CODEWITHSUNDEEP
creturnvoidundefined-behavior
lee77
lee77

Reputation: 1503

In C, can the expression foo() be used as the empty expression if foo's return type is void?

With respect to the return statement, the Microsoft Visual Studio documentation says

Syntax:

return expression;

where expression is marked as optional. It continues

The value of expression, if present, is returned to the calling function. If expression is omitted, the return value of the function is undefined.

This is quite clear, but on the other hand, there is the notion of an empty expression. This makes me confuse. Thinking of the empty expression not as nothing, but as an expression which is empty, I would think that if we have a function

void foo(void)
{
     return;
}

then the expression foo() could be used wherever the empty expression is allowed. For example, the behaviour of the code

unsigned int i=0;
for(foo();i<10;i++) printf("%u",i);

would be defined. I am aware that this is probably of little practical relevance, but I would like to understand why, in this context, the empty expression is not an expression.

Upvotes: 1

Views: 123

Answers (2)

Yu Hao
Yu Hao

Reputation: 122383

It's called void expression. And you can use a void expression in for like you did. The void expression does nothing but its side effect, which is calling the function.

In fact, if the first clause of for is any type of expression, it's evaluated as a void expression:

C99 6.8.5.3 The for statement

... If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.

and

C99 6.3.2.2 void

The (nonexistent) value of a void expression(an expression that has type void) shall not be used in any way, and implicit or explicit conversions (except to void) shall not be applied to such an expression. If an expression of any other type is evaluated as a void expression, its value or designator is discarded. (A void expression is evaluated for its side effects.)

Upvotes: 4

user529758
user529758

Reputation:

I don't know if there actually is such a thing in C as "the empty expression", but if there is, then you are confusing it with the type void, and at the same time you are confusing the act of returning "nothing" from a non-void function (which is illegal) with leaving out the initializer expression of a for loop (which is legal).

Upvotes: 1

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