Arduino C++ classes needing each other

Question

Here an example relating to my problem:

class classA
{
private:
    int Value = 100;

public:
    int get_Value()
    {
        return Value;
    }
    void set_Value(int p)
    {
        if (p == 0) Value = B.get_Value();
        else Value = 5;
    }
};

classA A = classA();

class classB
{
private:
    int Value = 200;

public:
    int get_Value()
    {
        return Value;
    }
    void set_Value(int p)
    {
        if (p == 0) Value = A.get_Value();
        else Value = 5;
    }
};

classB B = classB();

The problem is that class A can't access class B because it's definition is below it's own. My question is how can I define it so that class A has access to class B. I tried to something like class classB; or class classB; classB B = classB; before classA begins.

Because I'm relatively new to programming I don't know how to solve such (probably easy) problems. Hope for some help!

Check out forward declarations: https://stackoverflow.com/questions/4757565/what-are-forward-declarations-in-c — Fruchtzwerg, Dec 20 '20 at 19:01

HerrNamenlos123 · Accepted Answer · 2020-12-20T20:35:01.583

This is a problem with circular dependencies. Luckily this one is pretty easy to fix: Just put the declaration and definition into separate files.

ClassA.h:

    // Include guards to prevent double includes
    #ifndef CLASS_A_H
    #define CLASS_A_H
    
    class classA
    {
    private:
        int Value = 100;
    
    public:
        // Here we only declare, but not define the functions
        int get_Value();
        void set_Value(int p);
    };
    
    #endif // CLASS_A_H

ClassB.h:

    // Include guards to prevent double includes
    #ifndef CLASS_B_H
    #define CLASS_B_H
    
    class classB
    {
    private:
        int Value = 200;
    
    public:
        // Here we only declare, but not define the functions
        int get_Value();
        void set_Value(int p);
    };
    
    #endif // CLASS_B_H

And now we need to actually define the functions, but in different files

ClassA.cpp:

    #include "A.h"
    #include "B.h"
    
    // Use extern to tell the compiler that this declaration can be
    // found in any of the other files and it will not complain as
    // long as it is found somewhere
    extern classB B;
    
    int classA::get_Value()
    {
        return Value;
    }
    
    void classA::set_Value(int p)
    {
        if (p == 0) Value = B.get_Value();
        else Value = 5;
    }

ClassB.cpp:

    #include "A.h"
    #include "B.h"
    
    // Use extern to tell the compiler that this declaration can be
    // found in any of the other files and it will not complain as
    // long as it is found somewhere
    extern classA A;
    
    int classB::get_Value()
    {
        return Value;
    }
    
    void classB::set_Value(int p)
    {
        if (p == 0) Value = A.get_Value();
        else Value = 5;
    }

Then, in your main file just include the two headers and you're good to go:

    #include "A.h"
    #include "B.h"
    
    classB B = classB();
    classA A = classA();

By the way, when you're on Arduino you can just create new .h and .cpp files and put them next to the .ino file and it should be found. You can even put them in folders, but then the include paths must be relative to each files, for example: #include "../include/file.h"

Arduino C++ classes needing each other

1 Answers1