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#include position matters?

This is a discussion on #include position matters? within the C Programming forums, part of the General Programming Boards category; I have read somewhere, it says that include file hierarchy/position matters. For example, #include <Winsock2.h> before #include <iphlpapi.h>. I would ...

  1. #1
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    #include position matters?

    I have read somewhere, it says that include file hierarchy/position matters.
    For example, #include <Winsock2.h> before #include <iphlpapi.h>.
    I would really appreciate if someone could explain this to me. Thank you.

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    C++ Witch laserlight's Avatar
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    It shouldn't, but in practice it can matter if the header file author made some assumptions that other header files would be included first, or if a header tile declares things like macros which may change the meaning of code that follows.
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    Sometimes one header relies upon definitions in a different header. In large systems situations like that do arrise.
    Eman likes this.

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    Thank you for answering.

    Let's say i have 2 headers.
    winsock.h and winsock2.h

    I include winsock2.h first then only winsock.h. Both of then contain a function named sockfoo() which were defined differently. In this case, which function will be chosen when I call sockfoo() in main(). Is it sockfoo() from winsock.h or winsock2.h ?

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    ATH0 quzah's Avatar
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    Quote Originally Posted by valthyx View Post
    I include winsock2.h first then only winsock.h. Both of then contain a function named sockfoo() which were defined differently. In this case, which function will be chosen when I call sockfoo() in main(). Is it sockfoo() from winsock.h or winsock2.h ?
    Neither. Your compiler will yell at you for having two functions called the same thing in the same scope.


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    Quote Originally Posted by valthyx View Post
    Thank you for answering.

    Let's say i have 2 headers.
    winsock.h and winsock2.h

    I include winsock2.h first then only winsock.h. Both of then contain a function named sockfoo() which were defined differently. In this case, which function will be chosen when I call sockfoo() in main(). Is it sockfoo() from winsock.h or winsock2.h ?

    Depends on the Compiler/Header files; in this case I think the first one include is used on certain MinGW GCC Compilers.

    How it is sometimes done on MinGW GCC, the header guard for winsock2 declares the header guard for both winsock.h and winsock2.h; never really checked what winsock.h declares.

    http://en.wikipedia.org/wiki/Include_guard

    Tim S.
    Last edited by stahta01; 07-22-2011 at 06:43 AM.

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    I know it's only an example but you should not need to include both winsock2 and winsock, since winsock2 includes winsock for you.

    The easiest way to track this stuff down is to open the header files in a text editor (NOT a word processor) and have a look for yourself.

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    Where is the location of the header files (ie. winsock.h) in MS visual studio 2008?

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    Registered User hk_mp5kpdw's Avatar
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    If you have your project open in Visual Studio 2008 and you want to see what is in the header files, just right click on the header in the source and a window will pop-up. The first option in that pop-up will say "Open Document <iomanip>" as an example. Choosing that option will open that particular header file in another tab of the IDE so you can see what it contains. No need to go searching for the actual folder where it is located just to view its contents.
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