Rules for managing header file includes in C++

C++ header file management can be very frustrating:

• Changing the order of header files often results in difficult to track compilation errors
• C++ project compilations take a long time due to complex header file inclusions

Here are a few rules to remedy this:

• A header file should be included only when a forward declaration would not do the job.
• The header file should be so designed that the order of header file inclusion is not important.
• This is achieved by making sure that x.h is the first header file in x.cpp
• The header file inclusion mechanism should be tolerant to duplicate header file inclusions.

An example illustrates the header file inclusion rules:

Header file

#ifndef _a_h_included_
#define _a_h_included_
#include "abase.h"
#include "b.h"

// Forward Declarations
class C;
class D;

class A : public ABase
{
  B m_b;
  C *m_c;
  D *m_d;

public:
  void SetC(C *c);
  C *GetC() const;

  void ModifyD(D *d);
};
#endif

Source file

#include "a.h"
#include "d.h"

void A::SetC(C* c)
{
  m_c = c;
}

C* A::GetC() const
{
  return m_c;
}

void A::ModifyD(D* d)
{
  d->SetX(0);
  d->SetY(0);
  m_d = d;
}

For details, refer to C++ Header File Include Rules