c++ - Assigning this pointer to rvalue reference to a pointer

Question

Should the following sample compile?

struct B;
struct A
{
  A(B*&&){}
};

struct B : A
{
  B() : A(this){}
};

int main(){}

On LWS with clang it compiles, but with gcc I get:

no known conversion for argument 1 from 'B* const' to 'B*&&'

and if I add a const it compiles.

I would like to also point out MSVC gets it wrong too:

cannot convert parameter 2 from 'B *const ' to 'B *&&'

so it looks like we have a bug in two compilers.

BUGS FILED

MSVC bug link

GCC bug link

Answer 1

Yes, that should compile.

It is incorrect to implement this as cv T* const (where cv is the cv-qualifiers for the function, if any, and T is the class type). this is not const, merely a prvalue expression of a built-in type (not modifiable).

Many people think that because you can't modify this it must be const, but as Johannes Schaub - litb once commented long ago, a much better explanation is something like this:

// by the compiler
#define this (__this + 0)

// where __this is the "real" value of this

Here it's clear that you can't modify this (say, this = nullptr), but also clear no const is necessary for such an explanation. (And the value you have in your constructor is just the value of the temporary.)