How to avoid cyclic dependecies in C++?

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how to solve the problem of cyclic dependency such as below

C++

class cfoobar
{
     // ... refers to class cgoobar members ...
}

class cgoobar
{
     // ... refers to class cfoobar members ...
}

What I have tried:

C++

// pass cgoobar as a template parameter

template<class T=cgoobar>
class cfoobar {}

C++

// refer to cgoobar member via a cgoobar pointer
class cgoobar;
class cfoobar
{
public:
	void foobar_method()
	{
		auto goobar_ptr = new cgoobar; // error: use of undefined type 'cgoobar'
		goobar_ptr->goobar_method();
	}
};

class cgoobar
{
public:
	void goobar_method() { }
};

Posted 15-Dec-20 5:47am

BernardIE5317

Updated 15-Dec-20 16:49pm

v3

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3 solutions

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steveb · Answer 1 · 2020-12-15T07:27:00

Solution 1

You need to use forward declaration:

C++

class cgoobar;
class cfoobar
{
     // ... refers to class cgoobar ...
public:
    cgoobar* ptr;
}

class cfoobar; // not exactly needed but required 
//if the class declared in different translation unit
class cgoobar
{
     // ... refers to class cfoobar ...
public:
    cfoobar* ptr;
}

Posted 15-Dec-20 7:27am

steveb

Updated 15-Dec-20 7:31am

v2

Comments

CPallini 15-Dec-20 16:54pm

5.

Shao Voon Wong · Answer 2 · 2020-12-15T14:48:00

This is because you instantiate cgoobar, it needs cgoobar information such as its class size, forward declaration cannot help you. Forward declaration can only help when declaring cgoobar pointer in header because pointer size is always the same (32-bit or 64-bit depending on the platform). One solution is to put your class in header and functions in their own cpp file.

C++

// put this code inside cfoobar.h
class cfoobar
{
public:
	void foobar_method();
};

C++

// put this code inside cgoobar.h
class cgoobar
{
public:
	void goobar_method();
};

C++

// put this code inside cfoobar.cpp
#include "cfoobar.h"
#include "cgoobar.h"

void cfoobar::foobar_method()
{
	auto goobar_ptr = new cgoobar;
	goobar_ptr->goobar_method();
}

C++

// put this code inside cgoobar.cpp
#include "cgoobar.h"

void cgoobar::goobar_method()
{
}

Shao Voon Wong · Answer 3 · 2020-12-15T16:49:00

Solution 3

Define cgoobar before cfoobar.

C++

class cgoobar
{
public:
	void goobar_method() { }
};

class cfoobar
{
public:
	void foobar_method()
	{
		auto goobar_ptr = new cgoobar; // error: use of undefined type 'cgoobar'
		goobar_ptr->goobar_method();
	}
};

Posted 15-Dec-20 16:49pm

Shao Voon Wong