The address of a function matching a bool vs const void* overload

Question

I'm reading Unexpected value using random number generator as a function in C++ and the comments and current answer say that the user is outputting the address of the function. That sounded reasonable. I assumed that a function-to-pointer conversion was occurring and therefore matching the const void* overload, however upon testing it myself, I get different results in GCC/Clang vs MSVC. The following test program:

#include <iostream>

void test()
{
}

void func(bool)
{
    std::cout << "bool";
}

void func(const void*)
{
    std::cout << "const void*";
}

int main()
{
    func(test);
}

outputs bool in GCC/Clang (coliru)

and const void* in MSVC (rextester warning live collaboration link)

N3337 says:

[conv.func]

An lvalue of function type T can be converted to a prvalue of type "pointer to T." The result is a pointer to the function.

[conv.bool]

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

So a pointer which is not a null pointer value converted to bool should equal true, explaining the warning given by GCC/Clang.

Then Table 12 Conversions under [over.ics.scs] gives a function-to-pointer conversion an "Exact Match" rank and boolean conversions "Conversion" rank. [over.ics.rank]/4 then says:

Standard conversion sequences are ordered by their ranks: an Exact Match is a better conversion than a Promotion, which is a better conversion than a Conversion. Two conversion sequences with the same rank are indistinguishable unless one of the following rules applies:

— A conversion that does not convert a pointer, a pointer to member, or std::nullptr_t to bool is better than one that does.

— [...]

I am not a language lawyer so I hope that I quoted the right sections.

However MSVC will call the const void* overload even if the bool overload is absent, and vice versa: GCC/Clang will call the bool overload even if the const void* overload is absent. So I'm not clear on the conversions here. Can somebody clear this up for me?

Angew is no longer proud of SO Angew is no longer proud of SO · Accepted Answer · 2014-12-01T10:55:11

Seems like a bug (or extension) in MSVC. The standard does not define any standard conversions from a "pointer to function" to a "pointer to void."

It's hard to provide a quote for the absence of something, but the closest I can do is C++11 4.10/2 [conv.ptr]:

A prvalue of type “pointer to cv T,” where T is an object type, can be converted to a prvalue of type “pointer to cv void”. The result of converting a “pointer to cv T” to a “pointer to cv void” points to the start of the storage location where the object of type T resides, as if the object is a most derived object (1.8) of type T (that is, not a base class subobject). The null pointer value is converted to the null pointer value of the destination type.

Together with 3.9/8 [basic.types]:

An object type is a (possibly cv-qualified) type that is not a function type, not a reference type, and not a void type.

(emphasis mine)

Using /Za to disable extensions will disable the non-standard conversion.

The address of a function matching a bool vs const void* overload

1 Answers