a .cpp file always include the corresponding header file first

In many articles, there is such a coding guideline:

>> On a .cpp file always include the corresponding header file first, like this:

Code:

// This is Foo.cpp
 
#include "Foo.h"  
#include "tao/Bar.h" 
#include "ace/Baz.h" 
 
// Here comes the Foo.cpp code....

>> In this way we are sure that the header file is self-contained and can be safely included from some place else.

Why it can make sure the header file is self-contained by including the header file first in the corresponding implementation file? Can anybody explain it a little bit more?

 

By making sure that foo.h includes gromit.h and widget.h automatically, you save having to explain to all users of foo.h the need to include the other two files, and save having to remind them should they accidentally forget.

If at some future time, foo.h removed gromit.h and adds wallace.h, then all the users of foo.h don't have to go round changing many thousands of instances of including foo.h to fix the pre-requisite headers.

 

Salem, thanks for your answer. But it seems you explained the advantage of making the header file "self-contained".

My question is:
Why "a .cpp file always include the corresponding header file first (see example above)" is able to check whether the header file is self-contained?
What will happen if I include other header files before the corresponding header file of that .cpp file?

 

By being included first is how it checks.

 

Salem, do you mean if the cpp file does not include the corresponding header file in the first line (e.g. include other .h files, then its corresponding header file), it has no chance to check whether the corresponding header file is self-contained?

Shabbir, seems your answer conflicts with Salem's.

I get a little confused.

 

Well if you had this

Code:

#include "other.h"
#include "myHeader.h"

Then there may be the possibility that myHeader.h depends on other.h in some way.

But by having this

Code:

#include "myHeader.h"
#include "other.h"

There is no possibility of a hidden dependency.
Either myHeader.h doesn't depend on other.h, or it does depend on other.h and it (myHeader.h) includes other.h as well.

So by including it first, you guarantee that it has no hidden dependencies (otherwise it would fail to compile). This gives confidence that anyone else who needs myHeader.h can include it without having to worry about dependencies.

 

use
#pragma once

In case of to avoid too many times including the same header file

 

Macro guards are the preferred method for ensuring single inclusion.
It is more portable and does not have the problems of #pragma once
(cannot know that the same file is included under two different names
as can occur with links on unix, etc.).
gcc considers #pragma once to be deprecated.
These are macro (or include) guards:

Code:

// header.h
#ifndef HEADER_H
#define HEADER_H
...
#endif

 

In pragma once same code is written as you have written.

 

Salem, thank you very much for the explanation.

Shabbir and others, also thanks for your comments.