Use child method from the parent class c++

Question

Here's the deal. Imagine your as follows bellow. After loading the files to the algorithm layer I want to be able to apply the 4 different algorithms. So the Algorithm class has 4 children and I want to do something like this:

int main(int argc, char* argv[]) {
    Data *data = new Data();
    Parent *alg = new Parent(data);

    alg->start();

    //and this will call the start methods defined in each child. 
}

The children are like this:

class Child1: public Parent {
public:
   int start();
}

class Child2: public Parent {
public:
   int start();
}
//etc...

And what I do in the Parents start() method is this:

int Parent::start() {
    Child1::start();
    Child2::start();
    //etc....
    return 0;
}

But I'm getting a 'cannot call member function 'Child1::start()', etc.. without object.' Can this thing be done somehow? Calling a child method from the parent class?

Answer 1

Take a look at the "Gang of Four" composite pattern:

The Child classes should not be derived from the Parent class. Instead, Child and Parent classes should implement the same Interface, let's call it IAlgorithm. This Interface should have a pure virtual method start()...

Here is an example (using C++11 auto and Range-based for loop):

#include <iostream>
#include <vector>

/* parents and children implement this interface*/
class IAlgorithm {
public:
   virtual int start() = 0;
};

class Parent : public IAlgorithm {
private:
    std::vector<IAlgorithm*> children;
    
public:
    void addChild(IAlgorithm* child) {
        children.push_back(child);
    }

    int start() {
        std::cout << "parent" << std::endl;
        for (auto child: children)
            child->start();
        return 0;
    }
};

class Child1 : public IAlgorithm {
public:
    int start() {
        std::cout << "child 1" << std::endl;
        return 1;
    }
};

class Child2 : public IAlgorithm {
public:
    int start() {
        std::cout << "child 2" << std::endl;
        return 2;
    }
};

class Child3 : public IAlgorithm {
public:
    int start() {
        std::cout << "child 3" << std::endl;
        return 3;
    }
};

int main()
{
   Parent parent;
   Child1 child_1;
   Child2 child_2;
   Child3 child_3;
   
   parent.addChild(&child_1);
   parent.addChild(&child_2);
   parent.addChild(&child_3);
   
   parent.start();
   
   return 0;
}

The output:

parent
child1
child2
child3