Header in header

This is a discussion on Header in header within the C++ Programming forums, part of the General Programming Boards category; Hello, I'd like to know if it is considered bad practice to include a header (.h) in another header. Is ...

  1. #1
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    Header in header

    Hello, I'd like to know if it is considered bad practice to include a header (.h) in another header. Is it? Why?

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    My opinion is like this.
    My personnal rule is simple :

    My headers are guarded (Like those from the library)
    I include a header when I need it. (Can be in a source or a header)
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
    CSharpener vart's Avatar
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    I'd like to know if it is considered bad practice to include a header
    No - it is not...

    I prefer header to be complete - so I can include it in any c-file without including any other header before it...

    On the other hand - I prefer to make header dependencies - minimal, so If the forward declaration is enough - I use it rather to include the other header with the full type definition.
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
    C++まいる!Cをこわせ! Elysia's Avatar
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    My own preference is this:
    Include my own needed headers from my headers (if A.h needs B.h, include B.h).
    Any system headers (iostream, windows.h, dshow.h, etc, etc) are not included. The project that includes the header is expected to have put those headers inside its stdafx.h - precompiled header.
    Last edited by Elysia; 03-27-2008 at 01:46 PM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  5. #5
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    I think most people follow vart's usage. #include everything in your header that needs to be included so that it will compile no matter what. Prefer forward declarations over #include's when they will work.

    If you use precompiled headers, then of course common headers placed there don't need to be included.

  6. #6
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    Thanks for the super fast replies, guys. I'll follow vart's (and others') advice.

  7. #7
    and the hat of sweating
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    No, not if the header file is directly using something declared in another header, otherwise, if it's only used in the implementation rather than the interface, the other headers should be included in the .cpp files.

    Code:
    #ifndef MYCLASS_H
    #define MYCLASS_H
    
    #include <string>
    
    class MyClass
    {
    public:
        MyClass( const std::string&  name );  // std::string is referenced here, so we need to include <string>
    ...
    };
    
    #endif  // MYCLASS_H
    Code:
    #include "MyClass.h"
    #include <algorighm>
    
    MyClass::MyClass( const std::string&  name )
    {
        // std::transform is only referenced in the .cpp file, so <algorithm> should be included here, not in the .h file.
        std::transform( name.begin(), name.end(), name.begin(), ToLowerFunc );
    }

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