include library header in header files

This is a discussion on include library header in header files within the C++ Programming forums, part of the General Programming Boards category; I was working with xemacs in unix. One of my c++ class header file has a string variable. E.g.: document.h: ...

  1. #1
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    include library header in header files

    I was working with xemacs in unix. One of my c++ class header file has a string variable. E.g.:

    document.h:
    Code:
    #ifndef DOC_H
    #define DOC_H
    
    #include <string>
    using std::string;
    
    class Document
    {
    public:
    /* ... */ 
    private:
    String name;
    };
    
    #endif
    Notice the #include <string>. I had to place that or else the compiler doesnt recognize the string variable. Why is it so? I never had to do that in VC++. Can't i just include the string header before this header file in the implementation file?

    E.g:
    document.cpp:

    Code:
    #include <iostream>
    #include <string>
    #include "document.h"
    
    using namespace std;
    
    /*.........Implementations of class document....*/

  2. #2
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    When i use a string the same way you described, i had to use the using-stament the same way in microsoft VC 6 too. But i must warn you, never to use a using-stament in header-files. Its a real bad habit, because you force every user of your header/lib to live together with your using-statement. Its much better to always use the complete "path" in your header, for example std::string, std::vector an so on. Use using-statments only in cpp-files.

    mfg JJ

  3. #3
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    Yes thanks, i am aware of that namespace danger. My program was just a small application, so i was lazy to specify the namespace. But it's really strange. I never had to include a library header in my own header files before. Really wish someone could tell me why.

  4. #4
    Registered User jlou's Avatar
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    Sometimes some library headers include other library headers inside them. However, this depends on each library implementation and cannot and should not be counted on.

    One example is in VC++ 6.0, <iostream> includes portions of <string>. Unfortunately, this leads people to believe that they don't need the <string> header to use std::string, and later on they get errors trying to output the string without using c_str(), among other potential problems.

    So you should include the header file that contains the declarations you need even if it works without it.

  5. #5
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    document.cpp:
    Code:
    #include <iostream>
    #include <string>
    #include "document.h"
    
    using namespace std;
    
    /*.........Implementations of class document....*/
    I included it in document.cpp. But my question is why must i include it in the class header "document.h" in order for it to recognize string variables.

  6. #6
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    Quote Originally Posted by Raison
    document.cpp:
    Code:
    #include <iostream>
    #include <string>
    #include "document.h"
    
    using namespace std;
    
    /*.........Implementations of class document....*/
    I included it in document.cpp. But my question is why must i include it in the class header "document.h" in order for it to recognize string variables.
    When the compiler reads document.h, it doesn't know about <string>

    Each file is treated separately by the compiler (preprocessor, in this case) and only knows what you tell it in that file. So in document.h, if you tell it to #include <string>, now it knows everything in <string> and all is OK, otherwise: no joy.

    Regards,


    Dave
    Last edited by Dave Evans; 09-26-2004 at 02:08 PM.

  7. #7
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    I always tot the compiler knows how to link them together. Guess I will start including headers inside the headers from now on

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