If a base class is used as an interface, make it a pure abstract class

Reason

A class is more stable (less brittle) if it does not contain data. Interfaces should normally be composed entirely of public pure virtual functions and a default/empty virtual destructor.

Example

class My_interface {
public:
    // ... only pure virtual functions here ...
    virtual ~My_interface() {}   // or =default
};

Example, bad

class Goof {
public:
    // ... only pure virtual functions here ...
    // no virtual destructor
};

class Derived : public Goof {
    string s;
    // ...
};

void use()
{
    unique_ptr<Goof> p {new Derived{"here we go"}};
    f(p.get()); // use Derived through the Goof interface
    g(p.get()); // use Derived through the Goof interface
} // leak

The Derived is deleted through its Goof interface, so its string is leaked. Give Goof a virtual destructor and all is well.

Enforcement

• Warn on any class that contains data members and also has an overridable (non-final) virtual function that wasn't inherited from a base class.