In support of #defines

This is a discussion on In support of #defines within the C++ Programming forums, part of the General Programming Boards category; They aren't good practice in general, but there is one case in which they can be justified (I think): Let's ...

  1. #1
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    In support of #defines

    They aren't good practice in general, but there is one case in which they can be justified (I think):

    Let's say you have a program that has multiple versions. You want to compare them, and don't want to have lots of windows/tabs up (too lazy or something). You can try this:

    Code:
    #DEFINE VER1 0
    #DEFINE VER2 0
    #DEFINE VER3 1
    
    #IF VER1
    //version 1 code here
    #ENDIF
    
    #IF VER2
    //version 2 code here
    #ENDIF
    
    #IF VER3
    //version 3 code here
    #ENDIF
    And I can't think of a better way to switch versions on the go. Just change the version numbers at the top of your code and you're set to compile. It even works when there's version-specific code littered all over the place. Just make sure that you make your labels something obscure that you won't use in your actual code.

    Any objections?

  2. #2
    Sweet
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    Thats a decent way to use defines. But that can become real complicated real fast.

  3. #3
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    Quote Originally Posted by prog-bman View Post
    Thats a decent way to use defines. But that can become real complicated real fast.
    Most probably. How else would you do it, though?

    Comments don't stack

    #IF 0's get lost all over your code (plus you can't keep track of what the code you're 0'ring out does, and whether you should compile it or not)

    If you erase it, you might have erased something you might want later

    If you make separate files, it's a pain to open all of them
    Last edited by Differ; 07-30-2007 at 08:00 PM.

  4. #4
    Sweet
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    I am just saying that it can become real complicated not that it's bad. The only other way is to implement some sort of version system where you enable/disable things in code depending on the version. Both ways aren't that fun.

  5. #5
    Kiss the monkey. CodeMonkey's Avatar
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    This idea is perhaps the sole solution to certain implementation dependent tasks. However, most often you'd test for whether something was defined, rather than defining them all with a true or false value.
    Code:
    //#define VERSION1
    #define VERSION2
    //#define VERSION3
    
    . . . .
    
    #ifdef VERSION1
    //////
    #endif
    #ifdef VERSION2
    /////
    #endif
    //etc...
    I guess the idea is that compilers define certain macros that indicate the compiler and environment, so that code can be compiled all over the place.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  6. #6
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    Quote Originally Posted by CodeMonkey View Post
    This idea is perhaps the sole solution to certain implementation dependent tasks. However, most often you'd test for whether something was defined, rather than defining them all with a true or false value.
    Code:
    //#define VERSION1
    #define VERSION2
    //#define VERSION3
    
    . . . .
    
    #ifdef VERSION1
    //////
    #endif
    #ifdef VERSION2
    /////
    #endif
    //etc...
    I guess the idea is that compilers define certain macros that indicate the compiler and environment, so that code can be compiled all over the place.
    I didn't even know that function existed. It certainly looks cleaner than my way. =)

  7. #7
    Captain - Lover of the C
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    I didn't even know that function existed. It certainly looks cleaner than my way. =)
    That function certainly does exist. In fact, if you look in any of the standard header files, you should find that it is used quite frequently for the purpose you are describing. An portion of <iostream>:
    Code:
    #ifndef _IOSTREAM_
    #define _IOSTREAM_
    #include <istream>
    
    #ifdef  _MSC_VER
    #pragma pack(push,8)
    #endif  /* _MSC_VER */
    _STD_BEGIN
    		// OBJECTS
    static ios_base::Init _Ios_init;
    extern _CRTIMP istream cin;
    extern _CRTIMP ostream cout;
    extern _CRTIMP ostream cerr, clog;
    		// CLASS _Winit
    class _CRTIMP _Winit {
    public:
    	_Winit();
    	~_Winit();
    private:
    	static int _Init_cnt;
    	};
    		// WIDE OBJECTS
    static _Winit _Wios_init;
    extern _CRTIMP wistream wcin;
    extern _CRTIMP wostream wcout, wcerr, wclog;
    _STD_END
    #ifdef  _MSC_VER
    #pragma pack(pop)
    #endif  /* _MSC_VER */
    
    #endif /* _IOSTREAM_ */
    Don't quote me on that... ...seriously

  8. #8
    Algorithm Dissector iMalc's Avatar
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    Sounds like a poor substitute to version control systems, to me.
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  9. #9
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    It's not intended to be a substitute at all.

    Another example is platform specific code that you place in a single file. You can use #defines to separate windows code from unix/linux code. Obviously keeping separate branches for this file in a version control system makes no sense, since the code is the same for each version of your app.

  10. #10
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    Quote Originally Posted by Daved View Post
    Another example is platform specific code that you place in a single file. You can use #defines to separate windows code from unix/linux code. Obviously keeping separate branches for this file in a version control system makes no sense, since the code is the same for each version of your app.
    A better approach is to encapsulate the platform specific code in their own modules and build using the appropriate module for the platform. As the number of platforms you support grows, you don't want all platform-specific details shoved into a single source file.

    For instance, you have "linux-io.cpp", "win32-io.cpp," etc. which implement your platform-specific I/O routines. The makefile for Linux compiles the linux-io.cpp file and ignores win32-io.cpp. Vice versa for the Windows makefile.

  11. #11
    Frequently Quite Prolix dwks's Avatar
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    Just as a side note: GCC doesn't support uppercase preprocessor directives, and I'm sure that other compilers do not either. That is, you should use
    Code:
    #define X
    instead of
    Code:
    #DEFINE X
    Another legitimate use of #defines: debugging. When debugging mode is on, some code is compiled which makes the program easier to debug, but perhaps slows it down a lot. When you turn off debug mode, the unneccesary code can be completely skipped over by the compiler if it's inside a preprocessor #if. Something like this:
    Code:
    #ifdef DEBUG
        if(memory == NULL) std::clog << "First call or out of memory, allocating memory\n";
    #endif
    Assertions (from <cassert>) are a form of this.

    Lastly, #defines are essential as inclusion guards.
    dwk

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  12. #12
    Just Lurking Dave_Sinkula's Avatar
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    In many cases, things are worded in the reverse. That is, #if NDEBUG is not defined...
    7. It is easier to write an incorrect program than understand a correct one.
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  13. #13
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Daved View Post
    It's not intended to be a substitute at all.

    Another example is platform specific code that you place in a single file. You can use #defines to separate windows code from unix/linux code. Obviously keeping separate branches for this file in a version control system makes no sense, since the code is the same for each version of your app.
    Totally different ballgame. I would never suggest there is anything wrong with doing that.
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  14. #14
    Super Moderator VirtualAce's Avatar
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    If you want an example of how complicated versioning in this manner can become just check the windows.h header file.

    It's a mess.

  15. #15
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    >> Totally different ballgame. I would never suggest there is anything wrong with doing that.
    I was under the impression that something like my example was what the OP was talking about. I completely agree with you if the OP really did want to use #defines for versioning.

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