For example in std.container.rbtree:

---
    auto equalRange(this This)(Elem e)
    {
        auto beg = _firstGreaterEqual(e);
        alias RangeType = RBRange!(typeof(beg));
        if (beg is _end || _less(e, beg.value))
            // no values are equal
            return RangeType(beg, beg);
        static if (allowDuplicates)
        {
            return RangeType(beg, _firstGreater(e));
        }
        else
        {
            // no sense in doing a full search, no duplicates are allowed,
            // so we just get the next node.
            return RangeType(beg, beg.next);
        }
    }
---

On 12/02/2018 8:35 AM, Nathan S. wrote:
> For example in std.container.rbtree:
> 
> ---
>      auto equalRange(this This)(Elem e)
>      {
>          auto beg = _firstGreaterEqual(e);
>          alias RangeType = RBRange!(typeof(beg));
>          if (beg is _end || _less(e, beg.value))
>              // no values are equal
>              return RangeType(beg, beg);
>          static if (allowDuplicates)
>          {
>              return RangeType(beg, _firstGreater(e));
>          }
>          else
>          {
>              // no sense in doing a full search, no duplicates are allowed,
>              // so we just get the next node.
>              return RangeType(beg, beg.next);
>          }
>      }
> ---

https://dlang.org/spec/template.html#TemplateThisParameter

On Monday, 12 February 2018 at 08:35:05 UTC, Nathan S. wrote:
> For example in std.container.rbtree:
>
> ---
>     auto equalRange(this This)(Elem e)
>     {
>         auto beg = _firstGreaterEqual(e);
>         alias RangeType = RBRange!(typeof(beg));
>         if (beg is _end || _less(e, beg.value))
>             // no values are equal
>             return RangeType(beg, beg);
>         static if (allowDuplicates)
>         {
>             return RangeType(beg, _firstGreater(e));
>         }
>         else
>         {
>             // no sense in doing a full search, no duplicates are allowed,
>             // so we just get the next node.
>             return RangeType(beg, beg.next);
>         }
>     }
> ---

It's a template this parameter:
https://dlang.org/spec/template.html#template_this_parameter

Simply put, it gets the type of the 'this' parameter when the function is invoked, with all type modifiers. For a class A with a subclass B, 'This' would be B if called from B, and const(A) when called from const(A).

The type of the 'this' parameter is the statically known type, not the dynamic type. This creates some limitations, as shown below.

Some examples:

struct S {
   void foo(this This)() {
       writeln(This.stringof);
   }
}

class A {
    void foo(this This)() {
        writeln(This.stringof);
    }
    void bar() {
        foo();
    }
}

class B : A{}

unittest {
    S s;
    const shared S css;
    immutable S iss;
    s.foo(); // S
    css.foo(); // shared(const(S))
    iss.foo(); // immutable(S)
    B b = new B();
    A a = b;
    b.foo(); // B
    a.foo(); // A, since typeof(a) == A.
    b.bar(); // A, since bar() calls foo() in a context where the type of 'this' is A.
}

--
  Simen

On Monday, 12 February 2018 at 08:42:42 UTC, rikki cattermole wrote:
> https://dlang.org/spec/template.html#TemplateThisParameter

Cheers.