What is the feature to use for( ;; )?

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Is “for(;;)” faster than “while (TRUE)”? If not, why do people use it?

Found code with this for-loop. What exactly feature to use it insted of while(true) for example? Is it uses less memory ?

for (;;) is just an empty loop that reserves some space for more complex conditionals than while. Otherwise, not much benefit. — ActiveTrayPrntrTagDataStrDrvr, Jan 18 '13 at 13:06
Compared with `while(true)`, `for(;;)` uses 4 bytes less memory on your hard drive. — fredoverflow, Jan 18 '13 at 13:29
@FredOverflow: What's more important is that some compilers (used to?) warn about `while(true)` ("constant expression in condition") but none I came across warned about `for(;;)`. — sbi, Jan 18 '13 at 13:31
@FredOverflow: Readability is in the eye of the beholder. `:)` — sbi, Jan 18 '13 at 13:35
@sbi Right, you could `#define ever (;;)` and then `for ever` is the most readable ;) — fredoverflow, Jan 18 '13 at 13:37

simonc · Accepted Answer · 2013-01-18T13:42:14.777

11

for(;;) is functionally equivalent to while(true) but avoids a "conditional expression is constant" warning with some compilers (including MSVC's cl)

edited Jan 18 '13 at 13:42

answered Jan 18 '13 at 13:03

simonc

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Surely `nmake` is not a compiler, is it? – Daniel Roethlisberger Jan 18 '13 at 13:04
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@DanielRoethlisberger clarified as `cl` – simonc Jan 18 '13 at 13:06
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The compiler is called _MSVC_. `cl.exe` is just one of the command line tools (there's also `link.exe`, for example) coming with it. So, "including MSVC" should do. – sbi Jan 18 '13 at 13:33

score 3 · Answer 2 · answered Jan 18 '13 at 13:04

This construct has been popularized by Kernighan and Ritchie in their C Programming Language book (section 3.5)

The for statement

for (expr1; expr2; expr3)
    statement

If expr1 or expr3 is omitted, it is simply dropped from the expansion. If the test, expr2, is not present, it is taken as permanently true, so

for (;;) {
    ...
}

is an "infinite" loop, presumably to be broken by other means, such as a break or return.

There is absolutely no difference between for(;;) and while(true).

score 1 · Answer 3 · answered Jan 18 '13 at 13:04

while(true)
{

}

Is always what I've used and what I've seen others use for a loop that has to be broken manually.

Some compilers are giving you wrnings that this while loom MAY be incorrect. But it is not.

The assembly code generated for both is exactly the same. No diferences.

score 1 · Answer 4 · answered Jan 18 '13 at 13:09

First, The use of loop depends on the requirement & based on that only we can decide when to use which one. Just check what @Luchian has asked, then you will come to know it.

Secondly, that clearly depends on the particular implementation of the interpreter/compiler of the specific language.

That said, theoretically, any sane implementation is likely to be able to implement one in terms of the other if it was faster so the difference should be negligible at most.

What is the feature to use for( ;; )?

4 Answers4