻辑上， - > 运算符确定一个类型，将由 auto ，但我想得到这个，但是找不到信息。

I figured with all details, that were new to me, but one. Tell me please, where can I read about, what does the arrow operator (->) mean in function heading? I guess purely logically, that -> operator determines a type, that will be gotten by auto, but I want to get this straight, but can't find information.

推荐答案

在C ++ 11中，函数声明的语法：

In C++11, there are two syntaxes for function declaration:

     return-type （ 参数声明... ）

    return-type identifier ( argument-declarations... )

和

     auto （ argument-declarations ... ） - & code> return_type

    auto identifier ( argument-declarations... ) -> return_type

它们是等价的。现在当他们是等同的，你为什么要使用后者？好吧，C ++ 11介绍了这个很酷的 decltype 东西，让你描述一个表达式的类型。所以你可能想从参数类型派生返回类型。所以你尝试：

They are equivalent. Now when they are equivalent, why do you ever want to use the later? Well, C++11 introduced this cool decltype thing that lets you describe type of an expression. So you might want to derive the return type from the argument types. So you try:

template <typename T1, typename T2>
decltype(a + b) compose(T1 a, T2 b);

，编译器会告诉你它不知道 code>和 b 位于 decltype 参数中。这是因为它们只是由参数列表声明。

and the compiler will tell you that it does not know what a and b are in the decltype argument. That is because they are only declared by the argument list.

你可以使用 declval 和已声明的模板参数。喜欢：

You could easily work around the problem by using declval and the template parameters that are already declared. Like:

template <typename T1, typename T2>
decltype(std::declval<T1>() + std::declval<T2>())
compose(T1 a, T2 b);

除非它现在变得很冗长。所以替代声明语法被提出和实现，现在你可以写

except it's getting really verbose now. So the alternate declaration syntax was proposed and implemented and now you can write

template <typename T1, typename T2>
auto compose(T1 a, T2 b) -> decltype(a + b);

它的简洁性和范围规则不需要改变。

and it's less verbose and the scoping rules didn't need to change.

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