C ++:(((A *)nullptr)-> foo();是合法的吗? [英] C++ : ((A*)nullptr)->foo(); is legal?

问题描述

在深入研究MFC时，我发现了以下代码:

While digging deep into MFC, I found this code:

_AFXWIN_INLINE HWND CWnd::GetSafeHwnd() const
    { return this == NULL ? NULL : m_hWnd; }

似乎是以这种方式使用的.

It seems to be used in this way.

CWnd *pWnd = nullptr;
pWnd->GetSafeHwnd(); // NULL
pWnd = /* something */;
pWnd->GetSafeHwnd(); // window handle

在这一点上，我很困惑-我们现在使用 NULL 对象调用成员函数！怎么会是合法的C ++?

At this point, I've got confused - we're now calling a member function with NULL object! How can it be legal C++?

推荐答案

就语言而言，这是最明确的未定义行为.§9.3.1[class.mfct.non-static]/p2:

This is the plainest of undefined behavior as far as the language is concerned. §9.3.1 [class.mfct.non-static]/p2:

如果为对象调用了类 X 的非静态成员函数该行为不是 X 类型，也不是从 X 派生的类型是未定义的.

If a non-static member function of a class X is called for an object that is not of type X, or of a type derived from X, the behavior is undefined.

想必，编写此函数的人都知道，在这种特殊情况下，Microsoft的编译器不会做任何疯狂的事情，因为安全"调用null CWnd * 似乎是重点它的存在(！).

Presumably, whoever wrote this function knows that Microsoft's compiler won't do anything crazy in this particular case, as being "safe" to call on a null CWnd * appears to be the whole point for its existence(!).

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