CODEWITHSUNDEEP
cc99c89
lastland
lastland

Reputation: 910

Would it be safe to do swap by "b=(a+b)-(a=b);"?

In Cprogramming.com I found this piece of code:

int a,b;
scanf("%d %d",&a,&b);
b=(a+b)-(a=b);
printf("%d %d",a,b);

It is claimed to be a tip/trick to "swap without using temporary". My tests on Linux gcc prove it. However, wouldn't the order how different compilers or platforms computing this expression matters here? Is it safe to use such code?

Upvotes: 4

Views: 172

Answers (2)

CB Bailey
CB Bailey

Reputation: 791691

No. In the expression (a+b)-(a=b) there is no sequence point between a being written to and a being read in the (a+b) sub-expression to determine the value to be stored to b so the behaviour is undefined.

Upvotes: 12

sepp2k
sepp2k

Reputation: 370112

However, wouldn't the order how different compilers or platforms computing this expression matters here?

Yes.

Is it safe to use such code?

No, it's undefined behavior.

Upvotes: 8

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