Saturday, December 26, 2015

Macro Functions

   >>> The #define directive can also be used to create macro functions.

   >>> A macro function is a symbol created using #defines and that takes an argument much like a function does. The preprocessor will substitute the substitution string for whatever argument its given.

   For Example, You can define the macro TWICE AS

        #define TWICE(x)                ((x) * 2)

        and then in your code you write

       The entire string TWICE(4) will be removed and the value 8 will be substituted!

   >>> When the pre compiler sees the 4, it will substitute ((4) * 2), which will then evaluate  to 4 * 22 or 8.

   >>> A macro can have more than one parameter and each parameter can be used repeatedly in the replacement text. Two common macros are MAX and MIN:

    #define MAX(x,y) ((x) > (y) ? (x) : (y))
    #define MIN(x,y) ((x) > (y) ? (x) : (y))

   >>> Note that in a macro function definition the opening parenthesis for the parameter list must immediately follow the macro name with no spaces.

   >>> The preprocessor is not as following of white space as in the compiler.

   >>> If you were to write
           #define MAX (x,y) ((x) > (y) ? (x) : (y))

  and then tried to use MAX like this,

        int x = 5, y = 7, z;

        z = MAX (x,y);

  the intermediate code would be
        int x = 5, y = 7, z;
        z = (x,y)  ((x) > (y) ? (x) : (y)) (x,y)

   >>> A simple text substitution would be done rather than invoking the macro function.

   >>> Thus, the token MAX would have substituted for it (x,y)  ((x) > (y) ? (x) : (y)) and then that would be followed by the (x,y) which followed MAX.

   >>> By removing the space between MAX and (x,y), however, the intermediate code becomes:

           int x = 5, y = 7, z;
           z = 7;

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