Macros Using #s

This is a discussion on Macros Using #s within the C++ Programming forums, part of the General Programming Boards category; NULL is constant, not temporary. Not really - NULL is a constant, but its use in general programming practice can ...

  1. #16
    C++ Witch laserlight's Avatar
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    NULL is constant, not temporary.
    Not really - NULL is a constant, but its use in general programming practice can be temporary.
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  2. #17
    Registered User major_small's Avatar
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    Quote Originally Posted by quzah
    NULL is constant, not temporary.


    Quzah.
    party pooper... if you think about it, NULL (along with all other 'constant' variables) is temporary, because it only exists as long as the program is running

    and no, I'm not talking about NULL as a universal thing, just as it applies to a C/C++ program.

    more fun:
    Code:
    #include<iostream>
    #undef NULL
    
    int main()
    {
            #ifdef NULL
            int*ptr=NULL;
            std::cout<<"NULL defined as "<<ptr<<std::endl;
            #else
            std::cout<<"NULL not defined"<<std::endl;
            #endif
    
            return 0;
    }
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  3. #18
    ATH0 quzah's Avatar
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    And...aren't there rules about white space, new lines, commas, semi-colons, etc. when defining macros? Could somoene explain these a little? Isn't it that you never use semi-colons, comments, or double-quotes?
    There aren't any white space rules other than the usual. The only "newline" issue is that you escape multi-line macros. Like so:
    Code:
    #define FOO(bar) line1 \
        line2
    The last line doesn't have a \, all others do.

    A macro is simply a compile time text subsutution. The ## macros smash things together. Like so:
    Code:
    #define FOO(bar,baz) bar ## baz
    
    int var1, var2, var3
    
    FOO( var, 1 ) = 10;
    Then you do something like this:
    Code:
    #include<iostream>
    
    #define FOO(bar,baz) bar ## \
        baz
    #define BAR(foo,baz) foo # \
        baz
    
    int main( )
    {
        int var1, var2, var3;
    
        FOO( var, 1 ) = 10;
        FOO( var, 2 ) = 20;
        FOO( var, 3 ) = 30;
    
        std::cout << BAR("var",1) << " is: " << FOO( var, 1 ) << std::endl;
        std::cout << BAR("var",2) << " is: " << FOO( var, 2 ) << std::endl;
        std::cout << BAR("var",3) << " is: " << FOO( var, 3 ) << std::endl;
    
        return 0;
    }
    They're multi-line just for the hell of it. They don't need to be. The only thing about white space is you need your #define or #include or what not to start on the left margin. Some compilers will let you get away with it indented for some of the things, but IIRC, th're supposed to start on the left most character.
    Code:
    #include <GOOD>
    Code:
        #include <bad>

    Quzah.
    Last edited by quzah; 07-17-2005 at 06:41 AM.
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  4. #19
    ATH0 quzah's Avatar
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    Quote Originally Posted by laserlight
    Not really - NULL is a constant, but its use in general programming practice can be temporary.
    NULL is a constant. You've just said so. Your use of it is temporary, but NULL remains constant. There's no two ways about it.

    Waxing philisophical...


    Quzah.
    Hope is the first step on the road to disappointment.

  5. #20
    C++ Witch laserlight's Avatar
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    NULL is a constant. You've just said so. Your use of it is temporary, but NULL remains constant. There's no two ways about it.
    We're just playing with language
    I didnt say NULL is a temporary, I said NULL is temporary.
    I didnt say NULL is constant, I said NULL is a constant.
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  6. #21
    the hat of redundancy hat nvoigt's Avatar
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    Macros, if used correctly and documented correctly, can save a lot of work. And no, if the macro isn't simple, it cannot be replaced by an inline function.

    Challenge 1: Write an inline function that does the same as

    Code:
    #define PRINT_NAME_AND_NUMERICAL_VALUE( name ) printf("%s is %d", #name, name );
    or

    Code:
    #define DECLARE_VARIABLE_AND_ACCESSORS( name ) int m_i##name; \
    int GetNumber##name() { return m_i##name; }\
    void SetNumber##name( int val ) { m_i##name = val; }
    Looks ugly for a little class, but write a class with 100 variables and accessors and we will see.

    Or write an inline function, that will be compiled conditionally

    Code:
    #ifdef _PERF_
    MEASURE_PERFORMANCE( statement, description ) [performance measuring and output code]
    #else
    MEASURE_PERFORMANCE( statement, description ) statement;
    #endif
    This might be the job of a profiler... but you don't want to have a profiler on your production system.

    I don't see any alternatives to Macros. You probably frown upon them while others get work done using them
    hth
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  7. #22
    Supermassive black hole cboard_member's Avatar
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    Actually the first macro:

    Code:
    #define PRINT_NAME_AND_NUMERICAL_VALUE( name ) printf("%s is %d", #name, name );
    Wouldn't be to hard - not that I've tried - as you can surely use a stringstream to get the numerical value of name? You can use a stringstream to write an itos function.

    Actually I'm going to try that now, I'll post it up if I have any joy.
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