Passing derrived classes as virtual base class without loss of changes

Question

I have 1 base class and a couple of derrived classes that are pretty identic to the base. They look kind of like that:

class Base
{
protected:
    data stuff;
    size_t length;
public:
    Base();
    ~Base();
    virtual void print() 
    {
       std::cout << "Base" << std::endl;
    }
    // Some more virtual functions
};

class Der1: public Base
{
public:
    void print()
    {
       std::cout << "Der1" << std::endl;
       Base::print();
    }
};

class Der2: public Base
{
public:
    void print()
    {
       std::cout << "Der2" << std::endl;
       Base::print();
    }
};

This example is kind of stupid, but what I want to say is that derived classes don't really affect the data itself - only 1 method that does something before actually printing data.

The problem I have is that I have some functions that get Base class as a parameter and does something with the data. The problem is - I can pass derived classes to those functions, but they are passes as Base class - so they lose their overloaded print, and if printed from inside of such function - it won't print any "Der1" or "Der2" strings to stdout.

Edit: They are passed as (const Base &source)

So my question is - what is a way to properly pass derived classes to such functions?

Answer 1

It looks like your functions get Base class as the parameter by value. If you use passing by reference instead - so function(Base& object) instead of function(Base object) - nothing will be lost.

Answer 2

In addition to previous answer, please see below your example:

#include <iostream>

class Base
{protected:
int stuff;
size_t length;
public:
Base(){};
~Base(){};
virtual void print()
{std::cout << "Base" << std::endl;}
// Some more virtual functions
};

class Der1: public Base
{public:
Der1(){};
~Der1(){};
void print()
{
   std::cout << "Der1" << std::endl;
   Base::print();
};
};
class Der2: public Base
{public:
void print()
{
   std::cout << "Der2" << std::endl;
   Base::print();
};
};

void function(Base& base)
{
  base.print();
}

int main(void)
{
 Der1 derived;
 function(derived);
 return 0;
}

Execution: Der1 Base