有一个没有构造函数的类有意义吗？

Question

I am wondering whether it makes sense to have a class without constructor if the derived classes have one. If not, is there a way of doing what I want differently?

My case is the following: I want the users to only use the derived classes, but those have data members and methods in common.

EDIT: as default constructors exist for classes, it means that the user can always create an instance of Instrument directly, then how can I do what I want?

class instrument{
    public:
        double get_price();
        std::string get_udl();

    private:
        double m_price;
        std::string m_udl;

};

class stock : public instrument{
    public:
        double get_dividend();

    private:
        double m_dividend;
};

class option : public instrument{
    public:
        double get_strike();
    private:
        double m_strike;
};

Answer 1

I want the users to only use the derived classes, but those have data members and methods in common

Have your default (or any other) c'tor, but declare it with protected access. Now only derived classes may invoke it. Effectively making it usable only when inherited.

class instrument{
    public:
        double get_price();
        std::string get_udl();

    protected:
        instrument() = default;

    private:
        double m_price;
        std::string m_udl;

};

This also avoids the overhead associated with making a class abstract. If you don't need polymorphism, you shouldn't use it just to make a class usable only as a base class.

Answer 2

Have thought of Abstract class ?

An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

Isn't this what you are looking for ?

Answer 3

When there's no default constructor defined, the compiler will automatically generate one for you, doing nothing for members of basic types and calls the default constructor for members of struct/class types.

So the conclusion is, it makes no sense to have a class without any constructor because whenever you define an instance of that class you have to call at least one (more may be called with deligate construction).

Answer 4

Yes! Definitely it make sense. Because when user create the instance of derived class , first of all parent class's constructor will be called from within the derived class constructor . Even without you explicitly call it. And parent class's default constructor will be called even if you didn't create one. By default

public constructor with no parameter is present in every class
if it has no overridden constructor.

And yes all the public members of parent class will be present in derived class after creating derived class instance .