RE: [boost] Avoiding warning on the do{...}while(0) macroimplementation trick

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-09-20 11:29:53

Ivan Vecerina wrote:
> I've noticed discussions about this a couple of years ago.
> For assert-style macros that include multiple statements,
> a common idiom is to enclose the statements in a do{...}while(0)
> dummy loop. This makes the macro behave like a single statement.
> 
> A problem with some compilers (notably VC7.x) is that this
> triggers a warning (C4127: conditional expression is constant).
> 
> The workaround adopted by boost is to automatically disable
> this warning -- which however can be legitimate and useful
> in user code.
> 
> I think I have found a better 'always-false' condition which
> does not trigger a warning, and works at least in VC7.1:
>     do{ }while(__LINE__==-1)
>
> Do you see a problem with this approach?
<snip>

It works in VC++ 7.1 if you enable "edit and continue", because
this results in non-standard expansion of the __LINE__ macro to
an expression that isn't a compile-time constant.

In general it doesn't work.