Question

I have this class:

class A : public B

and i need to add some protected field: _field, in A and I can’t touch/change B.
Now, all the function in B and A are virtual except the constructor. Obviously, _field is not part of class B.

I need to initialize _field. How can i do that if the only constructor is B’s?
Also, something like this:

unsigned long _field = 0;

gives me an error compilation.

I solve this by:

class A : public B
{
protected: 
  unsigned long _field;
public:
  void fooFunction(){
     ....do other stuff....
     static bool isInitField = false;
     if (!isInitField){
       _field = 0;
       isInitField = true;
     }
     ...rest of the function...
  }

Is there a better way to do that without using static?

Thanks,
Or

Answer 1

Use A constructor and call B constructor in the initialization list, this way you initialize all fields of class A:

A(unsigned long i, other_paramaters oth) : B(oth),_field(i) {

}