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January 03, 2012
opCast!bool
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
January 03, 2012
Re: opCast!bool
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
January 03, 2012
Re: opCast!bool
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.
January 03, 2012
Re: opCast!bool
On 01/03/2012 05:41 PM, 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'

There is no 'bool result expected': The relation of the two operands in 
== is symmetric. You could just as well say that the result of 'true' is 
expected to be of type typeof(e).


>
> is(typeof(e) : bool); // false

This tests whether or not typeof(e) implicitly converts to bool, which 
can be false even if an explicit cast would succeed.
January 03, 2012
Re: opCast!bool
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
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