c++ Call subclass method in vector of base class type

Question

I have the abstract class Currency and several subclasses like FiatMoney or Crypto. I want to store all of objects of subclasses in one vector and in the same time have access to methods, which are only in those subclasses.

My Currency.h file looks like this:

    class Currency
{
    public:
        virtual void to_String() = 0;
        float GetAmount();
        string GetShortname();
        void AddAmount(float);
        void RemoveAmount(float);
        virtual void UpdateRatio() =0;
        virtual float trade(bool, float, float, float, string) = 0;

    protected:
        string *name, *shortname;
        float *amount, *exchange_ratio;
};

and my FiatMoney.h file:

    class FiatMoney : public Currency
{
    public:
        FiatMoney(string, string, float, float);
        ~FiatMoney();
        void to_String();
        void UpdateRatio();
        float trade(bool, float, float, float, string);
        void com();
    private:

};

In that class I want to have additional method of this class called com(). Finally my main function looks like this:

vector<Currency*> Wallet;
Wallet.push_back(new FiatMoney("name", "shortname", 1.0, 1.0));
Wallet[0]->com();

At this point I of course have an error, that class Currency has not member call com(). How can I avoid this?

How can I get access to the function com() which is only in object of subclass FiatMoney?

Can I have method in class FiatMoney without previously declarate it as virtual function of Currency class?

Is this proper way of storing objects of different subclasses?

The shown code demonstrates that you are aware of what virtual functions are, and how they work, and that's how you do that. So, your question is unclear? Define it as just another virtual method? — Sam Varshavchik, Nov 13 '21 at 19:47
@SamVarshavchik but I dont want to create another virtual method, since then I need to declare it in every child class. I want this particual method inside only one child class. — Daniel Jankowski, Nov 13 '21 at 19:50
@DanielJankowski "*since then I need to declare it in every child class*" - no, you don't. You could give it a default implementation in the base class, and override it only in the one derived class you want. Only *pure virtual* (ie *abstract*) methods need to be implemented in each derived class (like your `to_String()` and `UpdateRatio()` methods). Virtual methods don't have to be abstract. — Remy Lebeau, Nov 13 '21 at 19:54

Remy Lebeau · Accepted Answer · 2021-11-13T20:04:11.323

How can I get access to the function com() which is only in object of subclass FiatMoney?

You would have to type-cast a Currency* pointer into a FiatMoney* pointer, but you can only do that when the pointer is actually pointing at a valid FiatMoney object.

When you know the Currency* pointer is pointing at a FiatMoney object, you can use static_cast at compile-time, eg:

static_cast<FiatMoney*>(Wallet[0])->com();

Otherwise, use dynamic_cast at runtime to test the type of object being pointed at:

FiatMoney *fm = dynamic_cast<FiatMoney*>(Wallet[0]);
if (fm) fm->com();

Can I have method in class FiatMoney without previously declarate it as virtual function of Currency class?

Technically yes, though some people would argue that is not a good design choice when dealing with polymorphic classes.

Is this proper way of storing objects of different subclasses?

Technically yes, although Currency should have a virtual destructor. You have to delete the objects you new, and a virtual destructor will allow you to call delete on a Currency* pointer to invoke derived class destructors without type-casting the pointer. And as such, you should consider storing smart pointers, like std::unique_ptr, instead of raw pointers, so that delete is called automatically for you.

c++ Call subclass method in vector of base class type

1 Answers1