Two ways to write multi-statement macros 
Author Message
 Two ways to write multi-statement macros

> I've noticed that whenever I see multi-statement macros in this
> newsgroup they're usually written with a "do {...} while(0)" construct
> to transform them into a single statement.  One of my colleagues asked
> a simple question that I can't answer: Why is this a better way to
> write multi-statement macros than simply enclosing the statements in
> braces without adding the "do" and "while(0)"?

The problem is that this program is not legal C:

    #include <stdio.h>
    #define macro(x) { if (x) puts("yes"); else puts("no"); puts("\n"); }

    int main()
        char s[100];
        puts("Type some stuff > ");
        if ((gets(s))[0] == 'a')
        return 0;

whereas the same program with "do" and "while(0)" around the braces is
legal.  The idea of using "do { ... } while(0)" is that the macro be
legal in all statement contexts and nowhere else; i.e. that it should
behave as though the macro were in fact a function.

Gareth Rees

Mon, 30 Jun 1997 01:43:26 GMT  
 [ 1 post ] 

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