opCast!bool

[Jonathan M Davis]

On Wednesday, January 04, 2012 00:35:20 Timon Gehr wrote:
> On 01/04/2012 12:31 AM, Jonathan M Davis wrote:
> > On Tuesday, January 03, 2012 17:41:12 simendsjo wrote:
> >> I guess this is as designed, but I'll ask anyway.
> >> 
> >> http://dlang.org/operatoroverloading.html#Cast says an expression is
> >> rewritten to opCast "whenever a bool result is expected".
> >> 
> >> This is true for
> >> if(e) somethingElse
> >> and e&& somethingElse
> >> 
> >> , but not for other parts.
> >> assert(cast(bool)e == true); // explicit cast works
> >> assert(e == true); // Error: incompatible types for ((s) == (false)):
> >> 'S' and 'bool'
> >> 
> >> is(typeof(e) : bool); // false
> > 
> > Yeah. It's the same for built-in types. Take arrays and pointers for
> > example. They don't implicitly convert to bool, but when you use them
> > in a condition, they implicitly convert to bool (true if they're
> > non-null, false if they're null). If you want implicit conversion in
> > general, then you need to use alias this.
> > 
> > - Jonathan M Davis
> 
> The conversion is explicit. if(x) is rewritten to if(cast(bool)x) and e
> && somethingElse is rewritten to cast(bool)e && cast(bool)somethingElse.

It's implicit as far as the programmer is concerned. You do

if(x)

without caring that x isn't a bool. That rewrite just explains why it works 
implicitly in that case but not in general.

- Jonathan M Davis