Re: Template return values?

[Dmitry Olshansky]

12/4/2012 4:08 PM, Jacob Carlborg пишет:
On 2012-12-04 08:48, Era Scarecrow wrote:
  A thought's been going through my head for a while now. I wonder about
template return values. Function signatures are usually the inputs and
NOT the return type. However for writing a variant type could be so very
useful. So unless I've misunderstood, this doesn't work (but it would be
cool if it did).
struct Variant {
   enum ValueType {Long, Float, Double}

   ValueType vt;
   union {
     long lng;
     float fl;
     double dbl;
   }

   T getValue(T)() @property
   if (T == long || T == float || T == double) { //allowable types
     static if (T == long) { //only long case written for brevity
       switch(vt) {
         //entries would likely be mixed in and not written manually...
         case Long: return lng;
         case Float:
           assert(lng >= float.min && lng <= float.max);
           return cast(float) lng;
         case Float:
           assert(lng >= double.min && lng <= double.max);
           return cast(double) lng;
       }
     }

     assert(0, "Fell through, should never get here");
   }
}


  So then in theory

  Variant v;
  int i = v.getValue; //calls getValue!int()

You can use "alias this", but that only works for one type.

http://dlang.org/class.html#AliasThis


Well TDPL claims multiple alias this is allowed so in some distant 
future it maybe possible for Varaint to alias this to all built-in types.

--
Dmitry Olshansky