成员函数指针的奇怪C ++规则？ [英] Strange C++ rule for member function pointers?

问题描述

可能重复：

包含成员函数地址的错误

在这个最近的问题中，OP遇到了一个奇怪的C ++语言配置，使它非法以获取成员函数的地址（如果该成员函数名称带括号）。例如，此代码是非法的：

In this recent question the OP ran into a strange provision of the C++ language that makes it illegal to take the address of a member function if that member function name is parenthesized. For example, this code is illegal:

struct X {
    void foo();
};

int main() {
    void (X::* ptr)();
    ptr = &(X::foo);   // Illegal; must be &X::foo
}

我看了一下，发现由于C ++ ISO规范的§5.3.1/ 3，它读取

I looked this up and found that it's due to §5.3.1/3 of the C++ ISO spec, which reads

指向成员的指针只有在显式& [...]

A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses [...]

有没有人知道为什么规范有这个规则吗？它特定于指向成员的指针，所以我怀疑有一些语法歧义，这个解决，但我真的没有最昏暗的想法它可能是什么。

Does anyone have any idea why the spec has this rule? It's specific to pointers-to-member, so I would suspect that there is some grammatical ambiguity that this resolves, but I honestly haven't the faintest idea what it might be.

推荐答案

这只是一个个人意见。
如果&（unary-expression）允许&（qualified-id） b $ b qualified-id必须是一个表达式，并且表达式应该具有类型
（即使它不完整）。
然而，C ++没有表示成员的类型，只有
是指向成员的指针。
例如，以下代码不能编译。

This is just a personal opinion. If &(qualified-id) is allowed as &(unary-expression), qualified-id has to be an expression, and an expression is expected to have a type (even if it is incomplete). However, C++ didn't have a type which denotes a member, had only a pointer to member. For example, the following code cannot be compiled.

struct A { int i; };

template< class T > void f( T* );

int main() {
  (void) typeid( A::i );
  f( &A::i );
}

为了使& ）有效，编译器必须在内部持有
a成员类型。
然而，如果我们放弃&（qualified-id）符号，编译器不需要
来处理成员类型。
作为成员类型总是以指向它的指针的形式处理，
我想标准优先简化编译器的类型
系统一点。

In order to make &(qualified-id) be valid, the compiler has to hold a member type internally. However, if we abandon &(qualified-id) notation, the compiler doesn't need to handle member type. As member type was always handled in the form of a pointer to it, I guess the standard gave priority to simplify the compiler's type system a little.

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