Checking that a template parameter is an enum

[Ali Çehreli via Digitalmars-d-learn]

On 10/01/2015 03:26 PM, Nordlöw wrote:
> On Thursday, 1 October 2015 at 02:06:48 UTC, Fusxfaranto wrote:
>>> /** Returns: true iff all values $(D V) are the same. */
>>> template allSame(V...)          // TODO restrict to values only
>>> {
>>>     static if (V.length <= 1)
>>>         enum bool allSame = true;
>>>     else
>>>         enum bool allSame = V[0] == V[1] && allSame!(V[1..$]);
>>> }
>>
>> std.traits to the rescue!
>>
>> http://dlang.org/phobos/std_traits.html#isExpressions
>>
>> Using isExpressions!V as a template constraint looks like the behavior
>> you're looking for.
>
> Thanks!
>
> BTW: Is there some way to turn the recursive definition of `allSame`
>
> template allSame(V...)
>      if (isExpressions!(V))
> {
>      static if (V.length <= 1)
>          enum allSame = true;
>      else
>          enum allSame = V[0] == V[1] && allSame!(V[1..$]);
> }
>
> into an iterative definition?

Very quickly:

import std.traits;

template allSame(V...)
     if (isExpressions!(V))
{
     bool impl_(V...)() {
         static if (V.length > 1) {
             foreach (i, _; V[0 .. $ - 1]) {
                 if (V[i] != V[i + 1]) {
                     return false;
                 }
             }

             return true;

         } else {
             return true;
         }
     }

     enum allSame = impl_!V();
}

unittest
{
     static assert( allSame!());
     static assert( allSame!(42));
     static assert( allSame!(42, 42, 42));
     static assert(!allSame!(42, 43, 42));
}

Ali