CODEWITHSUNDEEP
c++constructorpure-virtual
Yuan Wen
Yuan Wen

Reputation: 1701

function called by constructors of derived classes not supposed to be pure virtual in base class

I have a base class and many derived classes.

The constructor of derived classes all need to call a function reset() with the same signature. As a result, I want to declare a pure virtual function virtual void reset() = 0 in base class as an Interface.

However, the problem is :

It is not supposed to call a virtual function in Constructor.

But I want reset() to be an interface provided by the base class. So that all derived classes have to implement reset respectively.

What should I do?

Upvotes: 0

Views: 131

Answers (2)

Jarod42
Jarod42

Reputation: 217293

You might create factory:

struct Base
{
    virtual ~Base() = default;
    virtual void reset() = 0;

    template <typename Der, typename ... Ts>
    static std::unique_ptr<Der> Make(Ts&&... args)
    {
        static_assert(std::is_base_of<Base, Der>::value, "!");
        auto p = std::make_unique<Der>(std::forward<Ts>(args)...);
        p->reset();
        return p;
    }
};

struct Derived
{
    void reset() override { /*..*/ }
};

Upvotes: 1

TheOperator
TheOperator

Reputation: 6476

Instead of forcing them to call a function (which you anyway cannot guarantee), make the base class constructor expect a parameter that includes the behavior you need to have.

That way, the base class is self-contained and has all the required information by the time its constructor runs.

For example, if you were thinking of something like this:

class Base
{
public:
    Base()
    {
        reset();
    }

protected:
    virtual void reset() = 0;
    void setValue(int value);
};

class Derived : public Base
{
public:
    void reset() override
    {
        setValue(20);
    }
};

you can do this instead:

class Base
{
public:
    explicit Base(int value)
    {
        setValue(value);
    }

private: // can be private
    void setValue(int value);
};

class Derived : public Base
{
public:
    Derived()
    : Base(20)
    {
    }
};

Upvotes: 5

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