http://d.puremagic.com/issues/show_bug.cgi?id=8762

           Summary: instanceOf trait for static conditionals
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: monarchdodra@gmail.com


--- Comment #0 from monarchdodra@gmail.com 2012-10-05 02:23:10 PDT ---
As discussed here: http://forum.dlang.org/thread/ssqgyxzggmzugbfbboay@forum.dlang.org

The idea would be a trait "instanceOf!T" that returns a reference to a T. The advantage of this approach is that it allows "extracting" an instance out of T, without ever worrying about how or where said instance came from.

This is particularly important because some types don't have T.init, and immutables don't have T t = void. The intersection of both these groups is {0} ...

Example:
//----
template isAssignable(T, U)
{
    enum bool isAssignable =
        is(typeof(instanceOf!T = instanceOf!U));
}
//----

The "signature" (as improved on by Simen Kjaeraas) would be:
//----
@property ref T instanceOf( T )( );
//----
But remain un-implemented.

This would ensure it is not actually usable during run-time.

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http://d.puremagic.com/issues/show_bug.cgi?id=8762



--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2012-10-05 22:06:12 PDT ---
In std.traits module, similar utility already exists (but today, it is private
template).

And recently I've posted a pull request which updates it. https://github.com/D-Programming-Language/phobos/pull/842 https://github.com/D-Programming-Language/phobos/pull/842/files#L1R140

The name `defaultInit` comes from `default initializer` for T (, and the name is used in the dmd implementation).

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