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?

Only the literal 0 can be a null pointer constant. This shouldn't compile indeed. — Quentin
– Quentin, Commented Jul 1, 2015 at 14:02
Surprisingly GCC compiles it too, but not without spewing "warning: converting 'false' to pointer type for argument 1 of [...] [-Wconversion-null]" — Quentin
– Quentin, Commented Jul 1, 2015 at 14:04

Community · Accepted Answer · 2017-05-23 12:06:04Z

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.

I believe you are wrong because bool is an integral type (§3.9.1/7)
@KonradRudolph bool is an integral type, but false is not an integer literal.
@TartanLlama How so? It’s an integer and it’s a literal, and its value is 0.
@KonradRudolph [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."

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