Re: Checking that a template parameter is an enum

[John Colvin via Digitalmars-d-learn]

On Thursday, 1 October 2015 at 22:26:39 UTC, 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?

Why? To avoid slowing down compilation with all those template 
instantiations?

How about a O(log2(N)) depth recursive version, something like 
this:

template allSame(V ...)
    if (isExpressions!V)
{
    static if (V.length <= 1)
        enum allSame = true;
    else static if(V.length & 1)
        enum allSame = V[$-1] == V[0]
            && V[0 .. $/2] == V[$/2 .. $-1]
            && allSame!(V[0 .. $/2]);
    else
        enum allSame = V[0..$/2] == V[$/2 .. $]
            && allSame!(V[0 .. $/2]);
}