Return derived class object in the script whereas original function declaration returns parent class-CodePudding

A function GetCarPrice() has a return type of a class Money where the function is declared, and this method is not declared within Money, meaning it's not a member function of this class.

Later, I derive another class from Money, defined as Dollar, with some additional attributes and methods.

Now, I want to return Dollar type object when using the function GetCarPrice() inside the main function, keeping the declaration of this function the same.

Here is a brief pseudo code:

class Money {...}; `
class Dollar: public Money {...};

Money* GetCarPrice () {...};

int main()
{
   Dollar* D1;
   ...
   D1 = GetCarPrice(); // Here I get error: invalid conversion from Money to Dollar
   ...
}

Is there any way to do this in C ?

N.B. I am new to this platform, please pardon my mistakes. Please do not close it, I need help. TIA.

CodePudding user response：

Just create a Dollar instance and return the pointer of it in the GetCarPrice().

If there are some legacy code that creates a Money instance and you want to keep it, you should convert the object like this.(If the Money class has a copy constructor, it will be easy because you can call it at the constructor of the Dollar.)

class Dollar : public Money {
...
    Zzz zzz;
public:
    Dollar(const Money &money) : Money(money) {
        zzz = transform_xxx_yyy_to_zzz(money.xxx, money.yyy);
        ...
    }
}


Money* GetCarPrice ()
{
    ...
    Money* money = ...;
    if (...) { // If you should convert to a Dollar.
        Dollar* dollar = new Dollar(*money);
        delete money; // This prevents a memory leak.
        return dollar;
    }
    return money;
}

And you can safely cast a Money pointer to a Dollar pointer like this.

Money* m = GetCarPrice();
Dollar* d = dynamic_cast<Dollar*>(m);
if (d) {
    // the m is actually a Dollar, so do whatever with the d.
}