Thread: Question in including tempolate header files.

Originally Posted by plutino

... Then there might be a duplicated definition error. How to avoid this problem?

Use Header Guards:

Code:

#ifndef SOME_UNIQUE_NAME
#define SOME_UNIQUE_NAME

// Put your header stuff here.

#endif  // SOME_UNIQUE_NAME

Every header should have the guard that cpjust posted. Alternatively you can use #pragma once inside of MSVC 2003+ which is not portable but very convenient.

You should also refrain from placing all your classes in the global namespace. Prefer something like:

Code:

#ifndef SOME_UNIQUE_NAME
#define SOME_UNIQUE_NAME

namespace some_namespace
{
   struct SomeStruct
   {
   };

   class SomeClass
   {
   };
}

#endif

As well you should include only what is absolutely necessary in header files. I prefer to include in the cpp file if I can get away with it. Sometimes though your class function prototypes may require you to include a header from another object, or another object in another project.

There is a way to include in such a way that you do not have to include in the header itself. For instance if you needed to include <windows.h> but didn't want to bring in the entire header into your header you could do this:

SomeClass.h

Code:

#ifndef SOME_UNIQUE_NAME
#define SOME_UNIQUE_NAME

namespace some_namespace
{
   struct SomeStruct
   {
   };

   class SomeClass
   {
       ...
       private:
          CRITICAL_SECTION m_CriticalSection;
   };
}

#endif

SomeClass.cpp

Code:

#include <windows.h>
#include "SomeClass.h"
...

This is prone to abuse and confusion but it is a valid practice in certain circumstances. As long as windows.h is included prior to SomeClass.h, SomeClass.h will not complain about undefined Win32 symbols.

Just keep in mind that if you include unecessary stuff in your headers everyone who includes your header will get all of the unecessary stuff.
Try to stay away from cyclic dependencies where object A needs object B but A also needs B. Sometimes this is unavoidable in which case you must use forward declares.

I think I covered most of what can cause some major header headaches.

Templates bring in a new headache since the code must be in the header. I normally bring in the code as a separate cpp and this is the only time I actually include the cpp in a header. The other alternative is to write the template code in the header.

There is a way to include in such a way that you do not have to include in the header itself. For instance if you needed to include <windows.h> but didn't want to bring in the entire header into your header you could do this:

But then, wouldn't every file that includes "someclass.h" have to include <windows.h> first - even if that file doesn't need that header - creating nothing but confusion as the dependency is still there?

Originally Posted by anon

But then, wouldn't every file that includes "someclass.h" have to include <windows.h> first - even if that file doesn't need that header - creating nothing but confusion as the dependency is still there?

Agreed. I would say that's a very bad thing to do because it requires everyone else to know that they need to include certain header files before including yours instead of just including it in one place.

Chapter 23. Make header files self-sufficient