Question

I came across this due to a bug in my code and I’m curious why it’s allowed. What reason is there that allows object members to be visible in the constructor initialization list?

#include <stdio.h>

class derived {
  private:
    int * value2;
  public:
    derived();
};

derived::derived()
 : value2(value2){} // Uninitialized self-assignment

int main()
{
  derived thisChild;
}

Clang gives a warning about this but unfortunately g++ does not.

Answer 1

So you can initalise one member using another; this is perfectly fine if the other has already been initialised, or if you’re just using its address to initialise a pointer or reference. For example:

struct Thingy
{
    int & r;
    int a;
    int b;

    Thingy(int x) :
        r(a),   // OK - a has a valid address, and we're not using the value
        a(x),
        b(a)    // OK - a has been initialised
    {}
};

It would be rather tricky to allow that and disallow your example.