c++ abstract base class private members

c++ class abstract base
valmo picture valmo · Apr 6, 2011 · Viewed 17.4k times · Source

Just wanted some clarification. Should abstract base classes never have private members? For example

class abc{
public:
  virtual void foo()=0;
private:
  int myInt;
}

you can never access myInt since you cannot create an instance of abc and it will not be in a derived class since its private. Is there any situation where you would use private members in abstract base classes or is this just wrong?

Answer

David Rodríguez - dribeas picture David Rodríguez - dribeas · Apr 6, 2011

In C++ you can have an abstract class that has non pure virtual methods. In that case, and depending on the design it can make sense to have private members:

class base {
   std::string name;
public:
   base( std::string const & n ) : name(n) {}
   std::string const & getName() const { return name; }

   virtual void foo() = 0;
};

That code ensures that every object that derives from base has a name, that is set during construction and never changes during the lifetime of the object.

EDIT: For completion after Charles Bailey reminded me of it in his answer

You can also define pure-virtual functions, and in that case, private attributes could also make sense:

// with the above definition of base
void base::foo() {
   std::cout << "My name is " << name << std::endl;
}

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