打印功能地址...声明？

Question

Consider the following MCVE:

#include <iostream>

int main() 
{
    void foo(int);
    std::cout << foo << std::endl;

    return 0;
}

Here I intentionally try to print a pointer to a function in a wrong way so that the operator<< overload which accepts bool is chosen.

 basic_ostream& operator<<( bool value );

The thing that puzzles me is that both gcc 7.2 and clang 5.0 produce a warning, but compile and link the program.

In the same time, Visual Studio 15.5.6 doesn't link this example.

Personally, I expected, that this code won't link at all despite the compiler used as foo seems to be ODR-used .

Could anybody explain why gcc and clang are able to link the program?

Answer 1

It is an ODR violation. But according to [basic.def.odr]/10 , emphasis mine:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required . The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined (see [class.ctor], [class.dtor] and [class.copy]). An inline function or variable shall be defined in every translation unit in which it is odr-used outside of a discarded statement.

And we must recall that compilers are free to assume you don't write code that exhibits undefined behavior or is otherwise ill-formed in ways they don't need to diagnose. Because each function must have an address that isn't null, the bool overload can just be called with true , since that's what the conversion must yield in a valid program .

We can see GCC 7.3 doing just that. It passes 1 for what is meant to be the result of the conversion, even at -O0 .