Hello,

I have a structure with two fields ad defined as

struct Info {
    this(int value, Category category)
    {
        category_ = category;
        value_ = category ? value : 0;
    }
    ...
private:
    Category category_ = null;
    int value_ = 0;
}


I would like an implicit conversion of Info to bool that return false if category_ is null so that I can write

Info s = foo();
if (s) {
   ...
}

and where

Info s2;
assert(!s2);

On Monday, 6 June 2016 at 15:23:50 UTC, chmike wrote:
> Hello,
>
> I have a structure with two fields ad defined as
>
> struct Info {
>     this(int value, Category category)
>     {
>         category_ = category;
>         value_ = category ? value : 0;
>     }

      // This converts implicitly to bool.
      T opCast(T : bool)() {
          return category_ !is null;
      }

>     ...
> private:
>     Category category_ = null;
>     int value_ = 0;
> }
>
>
> I would like an implicit conversion of Info to bool that return false if category_ is null so that I can write
>
> Info s = foo();
> if (s) {
>    ...
> }
>
> and where
>
> Info s2;
> assert(!s2);

See the inserted code above.

On Monday, 6 June 2016 at 15:23:50 UTC, chmike wrote:
> I would like an implicit conversion of Info to bool that return false if category_ is null so that I can write

add:

bool opCast(T : bool)() {
   return whatever;
}

to the struct and it should work.

On Monday, 6 June 2016 at 15:28:35 UTC, John wrote:
Thank you John and Adam. That was a quick answer !

On Monday, 6 June 2016 at 15:34:18 UTC, chmike wrote:
> On Monday, 6 June 2016 at 15:28:35 UTC, John wrote:
> Thank you John and Adam. That was a quick answer !

Too late but another option would have been to put an alias this on a bool getter:

struct Info
{
    bool getStuff()
    {
        return true;
    }

    alias getStuff this;
}

On 06/06/2016 08:28 AM, Adam D. Ruppe wrote:
> On Monday, 6 June 2016 at 15:23:50 UTC, chmike wrote:
>> I would like an implicit conversion of Info to bool that return false
>> if category_ is null so that I can write
>
> add:
>
> bool opCast(T : bool)() {
>    return whatever;
> }
>
> to the struct and it should work.

It's news to me that while opCast for all other types is for explicit casting, opCast for bool works for implicit casting.

Ali

On 6/7/16 6:15 PM, Ali Çehreli wrote:
> On 06/06/2016 08:28 AM, Adam D. Ruppe wrote:
>> On Monday, 6 June 2016 at 15:23:50 UTC, chmike wrote:
>>> I would like an implicit conversion of Info to bool that return false
>>> if category_ is null so that I can write
>>
>> add:
>>
>> bool opCast(T : bool)() {
>>    return whatever;
>> }
>>
>> to the struct and it should work.
>
> It's news to me that while opCast for all other types is for explicit
> casting, opCast for bool works for implicit casting.

as ag0... mentioned in another thread, opCast is NOT implicitly being invoked here, but rather explicitly.

That is:

bool x = someStruct; // error
if(someStruct) // explicit opCast!bool

-Steve

On Tuesday, 7 June 2016 at 22:28:57 UTC, Steven Schveighoffer wrote:
>>
>> It's news to me that while opCast for all other types is for explicit
>> casting, opCast for bool works for implicit casting.
>
> as ag0... mentioned in another thread, opCast is NOT implicitly being invoked here, but rather explicitly.
>
> That is:
>
> bool x = someStruct; // error
> if(someStruct) // explicit opCast!bool
>

Documented under 'Boolean Operations' at http://dlang.org/spec/operatoroverloading.html#cast