Re: defining static variables inside 'for' loops

"Alf P. Steinbach" <>
Tue, 21 Jul 2009 03:49:01 CST
* petrum:

I noticed you can define 'static' variables inside 'for' loops, but
only local variables in 'if' (or 'while') statements.

For example all these 3 x constructions are valid:

     for (static int i = 0;;)
     if (int i = 0)
     while (int i = 0)

I was wondering where in the standard C++ is this specified.

In the syntax for the loops.

'if' and 'while' only have a 'condition' (IIRC), which has a special purpose
syntax for emulating a limited kind of declaration, marginally useful for e.g. a
dynamic_cast as condition.

In the 'for' loop the second element of the loop head, the loop continuation
condition, is such a 'condition'. The first element can be a more general
declaration. Thus, you can have a general + a limited declaration :-), like

     for( int a = 10; int b = 10*a; --a )

I'd consider that as abuse of the construct, and it's probably not intended.

Many thanks,
Petru Marginean

Hm, it would be really nice with a public (e.g. SourceForge) version of
ScopeGuard adapted for Visual C++. The problem with that compiler is that its
__LINE__ doesn't work properly when you compile with flag for edit-and-continue
or whatever it was, which of course is a default setting in Visual Studio;
instead of standard __LINE__ one must use Microsoft-specific __COUNTER__. I
think more widespread usage of ScopeGuard would be a very good thing. :-)


- Alf

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