How to resolve build errors due to circular dependency amongst classes? [Answered]

Sample problem:

I often find myself in a situation where I am facing multiple compilation/linker errors in a C++ project due to some bad design decisions (made by someone else 🙂 ) which lead to circular dependencies between C++ classes in different header files (can happen also in the same file). But fortunately(?) this doesn’t happen often enough for me to remember the solution to this problem for the next time it happens again.

So for the purposes of easy recall in the future I am going to post a representative problem and a solution along with it. Better solutions are of-course welcome.


  • A.hclass B; class A { int _val; B *_b; public: A(int val) :_val(val) { } void SetB(B *b) { _b = b; _b->Print(); // COMPILER ERROR: C2027: use of undefined type 'B' } void Print() { cout<<"Type:A val="<<_val<<endl; } };

  • B.h#include "A.h" class B { double _val; A* _a; public: B(double val) :_val(val) { } void SetA(A *a) { _a = a; _a->Print(); } void Print() { cout<<"Type:B val="<<_val<<endl; } };

  • main.cpp#include "B.h" #include <iostream> int main(int argc, char* argv[]) { A a(10); B b(3.14); a.Print(); a.SetB(&b); b.Print(); b.SetA(&a); return 0; }

Resolve build errors due to circular dependency amongst classes- Answer #1:

The way to think about this is to “think like a compiler”.

Imagine you are writing a compiler. And you see code like this.

// file: A.h
class A {
  B _b;
};

// file: B.h
class B {
  A _a;
};

// file main.cc
#include "A.h"
#include "B.h"
int main(...) {
  A a;
}

When you are compiling the .cc file (remember that the .cc and not the .h is the unit of compilation), you need to allocate space for object A. So, well, how much space then? Enough to store B! What’s the size of B then? Enough to store A! Oops.

Clearly a circular reference that you must break.

You can break it by allowing the compiler to instead reserve as much space as it knows about upfront – pointers and references, for example, will always be 32 or 64 bits (depending on the architecture) and so if you replaced (either one) by a pointer or reference, things would be great. Let’s say we replace in A:

// file: A.h
class A {
  // both these are fine, so are various const versions of the same.
  B& _b_ref;
  B* _b_ptr;
};

Now things are better. Somewhat. main() still says:

// file: main.cc
#include "A.h"  // <-- Houston, we have a problem

#include, for all extents and purposes (if you take the preprocessor out) just copies the file into the .cc. So really, the .cc looks like:

// file: partially_pre_processed_main.cc
class A {
  B& _b_ref;
  B* _b_ptr;
};
#include "B.h"
int main (...) {
  A a;
}

You can see why the compiler can’t deal with this – it has no idea what B is – it has never even seen the symbol before.

So let’s tell the compiler about B. This is known as a forward declaration, and is discussed further in this answer.

// main.cc
class B;
#include "A.h"
#include "B.h"
int main (...) {
  A a;
}

This works. It is not great. But at this point you should have an understanding of the circular reference problem and what we did to “fix” it, albeit the fix is bad.

The reason this fix is bad is because the next person to #include "A.h" will have to declare B before they can use it and will get a terrible #include error. So let’s move the declaration into A.h itself.

// file: A.h
class B;
class A {
  B* _b; // or any of the other variants.
};

And in B.h, at this point, you can just #include "A.h" directly.

// file: B.h
#include "A.h"
class B {
  // note that this is cool because the compiler knows by this time
  // how much space A will need.
  A _a; 
}

Answer #2:

You can avoid compilation errors if you remove the method definitions from the header files and let the classes contain only the method declarations and variable declarations/definitions. The method definitions should be placed in a .cpp file (just like a best practice guideline says).

The down side of the following solution is (assuming that you had placed the methods in the header file to inline them) that the methods are no longer inlined by the compiler and trying to use the inline keyword produces linker errors.

//A.h
#ifndef A_H
#define A_H
class B;
class A
{
    int _val;
    B* _b;
public:

    A(int val);
    void SetB(B *b);
    void Print();
};
#endif

//B.h
#ifndef B_H
#define B_H
class A;
class B
{
    double _val;
    A* _a;
public:

    B(double val);
    void SetA(A *a);
    void Print();
};
#endif

//A.cpp
#include "A.h"
#include "B.h"

#include <iostream>

using namespace std;

A::A(int val)
:_val(val)
{
}

void A::SetB(B *b)
{
    _b = b;
    cout<<"Inside SetB()"<<endl;
    _b->Print();
}

void A::Print()
{
    cout<<"Type:A val="<<_val<<endl;
}

//B.cpp
#include "B.h"
#include "A.h"
#include <iostream>

using namespace std;

B::B(double val)
:_val(val)
{
}

void B::SetA(A *a)
{
    _a = a;
    cout<<"Inside SetA()"<<endl;
    _a->Print();
}

void B::Print()
{
    cout<<"Type:B val="<<_val<<endl;
}

//main.cpp
#include "A.h"
#include "B.h"

int main(int argc, char* argv[])
{
    A a(10);
    B b(3.14);
    a.Print();
    a.SetB(&b);
    b.Print();
    b.SetA(&a);
    return 0;
}

Answer #3:

Best practice: forward declaration headers

As illustrated by the Standard library’s <iosfwd> header, the proper way to provide forward declarations for others is to have a forward declaration header. For example:

a.fwd.h:

#pragma once
class A;

a.h:

#pragma once
#include "a.fwd.h"
#include "b.fwd.h"

class A
{
  public:
    void f(B*);
};

b.fwd.h:

#pragma once
class B;

b.h:

#pragma once
#include "b.fwd.h"
#include "a.fwd.h"

class B
{
  public:
    void f(A*);
};

The maintainers of the A and B libraries should each be responsible for keeping their forward declaration headers in sync with their headers and implementation files, so – for example – if the maintainer of “B” comes along and rewrites the code to be…

b.fwd.h:

template <typename T> class Basic_B;
typedef Basic_B<char> B;

b.h:

template <typename T>
class Basic_B
{
    ...class definition...
};
typedef Basic_B<char> B;

…then recompilation of the code for “A” will be triggered by the changes to the included b.fwd.h and should complete cleanly.


Poor but common practice: forward declare stuff in other libs

Say – instead of using a forward declaration header as explained above – code in a.h or a.cc instead forward-declares class B; itself:

  • if a.h or a.cc did include b.h later:
    • compilation of A will terminate with an error once it gets to the conflicting declaration/definition of B (i.e. the above change to B broke A and any other clients abusing forward declarations, instead of working transparently).
  • otherwise (if A didn’t eventually include b.h – possible if A just stores/passes around Bs by pointer and/or reference)
    • build tools relying on #include analysis and changed file timestamps won’t rebuild A (and its further-dependent code) after the change to B, causing errors at link time or run time. If B is distributed as a runtime loaded DLL, code in “A” may fail to find the differently-mangled symbols at runtime, which may or may not be handled well enough to trigger orderly shutdown or acceptably reduced functionality.

If A’s code has template specialisations / “traits” for the old B, they won’t take effect.

Answer #4:

I once solved this kind of problem by moving all inlines after the class definition and putting the #include for the other classes just before the inlines in the header file. This way one make sure all definitions+inlines are set prior the inlines are parsed.

Doing like this makes it possible to still have a bunch of inlines in both(or multiple) header files. But it’s necessary to have include guards.

Like this

// File: A.h
#ifndef __A_H__
#define __A_H__
class B;
class A
{
    int _val;
    B *_b;
public:
    A(int val);
    void SetB(B *b);
    void Print();
};

// Including class B for inline usage here 
#include "B.h"

inline A::A(int val) : _val(val)
{
}

inline void A::SetB(B *b)
{
    _b = b;
    _b->Print();
}

inline void A::Print()
{
    cout<<"Type:A val="<<_val<<endl;
}

#endif /* __A_H__ */

…and doing the same in B.h .

Hope you learned something from this post.

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Linux and Python enthusiast, in love with open source since 2014, Writer at programming-articles.com, India.

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