Libraries and headers

This is a discussion on Libraries and headers within the C++ Programming forums, part of the General Programming Boards category; Sorry for this newbie question, but what do libraries exactly do ? Why do I have to #include a header ...

  1. #1
    *******argv[] - hu? darksaidin's Avatar
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    Libraries and headers

    Sorry for this newbie question, but what do libraries exactly do ? Why do I have to #include a header file *AND* at the same time tell the compiler to link some libraries?

    In the past I've been coding in Delphi and there was no need for libraries, at least not that I am aware of.
    i.e. to get access to all external winsock.dll functions, all I had to do is inlcude the winsock.pas file. This file contained all neccessary data types as well as procedure prototypes that were linked to the appropriate DLL function (i.e. function accept; external WINSOCK_DLL name 'accept'; )

    As far as I can tell with my limited C++ knowledge, a header file contains almost the same data, so what are those libraries used for ? What kind of additional code is neccessary in C++ to access a DLL?

  2. #2
    Cat
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    A header file only tells the compiler what functions exist inside the libraries. The .lib file will do one of two things. In a static linked library, it will actually contain all of the functions you are trying to access. In a DLL, the .lib will contain the code needed to load the DLL and call the function.

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    The header files include very often used methods. Say you want to write some text to the console screen you need to #include <iostream> or if you want to open a file you need to #include <fstream>. if you got alot of spare time you could write your own version but why bother.

  4. #4
    *******argv[] - hu? darksaidin's Avatar
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    A .pas is split in interface- and implementation-section. So a c++ header file represents the interface section (procedure prototype to be used in pascal) and the library the implementation part (procedure; external; to link to the library).

    I think I understand that. ... think...

    Oh, just one more thing: Those libraries, can the the compiler extract only the needed data or is the entire thing linked into the exe?

  5. #5
    C++ Developer XSquared's Avatar
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    Compilers generally only extract the needed data AFAIK.
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  6. #6
    *******argv[] - hu? darksaidin's Avatar
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    Ok..... my problem here is: The more I understand, the more questions I have...

    Next problem: Some includes are postfixed with a .h, some are not. Does the .h mean there is nothing but prototypes in the include ? The mingw isn't exactly what I'd call well structured code (at least not for a c newbie), but that seems to be the only difference.

    Another thing is the library directory. It contains files with different extensions. What does .a mean? .o? .la?

  7. #7
    C++ Developer XSquared's Avatar
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    The only header files that are not prefixed with .h are the new standard C++ libraries, which always use the std namespace. You should never have code in an include anyways.

    .o and .a are compiled files which are ready to be linked. The other one I've never heard of.
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  8. #8
    Cat
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    Originally posted by XSquared
    You should never have code in an include anyways.
    Actually, in some cases it's vital to put the code in the headers. Templates require this, as do inline functions with many compilers.

  9. #9
    Super Moderator VirtualAce's Avatar
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    Ok..... my problem here is: The more I understand, the more questions I have...
    Welcome to the wonderful world of programming.



    Libraries are basically code repositories - functions with no main function in them.

    Headers are where function prototypes, class definitions, typdefs, #define's, etc. are.

    Header: foo.h
    Code:
    #ifndef _FOO_
    #define _FOO_
    
    #define TRUE 1
    #define FALSE 0
    
    typedef unsigned int WORD;
    
    void Foo(void);
    int Foo2(int x,int y);
    ...
    ...
    ...
    #endif  //end of foo.h
    Library:
    Code:
    #include "foo.h"
    
    void Foo(void)
    {
    ...
    ...
    ...
    }
    
    int Foo2(int x,int y)
    {
      return (x+y);
    }
    And again Cat is correct. Templates do require code in the headers - at least most of the compilers I've worked with do. I have a post on here about this very thing - somewhere deep down in the dark damp dungeons of the board.

  10. #10
    *******argv[] - hu? darksaidin's Avatar
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    ok, thanks for all the replies


    I've come to the point where my program (actually it's the code of NeHe's OpenGL tutorial) has become pretty... large.
    So I'd like to move some functions to a different file. Is there an easier way then splitting up this code into a header and a cpp? Is there any way to include a file "as it is" without the compiler messing around with it?

    I tried to store all functions but main in a different cpp and used include to "link" it to my main app. I got a lot of error messages because of missing datatypes in that new cpp file so I included those standard headers there, too. Now the compiler told me that those types are already defined :/

    Is there something like "uses ..." in c?

  11. #11
    Super Moderator VirtualAce's Avatar
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    Code:
    #ifndef _FOO_
    #define _FOO_
    
    #define TRUE 1
    #define FALSE 0
    
    typedef unsigned int WORD;
    
    void Foo(void);
    int Foo2(int x,int y);
    ...
    ...
    ...
    #endif  //end of foo.h
    This #ifndef, #define, #endif system will stop the multiple include problem. Your compiler is including files more than once which results in the error you are getting.

    #ifndef _SOME_VARIABLE_ - if this does not evaluate to 1

    #define _SOME_VARIABLE_ - define it and make it evaluate to 1

    ...this will be only included if SOME_VARIABLE evaluates to 0

    #endif - end of the ifndef conditional block

    This ensures that you only include a file when it is needed. If this file has already been included once, _SOME_VARIABLE_ will evaluate to 1 and the code between #ifndef and #endif will not be included again thus preventing multiple declarations.

    Remember that headers only contain declarations for the most part and not actual definitions or bodies of functions - exceptions are templates and inline functions.

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