There’s a series of C++ FAQ: http://www.parashift.com/c++-faq/pointers-to-members.html. And one of it addresses some technical details:
Pointers to member functions and pointers to data are not necessarily represented in the same way. A pointer to a member function might be a data structure rather than a single pointer. Think about it: if it’s pointing at a virtual function, it might not actually be pointing at a statically resolvable pile of code, so it might not even be a normal address – it might be a different data structure of some sort.
Let’s write some demo code:
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#include <iostream> using namespace std; class A { public: void fun1(); }; class B { public: virtual void fun2(); }; class C : public A, public B { }; int main() { typedef void (A::*pfnAFun1)(); typedef void (B::*pfnBFun2)(); typedef void (C::*pfnCFun1)(); typedef void (C::*pfnCFun2)(); cout << "sizeof(pfnAFun1) is " << sizeof(pfnAFun1) << endl; cout << "sizeof(pfnBFun2) is " << sizeof(pfnBFun2) << endl; cout << "sizeof(pfnCFun1) is " << sizeof(pfnCFun1) << endl; cout << "sizeof(pfnCFun2) is " << sizeof(pfnCFun2) << endl; //delete cout; return 0; } |
Output on WinXP/VS2005:
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sizeof(pfnAFun1) is 4 sizeof(pfnBFun2) is 4 sizeof(pfnCFun1) is 8 sizeof(pfnCFun2) is 8 |
Output on Ubuntu12.04/gcc4.6:
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sizeof(pfnAFun1) is 8 sizeof(pfnBFun2) is 8 sizeof(pfnCFun1) is 8 sizeof(pfnCFun2) is 8 |
Both are run on 32bit systems. Sizes of pointer-to-member-function are not confirmed to be equal to sizeof(void *). And you are not allowed to convert a pointer-to-member-function to void * or a plain pointer-to-function type. Only equality comparisons(=, !=) are supported. Thus, it can be used to implement the “Safe Bool Idiom” here: http://www.artima.com/cppsource/safebool.html.
Perhaps the best-known use of this technique comes from the C++ Standard the conversion that allows the state of iostreams to be queried uses it.
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if (std::cin) { // Is the stream ok? } |
But at least in gcc/libstdc++, its implementation uses bool and void * conversion operations. In basic_ios class:
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public: //@{ /** * @brief The quick-and-easy status check. * * This allows you to write constructs such as * "if (!a_stream) ..." and "while (a_stream) ..." */ operator void*() const { return this->fail() ? 0 : const_cast<basic_ios*>(this); } bool operator!() const { return this->fail(); } //@} |
This allows std::cout to even be deleted. Hmmm…Just write down some details here.