C++ for loop with no conditions

Question

I ran across this for ( ; ; ) {}

A quick cursory search lead me nowhere in finding out what this means. It looks like another thread might be handling terminating this. Is this equivalent to 'while (TRUE) {}' ?

while(TRUE){} isn't valid C++. Both while(true){} and for(;;){} are undefined behaviour. — Kerrek SB
– Kerrek SB, Commented Oct 7, 2013 at 22:30
@KerrekSB How exactly are either of those undefined behavior? — Kevin
– Kevin, Commented Oct 7, 2013 at 22:31
@Kevin: 1.10/24: "The implementation may assume that every thread teminates." — Kerrek SB
– Kerrek SB, Commented Oct 7, 2013 at 22:36
@KerrekSB: That refers to 6.8.5p5, which says: "An iteration statement whose controlling expression is not a constant expression, that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.". That doesn't apply to for (;;) {}. (A footnote says that for (;;) is treated as having a constant controlling expression.) — Keith Thompson
– Keith Thompson, Commented Oct 7, 2013 at 22:42

Gavin Smith · Accepted Answer · 2013-10-07 22:33:47Z

This is an infinite loop. Any of the three parts of a for loop (initialization, condition and increment) can be missing. Specifically, if the condition in a for loop is missing, it is treated as being true. So it is equivalent to while(1) { ... }.

hazer_drum · Accepted Answer · 2013-10-07 22:33:27Z

2

Yes they are equivalent in functionalities.

answered Oct 7, 2013 at 22:33

hazer_drum

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