Self-initialization question

Question

This code fails at runtime in the copy constructor. But the compiler (MSVS2008) issues no warnings.

Could you explain (preferably cite the standard) whether this code is illegal or what?

I understand that A a = a; should never be written at the first place, but I am looking for a theoretical background.

 class A
 {
 public: 

    A()
    :p(new int)
    {
    }

    A(const A& rv)
    {
     p = new int(*rv.p);
    }

    ~A()
    {
     delete p;
    }


 private:

    int *p;
 };

 int main()
 {
    A a = a;
 }

Answer 1

Your code is not calling the standard constructor but the copy constructor, so you are accessing an uninitialized pointer.

Answer 2

A a = a; definitely should not be written. But a = a could be written. Your assignment operator must check for &rv == this and do nothing in case of a self-copy.

Oh, yes, you do need to write an assignment operator for class A.

Answer 3

According to the standard (12.6.1 [class.expl.init] ) self initialization is perfectly legal.

Therefore the following is legal.

A a = a;

You just need to write your copy constructor to deal with it correctly.

A(const A& rv)
{
    if(&rv == this) {
        p = new int(0);
        return;
    }

    p = new int(*rv.p);
}

Edit: Updated code based on comments.

Answer 4

Interesting reading on self-assignment: http://www.gotw.ca/gotw/011.htm

In particular, note "Postscript #1" in relation to this question and some of the answers given.