Access a member variable of a sister object in C++

Question

I am trying to access a variable which is inside of an object (TestClassThree.h) which is inside the parent class (TestClassOne.h). Each class is in its own file, and when I try to import the files to instantiate the classes it crashes. I suspect it is because of import loops. I don't think that I can use forward class declarations, because that would restrict the access to the variable. How can I access the variable inside of TestClassThree from TestClassTwo?

//--TestClassOne.h--
#include "TestClassTwo.h"
#include "TestClassThree.h"

class TestClassOne {
public:
    TestClassTwo *myVariable;
    TestClassThree *mySecondVariable;

    TestClassOne() {
        myVariable = new TestClassTwo(this);
        mySecondVariable = new TestClassThree();
    }
};

//--TestClassTwo.h--
#include "TestClassOne.h" //<-- ERROR

class TestClassTwo {
public:
    TestClassOne *parent;

    TestClassTwo(TestClassOne *_parent) : parent(_parent) {

    }

    void setValue() {
        parent->mySecondVariable->mySecondVariable->value = 10;
    }

};

Presumably when you say "crashes" you really mean "I get a compiler error message"? — Oliver Charlesworth, Nov 24 '13 at 11:02
I don't think you can include a.h in b.h and b.h in a.h simultaneously. It will lead to infinite depth dependence. — Abhishek Bansal, Nov 24 '13 at 11:04

score 1 · Answer 1 · answered Nov 24 '13 at 11:06

1

you can use forward class declarations and friend Keyword

answered Nov 24 '13 at 11:06

Patato

1,464
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I dont think he needs friend since all class members are public – Laurent Nov 24 '13 at 12:30

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

Try adding a so-called include guard (cf. this SO question). In TestClassOne.h add the following lines at the top and bottom of the file:

#ifndef TESTCLASSONE_H
#define TESTCLASSONE_H

[...]

#endif

Add this also to TestClassTwo.h, but change the name of the preprocessor macro to TESTCLASSTWO_H.

edited May 23 '17 at 11:56

Community

1
1

answered Nov 24 '13 at 11:44

herzbube

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Laurent · Answer 3 · 2013-11-24T12:40:37.567

Both what said herzbube and patato answered your question :

1 - Avoid "includes loops" using #ifndef / #define macros just like herzbube explained

2 - Use forward classes declarations to tell the compiler a class will be defined after

// 1- avoid "include loops"
#ifndef TESTCLASSONE_H
#define TESTCLASSONE_H

// 2- Forward classes declarations
class TestClassTwo;
class TestClassThree; // assuming TestClassThree needs TestClassOne.h

class TestClassOne{
...
};
#endif

Access a member variable of a sister object in C++

3 Answers3