C Preprocessor - example

Question

1.00/5 (1 vote)

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C#

#include<stdio.h>
#define SWAP(a, b, c)(c t; t=a, a=b, b=t)
int main()
{
    int x=10, y=20;
    SWAP(x, y, int);
    printf("%d %d\n", x, y);
    return 0;
}

why it is not allowing int to be declared?can we declare inside a paranthesis?whether it is allowed?

Posted 17-Jan-13 1:52am

mvigsnhwar

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4 solutions

Solution 4

I am not sure where you are going with these questions, but I would strongly recommend you get hold of a decent book on C++ and study it. Trying to solve programming problems with preprocessor macros is a bad idea and can lead to many difficult to diagnose problems. Use the language to write your code using the proper tools and constructs.

Posted 17-Jan-13 3:08am

Richard MacCutchan

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Mattias Högström · Accepted Answer · 2013-01-17T02:06:00

Solution 2

Try this instead

#define SWAP(a, b, c) {c t; t=a, a=b, b=t;}

I inserted a space, and exchanged parentheses for braces.

Posted 17-Jan-13 2:06am

Mattias Högström

Updated 17-Jan-13 2:07am

v2

Comments

nv3 17-Jan-13 8:12am

The space should not be necessary. Other than that you hit it!

To questioner: See the std::swap template function which does exactly that without even needing the type argument.

Espen Harlinn 17-Jan-13 8:39am

What happens if your macro is used in this fashion:
if(x < y)
SWAP(x,y,int);
else
y++;
// Hint: It will not compile ...

Mattias Högström 17-Jan-13 11:05am

You are right.
Hey!! Don't call it my macro. I just tried to correct it (but missed making it foolproof).
I hate macros. They are error prone and makes it harder to debug the code.
I would prefer an inlined function over a macro.

std::swap is the preferred alternative

ps. According to my coding style guideline
Even 1 line if else statements should use braces. :)

Espen Harlinn 17-Jan-13 14:41pm

Relax, IMHO this should not be implemented as a macro :)

Espen Harlinn · Accepted Answer · 2013-01-17T02:19:00

Solution 3

Use std::swap[^]
or if you're set on doing this using the preprocessor:

#define SWAP(a,b,c) do {c t=a; a=b; b=t;}while(0)

The do while construct ensures that you can do something like this:

if(x < y)
 SWAP(x,y,int);
else
 y++;
// rest of your code

Without undesired side effects - I would advice you to use std::swap.

Best regards
Espen Harlinn

Posted 17-Jan-13 2:19am

Espen Harlinn

Updated 17-Jan-13 2:35am

v2

Comments

H.Brydon 17-Jan-13 13:39pm

Correct and complete... +5 from me.

Espen Harlinn 17-Jan-13 14:43pm

Thank you, Brydon :-D

CPallini 18-Jan-13 7:41am

5.
I would advise the OP to follow your advice.

Espen Harlinn 18-Jan-13 11:37am

Thank you, Carlo :-D

*Suvabrata Roy* · Accepted Answer · 2013-01-17T02:02:00

Solution 1

Take a look Click[^]

Posted 17-Jan-13 2:02am

Suvabrata Roy

C Preprocessor - example

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C Preprocessor - example

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