C++ Implicit conversion from bool to string

Question

I have the following code, which compiles in Visual C++ 2012.

#include <string>

void func(std::string str)
{
}

void my_func()
{
    func(false);
}

The boolean 'false' is implicity passed into the string constructor

string(const char* _Ptr)

And then the pointer is null (because false = 0). Why does this compile, and should it compile according to the C++11 standard?

Answer 1

MSVC is mistakenly treating false as a null pointer constant. However, according to N4140, §4.10 [conv.ptr]/1 (emphasis mine):

A null pointer constant is an integer literal with value zero or a prvalue of type std::nullptr_t . A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type.

The wording changed a bit from C++11, and you can find that discussion here . The verdict there was that it was an error in C++11 as well.

For visibility, TartanLlama provided the definition of "integer literal" below, according to [lex.icon]/1:

An integer literal is a sequence of digits that has no period or exponent part, with optional separating single quotes that are ignored when determining its value.