Hi there,

I recently read the 'More Templates' chapter of Ali's book (<-- thanks
for that ;) ).
At the section 'Named constraints', there were a definition like this:

template isUsable(T)
{
     enum isUsable = is ( typeof(
     {
         T obj;
         obj.call();
         obj.otherCall(1);
         obj.ye tAnotherCall();
     }() ) );
}

But at Phobos I always see definitions like this (std.range : isInputRange):

template isInputRange(R)
{
enum bool isInputRange = is(typeof(
(inout int = 0)
{
R r = R.init; // can define a range object
if (r.empty) {} // can test for empty
r.popFront(); // can invoke popFront()
auto h = r.front; // can get the front of the range
}));
} What does (inout int = 0) mean/affect here? I created the same
template for myself just without the (inout int = 0) and it worked (at
least with a dummy struct).. - Tim

On 04/22/2014 07:07 AM, Tim Holzschuh via Digitalmars-d-learn wrote:

> read the 'More Templates' chapter of Ali's book (<-- thanks
> for that ;) ).

Yay! :)

> At the section 'Named constraints', there were a definition like this:
>
> template isUsable(T)
> {
>      enum isUsable = is ( typeof(
>      {
>          T obj;
>          obj.call();
>          obj.otherCall(1);
>          obj.ye tAnotherCall();
>      }() ) );
> }

That is how Phobos used to be.

> But at Phobos I always see definitions like this (std.range :
> isInputRange):
>
> template isInputRange(R)
> {
> enum bool isInputRange = is(typeof(
> (inout int = 0)

I had noticed that as well and am curious myself. My guess is that it is a syntax issue where that default parameter list makes it a stronger delegate syntax. :p

> {
> R r = R.init; // can define a range object
> if (r.empty) {} // can test for empty
> r.popFront(); // can invoke popFront()
> auto h = r.front; // can get the front of the range
> }));
> } What does (inout int = 0) mean/affect here? I created the same
> template for myself just without the (inout int = 0) and it worked (at
> least with a dummy struct).. - Tim

Same here: I cannot find an example where the first version does not work. (?)

Ali