Accessing inherited class methods from vector of base class

Question

I have a base class and several inherited classes.

class Base {
public:
void update();
};

class Inherited1 {
public:
void update(); // override base method
};

class Inherited2 {
public:
void update(); // override base method
};

I have a vector of the base class containing pointers to several instances of the subclasses, i.e.

vector<Base> myObjects;
Base *i1 = new Inherited1();
Base *i2 = new Inherited2();
myObjects.push_back(*i1);
myObjects.push_back(*i2);

I want to be able to go through each element in the vector and automatically call the subclass (i.e. overriden) methods, like:

for (int i=0; i<myObjects.size(); i++) {
    (*myObjects[i]).update();
}

But this doesn't work -- it just calls the superclass method. What is the proper way to achieve this?

Raul Andres · Accepted Answer · 2014-05-12 07:46:22Z

4

You must declare public inheritance, and then, declare memvbers as virtual

class Base {
public:
virtual void update();
};

class Inherited1: public Base{
public:
virtual void update(); // override base method
};

class Inherited2: public Base{
public:
virtual void update(); // override base method
};

answered May 12, 2014 at 7:46

Raul Andres

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Comments

Vlad from Moscow · Accepted Answer · 2014-05-12 07:47:43Z

4

The code snippets shall not be compiled. This

vector<Base> myObjects;

is not a vector of pointers to objects of the base class. It is a vector of objects of the base class.

You need to define the vector as

vector<Base *> myObjects;

As for function update then it must be defined as virtual in the base class

virtual void update();

Only in this case there will be a polymorphic hierarchy of classes.

I think there is a typo in the defining of the derived classes. There should be

class Inherited1 : public Base

instead of

class Inherited1

answered May 12, 2014 at 7:47

Vlad from Moscow

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2 Comments

Jonas Schäfer Over a year ago

The code compiles -- but implicitly copies(!) the Base part of the values when inserting.

Vlad from Moscow Over a year ago

@Jonas Wielicki The code can not be compiled because this (*myObjects[i]).update(); is invalid syntax.

quantdev · Accepted Answer · 2014-05-12 07:45:35Z

1

Your design is almost good, but you need to declare update() virtual in the Base class (and if using C++11, you may mark the overriding method with the new 'override' keyword).

Without the "virtual" keyword, polymorphism wont take place (you are simply redeclaring and hiding "update" in each subclass)

answered May 12, 2014 at 7:45

quantdev

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Comments

Levey · Accepted Answer · 2014-05-12 08:10:09Z

1

#include <iostream>
#include <vector>
using namespace std;

class Base{
public:
    virtual void update(){
        cout << "base" << endl;
    }
};

class Inherited1: public Base {
public:
    void update(){
        cout << "Inherited1" << endl;
    }
};

class Inherited2: public Base {
public:
    void update(){
        cout << "Inherited2" << endl;
    }
};

int main()
{
    vector<Base*> myObjects;
    Base *i1 = new Inherited1();
    Base *i2 = new Inherited2();
    myObjects.push_back(i1);
    myObjects.push_back(i2);
    for (auto iter = myObjects.begin(); iter != myObjects.end(); ++iter)
    {
        (*iter)->update();
    }
}

the output is

Inherited1
Inherited2

answered May 12, 2014 at 8:10

Levey

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