How can I initialize a const variable of a base class in a derived class' constructor in C++?

Question

I have an abstract C++ class with no constructor. It's supposed to be a base class so other classes can inherit from it. What I am trying to do is to declare a constant variable in the base class and initialize it in each derived class' constructor but nowhere else in each one of those classes. Is it legal in C++? If so, how can I do that?

Answer 1

Is it legal in C++?

No. The constant must be initialized in the base class constructor.

The solution is to provide an appropriate constructor in your base class – otherwise it cannot be used. Furthermore, there’s no reason not to provide that constructor.

class Base {
    int const constant;
public:
    virtual ~Base() = 0; // Makes this an abstract base class.
protected:
    Base(int c) : constant(c) { }
};

// Must be implemented!
Base::~Base() { }

class Derived : public Base {
public:
    Derived() : Base(42) { }
};