Can I force a derived class to define a copy/move operator/constructor?

[woohyeon]

Hi.

I want to force a derived class to define user defined copy constructor/assignment operator, and move too.

It will be good if acting like a pure virtual function.

Is there any way?

[thmm]

I don't think that there is a way.
Why do you want to do it? It would make the class much more difficult to use.

[ne555]

you could make your base class non-copiable/assignable/movable, then the derived classes would need to implement custom copy/assign/move constructor
to do that, either delete them or declare them private.

> It will be good if acting like a pure virtual function.
no sure if I understood you

class vehicle;
class car: public vehicle;
class bicycle: public vehicle;

bicycle b; car c;
b = c; //¿what do you expect to happen here? 

[thmm]

@ne555,
not sure if I understand you correctly. Even if you delete the copy and assignment ctors and operators you could still create a derived class without implementing these.

#include <iostream>

class Useless
{
public:
  Useless(){}
private:
  Useless(const Useless&) = delete;
  Useless(Useless&&) = delete;

  Useless& operator=(const Useless&) = delete;
  Useless& operator=(Useless&&) = delete;
};

class Derived: public Useless
{
public:
  Derived(): Useless()
  {
    std::cout << "Derived ctor";
  }
};

int main()
{
  Derived d;
}

Output:

Derived ctor

[keskiverto]

@thmm:
Derived e = d; generates:

 In function 'int main()':
27:15: error: use of deleted function 'Derived::Derived(const Derived&)'
15:7: note: 'Derived::Derived(const Derived&)' is implicitly deleted because the default definition would be ill-formed:
8:3: error: 'Useless::Useless(const Useless&)' is private
15:7: error: within this context
15:7: error: use of deleted function 'Useless::Useless(const Useless&)'
8:3: note: declared here

The point is that you can't copy or move Derived trivially.

Compiler can generate implicit copy/move for derived classes.
If that does the "right thing", then why should we have to write explicit constructors/assignments?

If the implicit implementation is not appropriate for a class, then the programmer should write explicit version.
However, there is no way for the author of base class to know whether the derived class will have such need.

[thmm]

The point is that you can't copy or move Derived trivially.

@keskiverto,
that I understand. You still could use Derived without copy or move, so it's not really enforced.
Maybe the OP tells us the reason behind, somehow I can't see a real use for it.