Thread: #define versus const (a beginner's question)

  1. #1
    Registered User
    Join Date
    Mar 2002

    #define versus const (a beginner's question)

    What is the difference between #define and const? And when do you use which?

  2. #2
    C > C++ duders ggs's Avatar
    Join Date
    Aug 2001
    a #define is handled by the preprocessor, which mashes up source files for the compiler to handle beforehand. const are like variables, but have a constant value. you can't use const to make macros such as

    #define MACRO(x) { x += 10 };

    if you wait, somebody will probably give a better answer. would probably give the best answer of all.
    .sect signature

  3. #3
    Registered User
    Join Date
    Oct 2001

    #define vs. const

    This as an incomplete answer but might help.
    #define is a preprocessor facility used in C to define constants and macros among other things. The preprocessor goes through the source file and does a literal replacement of the values defined. In C a const only prevents the program from changing the value of the const variable.
    In C++ a const is used as a preprocessor tool instead of a #define. It lets the programmer "type qualify" the variable where #define cannot do that. Ex. #define number 33 vs. const int number = 33;
    There is a lot more to this. Check a good book on C/C++.

  4. #4
    Join Date
    Aug 2001
    Groningen (NL)
    If you use #define, this will be processed by the preprocessor. So when you define

    #define MY_VALUE 10

    then everywhere in the code MY_VALUE appears, it is substituted by 10. But if you declare a const, you declare a constant variable and a variable takes memory. A constant variable can be used in almost the same way as a normal variable, but its value can't be changed. A defined value can't be used as a variable.

  5. #5
    The Artful Lurker Deckard's Avatar
    Join Date
    Jan 2002
    One key difference between using preprocessor defines and constants is type checking. I'll take compile-time warnings over run-time errors anyday :)
    Jason Deckard

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