At
https://dlang.org/library/std/traits/is_boolean.html

it has:


enum isBoolean(T) = is(BooleanTypeOf!T) && !isAggregateType!T;

per:
https://dlang.org/library/std/traits/is_aggregate_type.html

isAggregateType is true for [struct, union, class, interface].

So BooleanTypeOf!T is true for structs, unions, classes and interfaces? And if yes, why is that so?

On Sunday, 18 February 2018 at 14:52:37 UTC, Tony wrote:
> At
> https://dlang.org/library/std/traits/is_boolean.html
>
> it has:
>
>
> enum isBoolean(T) = is(BooleanTypeOf!T) && !isAggregateType!T;
>
> per:
> https://dlang.org/library/std/traits/is_aggregate_type.html
>
> isAggregateType is true for [struct, union, class, interface].
>
> So BooleanTypeOf!T is true for structs, unions, classes and interfaces? And if yes, why is that so?

Generally, no. But with alias this, it can be:

=====
import std.traits : BooleanTypeOf;
import std.stdio : writeln;

struct NoBool {
    int x;
}

struct AliasThisBool {
    bool b;
    alias b this;
}

void main()
{
    static if(is(BooleanTypeOf!NoBool)) writeln("NoBool");
    static if(is(BooleanTypeOf!AliasThisBool)) writeln("AliasThisBool");
}
=====

On Sunday, 18 February 2018 at 15:12:50 UTC, Mike Parker wrote:

> Generally, no. But with alias this, it can be:
>
> =====
> import std.traits : BooleanTypeOf;
> import std.stdio : writeln;
>
> struct NoBool {
>     int x;
> }
>
> struct AliasThisBool {
>     bool b;
>     alias b this;
> }
>
> void main()
> {
>     static if(is(BooleanTypeOf!NoBool)) writeln("NoBool");
>     static if(is(BooleanTypeOf!AliasThisBool)) writeln("AliasThisBool");
> }

Thanks!

It doesn't appear that BooleanTypeof is documented on dlang.org (outside of it's placement on the isBooleanType page). At least it isn't coming up in a "BooleanTypeOf site:dlang.org" search and not on the traits page:

https://dlang.org/library/std/traits.html

On Monday, 19 February 2018 at 13:07:08 UTC, Tony wrote:
> It doesn't appear that BooleanTypeof is documented on dlang.org (outside of it's placement on the isBooleanType page). At least it isn't coming up in a "BooleanTypeOf site:dlang.org" search and not on the traits page:
>
> https://dlang.org/library/std/traits.html

Indeed but Phobos maintainers don't want the ...TypeOf family to be documented.
(https://github.com/dlang/phobos/pull/5747)

On Monday, 19 February 2018 at 13:47:15 UTC, Basile B. wrote:

>
> Indeed but Phobos maintainers don't want the ...TypeOf family to be documented.
> (https://github.com/dlang/phobos/pull/5747)

Ok, thanks.

But, assuming there is a use case for it, what if you want to restrict to a type that is either boolean, or a struct/class that can substitute for boolean - how do you do that without using the "private" TypeOfBoolean thing?

On Monday, 19 February 2018 at 15:12:15 UTC, Tony wrote:
> But, assuming there is a use case for it, what if you want to restrict to a type that is either boolean, or a struct/class that can substitute for boolean - how do you do that without using the "private" TypeOfBoolean thing?


In that case you can just write `is(T : bool)`.

On Monday, 19 February 2018 at 17:22:04 UTC, Nathan S. wrote:
> On Monday, 19 February 2018 at 15:12:15 UTC, Tony wrote:
>> But, assuming there is a use case for it, what if you want to restrict to a type that is either boolean, or a struct/class that can substitute for boolean - how do you do that without using the "private" BooleanTypeOf thing?
>
>
> In that case you can just write `is(T : bool)`.

Thanks.

Assuming it would substitute, that should probably be used on this page in place of BooleanTypeOf since BooleanTypeOf is not supposed to be public:

https://dlang.org/library/std/traits/is_boolean.html

"enum isBoolean(T) = is(BooleanTypeOf!T) && !isAggregateType!T;"