Named template constraints

[Ali Çehreli via Digitalmars-d-learn]

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