Question

i’m learning c++. i know quite well java and python, but i’m getting mad in using a little design pattern fancy in my c++ trials.

here are my files, i think that they are quite auto-explicative, if there are some question, comment are welcome!

i have a class Engine that has a Behaviour.. i want to sublass Behaviour into different specific behaviours.. but it’s quite easy also without subclass..

main.cpp :

#include "Engine.h"

int main() {
    Engine e;
    e.work();
    return 0;
};

Engine.h :

#ifndef ENGINE_H_
#define ENGINE_H_

#include <iostream>
#include "Behaviour.h"

class Engine {

public:

    Engine() {
        std::cout << "Engine constructor" << std::endl;
        this->b = new Behaviour(this);
    };
    virtual ~Engine(){};

    void work() {
        std::cout << "Engine work" << std::endl;
    };

    int getFoo() { return 42; };

private:
    Behaviour * b;
};

#endif /*  ENGINE_H_ */

Behaviour.h :

#ifndef BEHAVIOUR_H_
#define BEHAVIOUR_H_

#include <iostream>
#include "Engine.h"

class Behaviour {

public:

    Behaviour(Engine* e) {
        std::cout << "behaviour constructor, kind of abstract class" << std::endl;
        this->e = e;
    };

    virtual ~Behaviour(){};

    void work() {
        std::cout << "Behaviour work" << this->e->getFoo() << std::endl;
    };

protected:
    Engine * e;

};

#endif /*  BEHAVIOUR_H_ */

my compiler errors:

$ rm *.gch; c++ *
In file included from Behaviour.h:5:
Engine.h:26: error: ISO C++ forbids declaration of ‘Behaviour’ with no type
Engine.h:26: error: expected ‘;’ before ‘*’ token
Engine.h: In constructor ‘Engine::Engine()’:
Engine.h:14: error: ‘class Engine’ has no member named ‘b’
Engine.h:14: error: expected type-specifier before ‘Behaviour’
Engine.h:14: error: expected ‘;’ before ‘Behaviour’
In file included from Engine.h:5:
Behaviour.h:11: error: expected ‘)’ before ‘*’ token
Behaviour.h:23: error: ISO C++ forbids declaration of ‘Engine’ with no type
Behaviour.h:23: error: expected ‘;’ before ‘*’ token
Behaviour.h: In member function ‘void Behaviour::work()’:
Behaviour.h:19: error: ‘class Behaviour’ has no member named ‘e’
Engine.h: In constructor ‘Engine::Engine()’:
Engine.h:14: error: no matching function for call to ‘Behaviour::Behaviour(Engine* const)’
Behaviour.h:7: note: candidates are: Behaviour::Behaviour()
Behaviour.h:7: note:                 Behaviour::Behaviour(const Behaviour&)

i think i need some forward declarations but i’m not sure and looking through tutorials and make some trials does not resolve my problems at all.

Answer 1

In Engine.h replace the include with a forward declaration:

#include "Behaviour.h"

with

class Behaviour;

Same for class Engine; inside Behavior.h.

Because you have a cyclic dependency, you’ll need to use forward declarations instead of includes.

You’ll also need to separate the implementation in a cpp file. A forward declaration alone won’t let you call this->b = new Behaviour(this);.

Also, take some time to rethink your design. This sort of dependancy is usually a code smell.