Question

class ClassA {
public:
    ClassA() {} // when this function must be provided
    virtual ~ClassA() = 0 {}
};

class ClassB : public ClassA
{
    // ...        
};

I want to know when the default constructor of an abstract base class must be provided.

Answer 1

there is no connection between providing a default constructor, and the abstractness or not of the class.

provide a default constructor definition if you need to initialize things.

provide a (possibly non-implemented) non-public declaration if you want to prohibit default construction.

cheers & hth.,