Inheritance, Calling base class ctors

Question

class Base{
    public:
      Base(int val):_id(val){};
      int _id;
};

class Derived : Base {
    public:
      Derived(int val):Base(_id+val){};
};

int main(){
     Derived d(60);
}

why doesn't this give an error? Base class is still not constructed but I'm able to use '_id'?

thanks

Answer 1

It's not constructed but its memory was allocated and therefore _id exists and holds an uninitialized value.

Answer 2

I can't understand the question. It works because the c++ standard allows this kind of behaviour. The memory for the class has been already allocated at this moment so it is possible to use this variable.

Answer 3

A simpler example : int x = x + 1; show you that c++ compilers don't track variable initialization. In your example, _id exists in memory (it has an address), but was never initialized. However, since the compiler does not track, there is no error.

Answer 4

Imagine there was an invisible malloc at the point where you typed the : to start the initializer list. It would give you an uninitialized block of memory big enough to hold a Derived object, including the Base part. The compiler knows the offset that _id refers to, so it willingly gives you the junk value that lives there.

It's legal to reference _id after the call to Base() in the initializer list. Compilers don't make a special case for when the base class constructor hasn't been called yet.

Answer 5

It would be an error if you properly declared your member attribute private. Mixing public data with inheritance like this is probably only going to cause confusion or incorrect behavior such as you've observed. If your members were private, the compiler would prevent this mistake AND it enhances your class encapsulation.

EDIT: The reason _id is usable is because the memory for it has been allocated, it just hasn't been initialized yet. Think something along the lines of:

int x;
int y = x;

You have no idea what will be in y because x was never initialized.