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Reputation: 21366

Why do for(;;) loops behave like infinite loops?

The answers to a recent question about for(;;){} loops (What does a for (;;) loop do) did not seem to answer something for me, so I thought that I would try to refine the question a bit. In particular, beyond knowing that for loops without conditionals are infinite loops, I would like to know why they are infinite loops.

In the statement for (;_;){}, the _ is a conditional expression. My first guess would be that an empty expression might evaluate to 0 or NULL. But if you test:

for (;;){}

is an infinite loop, as everyone has pointed out.

for (;1;){}

is an infinite loop.

But neither of these loop bodies execute at all:

for (;0;){}
for (;NULL;){}

Thus, the empty conditional expression does not seem to evaluate to either 0 or NULL.

So, my question: is the behavior of the for (;;){} loop an artifact of the way that C evaluates expressions, or is it just a special implementation-defined case, because a loop body that never executes is not very useful?

UPDATE: After reading the comments and answers, I realize that my question wasn't as clearly formulated as it might have been. I suppose that the question was two-fold:

  1. Is the behavior of for(;;){} loops strictly a result of the way that C evaluates expressions in general, or is this behavior specific to the way that C evaluates for statements?

  2. Why was this behavior chosen for for loops lacking conditional expressions?

Upvotes: 7

Views: 450

Answers (2)

Martin Sugioarto
Martin Sugioarto

Reputation: 340

The conditional expression checks if the loop is to be continued, yes, and the default function of a loop is.. to loop. If it wasn't a special case having an empty test expression and continue the loop in this case, it would be consequently the opposite case (as you already noticed) and abort the loop which makes the entire for statement less powerful or even redundant.

The technique used here is called reasonable/sensible defaults.

Upvotes: 0

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385274

Both C and C++ guarantee this behaviour.

[C99: 6.8.5.3/1]: Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

[C++14: 6.5.3/1]: The for statement

for ( for-init-statement conditionopt; expressionopt) statement

is equivalent to

{
   for-init-statement
   while ( condition ) {
      statement
      expression ;
   }
}

[..]

[C++14: 6.5.3/2]: Either or both of the condition and the expression can be omitted. A missing condition makes the implied while clause equivalent to while(true).

Upvotes: 19

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