How can I extend a compiler generated copy constructor

Question

I frequently run into the problem, that I must extend a compiler generated copy constructor. Example:

class xyz;
class C
{
    ...
    int a, b, c; 
    std:         


        

Answer 1

The easiest way is to introduce a base class:

class xyz;

struct CDetail {
  //...
  int a, b, c; 
  std::set mySet;
  xyz *some_private_ptr;
};

struct C : private CDetail {
  C(C const &other)
  : CDetail(other)
  {
    if (!CanCopy(other.some_private_ptr))
      some_private_ptr = 0;
    // opposite case already handled
  }
};

This is an abuse of inheritance to an extent, but the advantages over a nested "impl" class are 1) you can access each member as "name" rather than "data.name" (reducing code changes when refactoring), and 2) (though only sometimes desired) you can "promote" individual members to protected or public without affecting other members:

struct C : private CDetail {
protected:
  using CDetail::a;
};

struct D : C {
  void f() {
    cout << a;
  }
};

int main() {
  D d;
  d.f();  // D can access 'a'
  cout << d.a;  // main cannot access 'a'
  return 0;
}