具有多态性的派生类中的重载函数（C++）

Question

Considering this code example:

#include <iostream>
using namespace std;

class Base
{
private:
    int number;

public:
    Base():number(10){}
    ~Base(){}
    virtual void print()
    {
        cout << "Base class" << endl;
    }
};

class Derived : public Base
{
public:
    Derived():Base(){}
    ~Derived(){}
    void print(int value)
    {
        //printing number in Base class and paramter value
        cout << "Derived with value " << value << " number is" << number << endl; 
    }
};

I wanted to use polymorphism and call theoverloaded print() function.
So use these classes as follows:

void somewhere_else()
{
    Base* polymorphism = new Derived();
    polymorphism->print(5); //Error indicating there are too many parameter
                            //thinking that I am trying to use print in Base class
    ((Derived*)polymorphism)->print(5) 
                       //This works as I am casting the variable as Derived variable
}

Unfortunately, I can't call print() from the base class pointer (compilation error, see comment above). I can only call it with a cast. Is there a better way to keep the polymorphism and still calls overloaded function based on derived class?

Answer 1

In your code you have two different member functions, that have different signatures:

• a virtual print() that takes no argument. It is declared and defined in Base , and inherited in Derived
• a non-virtual print() that takes one int argument. It is declared and defined ONLY for Derived

So the base object doesn't know a print function with an int parameter. This is why you need to cast (which is by the way a symptom that should ring alarm bells if you need it).

How to improve ?

First, if you want to override a virtual function in a derived class, use the keyword override :

class Derived : public Base
{
public:
    Derived():Base(){}
    ~Derived(){}
    void print(int value) override
    {
        ...
    }
};

This will ensure an error message in case of subtle mismatch in the function signature:

prog.cpp:23:10: error: ‘void Derived::print(int)’ marked ‘override’, but does not override
     void print(int value) override
          ^~~~~

Then make sure that the signatures are aligned in the base class and derived class (ie either both take an int argument or non of them.

Note that you can't access a private member of the base class in a derived class. You have to define number as protected to print it in Derived .

Finally, if you have a base class having a virtual member, it is a sound practice to systematically make the destructor virtual. This will avoid subtle bugs for more complex classes:

class Base
{
protected:
    int number;   
public:
    Base():number(10){}
    virtual ~Base(){}
    virtual void print(int value)
    {
        ...
    }
};

Here the online demo

Now that the things are working, here a short article making the difference between overload and override .