std :: add_pointer，在可能的实现中try_add_pointer的作用是什么? [英] std:: add_pointer, what the try_add_pointer for in the possible implementation?

问题描述

在cppreference add_pointer 中，我们可以将其实现为:

In cppreference add_pointer, it is said we may implement it as:

namespace detail {

template <class T>
struct type_identity { using type = T; }; // or use std::type_identity (since C++20)

template <class T>
auto try_add_pointer(int) -> type_identity<typename std::remove_reference<T>::type*>;
template <class T>
auto try_add_pointer(...) -> type_identity<T>;

} // namespace detail

template <class T>
struct add_pointer : decltype(detail::try_add_pointer<T>(0)) {};

我的问题是 try_add_pointer 是干什么的?我知道是SFINAE.但是为什么这里需要实现呢?

My question is what is the try_add_pointer for? I know it is SFINAE. But why the implementation need it here?

推荐答案

如果您阅读了有关cppreference的页面，您会注意到这句话

If you read over the page on cppreference, you'll notice this sentence

否则(如果T是cv或ref限定的函数类型)，则提供成员typedef类型，即类型T.

Otherwise (if T is a cv- or ref-qualified function type), provides the member typedef type which is the type T.

函数有自己的类型.通常，您会在正常代码中看到的是类似 int(int)的类型，即，采用单个整数并返回整数的函数的类型.这是参数 std :: function 所期望的类型，例如 std :: function< int(int)> .

Function have their own types. Normally, what you'll see in sane code is types like int(int), i.e. the type of a function taking a single integer and returning an integer. This is the type of argument std::function expects, e.g std::function<int(int)>.

但是，函数类型集还包含与成员函数有关的奇数.例如

However the set of function types also contains oddities that relate to member functions. For instance

struct foo {
  int bar(int) const;
};

int(int)const 是 bar 的函数类型.尽管此类型存在于类型系统中，但其用途受到限制.

int(int) const is the function type of bar. While this type exists in the type system, it is limited in its uses.

[dcl.fct]

6 一个cv-qualifier-seq或ref-qualifier只能是以下内容的一部分:

6 A cv-qualifier-seq or a ref-qualifier shall only be part of:

• 非静态成员函数的函数类型，
• 指向成员的指针所指向的函数类型，
• 函数typedef声明或别名声明的顶级函数类型，
• 类型参数([temp.param])的默认参数中的type-id，或者
• 类型参数([temp.names])的模板参数的type-id.

在函数声明器中cv-qualifier-seq的作用不是与在函数类型的顶部添加cv-qualification相同.在里面在后一种情况下，将忽略cv限定词.[注意:函数类型具有cv-qualifier-seq的不是cv-qualified类型；没有具有cv资格的函数类型.—尾注] [示例:

The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type. In the latter case, the cv-qualifiers are ignored. [ Note: a function type that has a cv-qualifier-seq is not a cv-qualified type; there are no cv-qualified function types. — end note ] [ Example:

typedef void F();
struct S {
  const F f;        // OK: equivalent to: void f();
};

—结束示例]返回类型，参数类型列表，ref-qualifier和cv-qualifier-seq，但不是默认参数([dcl.fct.default])或例外说明([except.spec])，是函数类型的一部分.

— end example ] The return type, the parameter-type-list, the ref-qualifier, and the cv-qualifier-seq, but not the default arguments ([dcl.fct.default]) or the exception specification ([except.spec]), are part of the function type.

因此trait允许您将其提供给 int()const 之类的函数类型，并且期望将其返回不变.

So the trait allows you to feed it a function type like int() const, and it's expected to return it unchanged.

那里是 try_add_pointer 的来源.由于从上面的列表中可以看到，没有常规的指针指向这些类型的函数，因此我们将在 typename std中得到替换失败::remove_reference< T> :: type * .但多亏SFINAE，回退存在.

That's where try_add_pointer comes in. Since as you can see from the list above, there are no regular pointers to these sort of functions, we will get a substitution failure in typename std::remove_reference<T>::type*. But thanks to SFINAE, a fallback exists.

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