Is using a member function as an argument to a constructor undefined behavior?

Question

#include <cstdio>

class A
{
public:
    A(int){puts("3");};

    int foo(){puts("4");return 10;}
};

int main()
{
    A a(a.foo());
    return 0;
}

Outputs 4 and 3.

It calls a member function before calling the constructor. Is the behavior defined by the standard?

Answer 1

§12.7 [class.cdtor]/p1:

For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.

A conforming compiler is allowed to emit code that blows your legs off.

Answer 2

Yes. In practice, it may work, because A::foo doesn't se any state from a instance. You should never write code like this (and you should probably correct it).