Question

I reduced this:

struct A
{
   int * x;
   A() : x( x = new int() )
   {
   }
};

to the following:

int m = m = 3;
//or
struct X;
//...
X x = x = X();

Seems legal to me. I don’t see why you’d want to do it, but is it legal? Are there cases where you’d want to do this (not the int case, I realize that’s completely useless)?

Answer 1

It depends on how you define “legal”. It will compile; that doesn’t mean that it is guaranteed to work.

Until the full statement X x = ... executes, x is uninitialized. It is not an X yet. Therefore, performing x = X() means to create a temporary X and call X::operator=(const X&) on the uninitialized variable x.

Calling a function on a non-POD class instance that has not been initialized (who’s constructor has not yet been called) yields undefined behavior. If X is a POD type (or trivial in C++11), then it will work. But otherwise no.