macro problem

This is a discussion on macro problem within the C Programming forums, part of the General Programming Boards category; if I have a macro defined as Code: #define swap(a,b,c) c t; t=a,a=b,b=t; The above would work if a,b were ...

  1. #1
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    macro problem

    if I have a macro defined as

    Code:
    #define swap(a,b,c) c t; t=a,a=b,b=t;
    The above would work if a,b were numbers and 'c' was a data type like int.but it wont work for float * or any other pointer type.it will give a compile error ,why?


    cheers!

  2. #2
    and the hat of wrongness Salem's Avatar
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    You would need to show an example use of "it doesn't work".
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    spurious conceit MK27's Avatar
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    Quote Originally Posted by Salem View Post
    You would need to show an example use of "it doesn't work".
    I'd be more interested in where it does work...

    Have you seen this one:
    Code:
    #define swap(a,b) {a ^= b; b ^= a; a ^= b;}
    That only works with ints, tho.
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  4. #4
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by MK27 View Post
    I'd be more interested in where it does work...

    Have you seen this one:
    Code:
    #define swap(a,b) {a ^= b; b ^= a; a ^= b;}
    That only works with ints, tho.
    So far as I can tell it works everywhere for anything, provided you are in a context where swap can safely expand to two statements (so if it's inside an if, you're going to need braces), at least assuming C99 so we can have variables declared anywhere.

  5. #5
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by MK27
    I'd be more interested in where it does work...
    I think that it should work in quite a few situations, but to enable it to work in more situations:
    Code:
    #define swap(a,b,c) do { c t; t=a,a=b,b=t; } while (0)
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  6. #6
    spurious conceit MK27's Avatar
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    Quote Originally Posted by laserlight View Post
    I think that it should work in quite a few situations, but
    How would I call that then? I almost never use macros, so I am interested in the syntax. But if I try:
    Code:
    swap(x,y,z);
    Where x y z are ints, I get:

    error: ‘t’ undeclared (first use in this function)

    for both versions of this. Of course, this will work:
    Code:
    #define swap2(a,b,t) {t=a,a=b,b=t;}
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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    swap generally involves 2 variables. In the macro, the third parameter specifies the type, so if x and y are ints, you would call it swap(x, y, int)

  8. #8
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by MK27
    How would I call that then?
    Eh, eklavya8 described an example: "The above would work if a,b were numbers and 'c' was a data type like int."

    Thus the example would be:
    Code:
    swap(x, y, int);
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  9. #9
    spurious conceit MK27's Avatar
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    Got it, thanks (I was thinking, "t" for temp, and that the OP meant "submit a tmp variable of the correct type")!

    So today is my day to learn about writing macros...
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
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  10. #10
    spurious conceit MK27's Avatar
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    Quote Originally Posted by tabstop View Post
    So far as I can tell it works everywhere for anything, provided you are in a context where swap can safely expand to two statements (so if it's inside an if, you're going to need braces), at least assuming C99 so we can have variables declared anywhere.
    error: invalid operands to binary ^ (have ‘float’ and ‘float’)

    Code:
    #include <stdio.h>
    
    #define swap(a,b) {a ^= b; b ^= a; a ^= b;}
    
    int main() {
    	float i=0.7f, j=22.11f;
    	swap(i,j);
    	printf("swap: %f %f\n",i,j);
    	return 0;
    }
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  11. #11
    C++ Witch laserlight's Avatar
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    MK27, tabstop's statement was in response to your comment about wanting to know where eklavya8's swap macro will work. It was not in response to your comment about XOR swap.
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  12. #12
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    Quote Originally Posted by MK27 View Post
    error: invalid operands to binary ^ (have ‘float’ and ‘float’)

    Code:
    #include <stdio.h>
    
    #define swap(a,b) {a ^= b; b ^= a; a ^= b;}
    
    int main() {
    	float i=0.7f, j=22.11f;
    	swap(i,j);
    	printf("swap: %f %f\n",i,j);
    	return 0;
    }
    Because the bitwise operators work only on integral operands (singed or unsigned).

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