Question

I could have also phrased this as What constitutes observable behavior?

The C++ Standard talks a lot about observable behavior, but I am not really sure if program termination is part of observable behavior.

That is, given a program such as:

int main() {
  for(;;) {}
  return 0;
}

is a conforming implementation allowed to ever terminate this program?

Answer 1

Yes it is legal for the compile to generate an empty main body for the above code (thus terminating almost immediately).

The C++0x FCD says 6.5 says (pay special attention to the note):

A loop that, outside of the for-init-statement in the case of a for statement,
* makes no calls to library I/O functions, and
* does not access or modify volatile objects, and
* performs no synchronization operations (1.10) or atomic operations (Clause 29)

may be assumed by the implementation to terminate. [ Note: This is intended to allow compiler transformations, such as removal of empty loops, even when termination cannot be proven. — end note ]

So the compiler can assume that your for loop will terminate and since the body is empty it can optimize it away altogether.

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The quotation from the draft was copied from this question and verified against my copy.