A class function that uses an instance of its child class as argument

[activecat]

I am facing a real-life problem, it can be simplified as below:

#include <iostream>
using namespace std;

class B;

class A
{
    public:
        void f1(A a) {}
        void f2(B b) {}
};


class B : public A
{
};


int main()
{
    A a;
    B b;
}

There is no problem at all with the f1(), it compiles and executes without any problem.
But f2() gives compilation error. How to solve this?

The error message is:

error: 'b' has incomplete type


This is just to define the function f2() in a class, that uses an instance of its child class as one of its arguments.
Please help, thanks in advance.

[ajh32]

How about this:

#include <iostream>
using namespace std;

// forward declares:
class B;

class A
{
public:
	A() {}
	virtual ~A() {}
	void f1(A* a) {}
	void f2(B* b) {}
};

class B : public A
{
public:
	B():A() {}
	virtual ~B() {}
};

int main()
{
    A a;
    B b;
}

[vlad from moscow]

#include <iostream>
using namespace std;

class B;

class A
{
    public:
        void f1(A a) {}
        void f2(B b);
};

class B : public A
{
};

inline void A::f2( B b ) {}

int main()
{
    A a;
    B b;
}

[vlad from moscow]

@ajh32

You declared different functions compared to functions in the original post. Also there is no need to specify explicitly the constructor of B and to call explicitly the constructor of A in the mem-intilaizer list of B().

[ajh32]

@vlad from moscow

Just because I pass the argument by a pointer doesn't mean that its wrong. It's also good practice to declare a default constructor and to call it's base class constructor. Why don't you tell me why not?

[MikeyBoy]

The default constructor for B will automatically call the default constructor for A, unless you specify different behaviour. There's no need to write it out in constructor for B - it's just code clutter.

[vlad from moscow]

@ajh32
Just because I pass the argument by a pointer doesn't mean that its wrong. It's also good practice to declare a default constructor and to call it's base class constructor. Why don't you tell me why not?

I did not say that your functions incorrect. I said that they are different functions. And I do not know about your "good practice" of declaring a default constructor and calling a base default constructor when there is no such a need. At least it would be much better if you would define the default constructor of class A as

A() = default;

[MikeyBoy]

At least it would be much better if you would define the default constructor of class A as

A() = default;

Isn't that a C++11 construct? Most people won't be using that yet.

[vlad from moscow]

@MikeyBoy

Isn't that a C++11 construct? Most people won't be using that yet.

Most people do not use what they do not know.:) And as it is known most people know little in C++.:)

[MikeyBoy]

True... but also, a lot of people reading this will be working in an environment where they can't use C++11, so I thought it was worth mentioning :)

[activecat]

Dear ajh32, vlad and Mikey Boy,

So far there are two options to solve it.
1). using a class pointer instead as the argument
2). declare the class function first, then define it outside the class definition.

Both works, thanks.
The problem is solved.

Thank you very much.

[MikeyBoy]

You're welcome!