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

You might place your special member in a base class, something like:

class xyz;

class SpecialCopyXYZ
{
public:
    SpecialCopyXYZ() = default;
    SpecialCopyXYZ(const SpecialCopyXYZ& rhs)
    {
       if (CanCopy(other.some_private_ptr)) {
          some_private_ptr = other.some_private_ptr;
       } else {
          some_private_ptr = nullptr;
       }
    }

    // SpecialCopyXYZ& operator=(const SpecialCopyXYZ& rhs)

protected:
    xyz *some_private_ptr = nullptr;
};


class C : private SpecialCopyXYZ
{
public:
    C(const C &other) = default;
private:
    int a, b, c; 
    std::set mySet;
};

If SpecialCopyXYZ need C data, you may use CRTP and downcast.