What does comma operator in C do when we return an integer with two values?

Question

I was actually returning a float value when I typed , instead of . but it did not give me any error. Then I tried running the below code.

#include<stdio.h>
#include<conio.h>
int getValue();
int main()
{
    int a = getValue();
    printf("%d", a);
    return 0;
}

int getValue()
{
    return 2, 3;
}

Now the output is 3, that is it returned the second value. This happened two years ago and was searching for the proper answer since then.

Studying answers to this question I came to know that it returns the second value after evaluating, but what does it do with the first one?

I studied the logic of the stack(how the values are pushed and pop internally) but I don't think this have anything to do with it.

Does it processes the two values or do something else?

I did mentioned the same in the description – Drag0nKn1ght May 26 '15 at 09:02 — Drag0nKn1ght, May 26 '15 at 09:02

Sourav Ghosh · Accepted Answer · 2015-05-26T09:34:11.610

4

Quoting C11 standard, chapter §6.5.17, Comma operator

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

Now, to answer your question

what does it do with the first one?

we can say, the first operand (left hand operand) is evaluated and the result is discarded.

^{NOTE: I mentioned the result, not the effect.}

Just to clarify, in your case, you won't notice the effect, but you can notice it's effect if both the left and right hand expressions are related to the same variable. For example, let's discuss a simple exmple

return p=3, p+2;

We can break down the return statement like

asign a value 3 to the variable p [left hand side operator of ,]
execute p+2, ehich generates a value of 5. [right hand side operator of ,]
return the value 5 as the value of second argument [following the clause : "the result (of , operator) has its (evaluation of right-operand) type and value."]

See a live demo.

edited May 26 '15 at 09:34

answered May 26 '15 at 08:58

Sourav Ghosh

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1

Won't the compiler optimize away the first operand because it has no impact and no use of its result? – Eregrith May 26 '15 at 09:03
@Eregrith Maybe, but that is not directly linked with the opearation of the `,` operator, right? – Sourav Ghosh May 26 '15 at 09:10
1

Yes, it's only compiler-dependent, and the OP should('ve) include(d) his compiler to the question, as he wanted to know "what really happens". – Eregrith May 26 '15 at 09:11
@Mat What? Is optimisation and removal of void useless impactless statements **not** compiler dependent? – Eregrith May 26 '15 at 09:23
@Eregrith: I had read only your second comment, sorry. Didn't understand you were talking about potential optimizations only and not the guaranteed behavior. – Mat May 26 '15 at 09:26
@Mat updated. Hope it should be simple to understand for all. Please review. :-) – Sourav Ghosh May 26 '15 at 09:30

score 2 · Answer 2 · answered May 26 '15 at 08:58

2

comma operator will evaluate from left to right and return the right most expression or value. This is equivalent to:

int getValue()
{
    (void)2;
    return 3;
}

answered May 26 '15 at 08:58

Mohit Jain

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What does comma operator in C do when we return an integer with two values?

2 Answers2