C++ Call subclass method from superclass

Question

I have code of the following style:

class SubClass;
class SuperClass;

class SuperClass {

private:

    void bar() {

        SubClass().foo();
    }
};

class SubClass : SuperClass {

public:

    void foo() {};
};

So basically I have a SuperClass from where I want to call a method foo() of the subclass. VS 2012 gives me the following errors:

Error 1 error C2514: 'SubClass' : class has no constructors.

Error 2 error C2228: left of '.foo' must have class/struct/union.

What is the correct structure for what I want to do?

Answer 1

You can't do this. You must (at least) declare the method in the base class. For example:

#include <iostream>

class SuperClass 
{
public:
    void bar() 
    {
        foo();
    }
private:
    virtual void foo() // could be pure virtual, if you like
    {
        std::cout << "SuperClass::foo()" << std::endl;
    }
};

class SubClass : public SuperClass // do not forget to inherit public
{
public:
    virtual void foo() { std::cout << "SubClass::foo()" << std::endl; }
};

int main()
{
    SuperClass* pTest = new SubClass;

    pTest -> bar();

    delete pTest;
}

will print SubClass::foo() .

Answer 2

First, you have to realise that calling SubClass().foo() has nothing to do with the current object - it will create a new SubClass object and call its foo member. If you want the bar function to call foo on the current object, you need to declare foo as a virtual function in the base class, as Kiril suggested.

However, if you do want to call foo on a new object (which doesn't make much sense, but whatever), you are doing it correctly. Your example didn't compile because SubClass is forward declared, but not defined before bar function - you might want to move the implementation of bar below the definition of SubClass , like so: class SubClass; class SuperClass;

class SuperClass {

private:

    void bar();
};

class SubClass : SuperClass {

public:

    void foo() {};
};

void SuperClass::bar() {
    SubClass().foo();
}

Answer 3

you can try with some CRTP, ie:

#include <iostream>


class SubClass;

template<typename T>
class SuperClass {
public:

    void bar() {

        ((T *) this)->foo();
    }
};

class SubClass : public SuperClass<SubClass> {

public:

    void foo() {
        std::cout << "hello crtp!" << std::endl;
    };
};

int main(void){

    SubClass a = SubClass();
    a.bar();

    return 0;
}