Array initialization with Struct templates

[Jonathan M Davis via Digitalmars-d-learn]

On Monday, August 31, 2015 04:57:05 WhatMeWorry via Digitalmars-d-learn wrote:
>
> This seemingly trivial array initialization has caused me hours
> of grief.
>
> enum Purpose { POSITIONAL, COLOR_ONLY, COLOR_AND_ALPHA,
> GENERIC_TRIPLE, GENERIC_QUAD }
> Purpose purpose;
>
> struct Chameleon(T, Purpose p)  // template
> {
>      static if (is (p == POSITIONAL)) {
>          T  x, y, z;
>      } else static if (is (p == COLOR_ONLY)) {
>          T  r, g, b;
>      } else static if (is (p == COLOR_AND_ALPHA)) {
>          T  r, g, b, a;
>      }  else static if (is (p == GENERIC_TRIPLE)) {
>          T  a, b, c;
>      } else static if (is (p == GENERIC_QUAD)) {
>          T  a, b, c, d;
>      }
> };
>
> struct VertexData
> {
>      Chameleon!(float, purpose.POSITIONAL)  position;
>      Chameleon!(float, purpose.COLOR_ONLY)  color;
> }
>
> alias Vert = VertexData;
>
> VertexData[] vertices =
> [
>      Vert(1.0, 1.0, 1.0, 0.0, 0.0, 0.0)  // compiler error here
> ];
>
> I keep getting:
>
> Error: cannot implicitly convert expression (1.00000) of type
> double to Chameleon!(double, cast(Purpose)0)
>
> I even tried  Vert(1.0f, 1.0f, 1.0f, 0.0f, 0.0f, 0.0f)
>
> but it has the exact same error.  Any ideas?  Thanks in advance.

VertexData doesn't have a constructor that takes 6 doubles or 6 floats. It
has a compiler-generated constructor that's equivalent to


this(Chameleon!(float, purpose.POSITIONAL) position,
     Chameleon!(float, purpose.COLOR_ONLY) color)
{
    this.position = position;
    this.color = color;
}

So, you're going to need to pass it a Chameleon!(float, purpose.POSITIONAL)
and a Chameleon!(float, purpose.COLOR_ONLY color), not 6 doubles - either
that, or you're going to need to declare a constructor for VertexData which
takes 6 doubles or floats and converts them to what's require to assign to
its member variables.

- Jonathan M Davis