Re: Array initialization with Struct templates

[Mike Parker via Digitalmars-d-learn]

On Monday, 31 August 2015 at 05:47:31 UTC, Ali Çehreli wrote:
On 08/30/2015 10:38 PM, Jonathan M Davis via 
Digitalmars-d-learn wrote:
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 would drop chameleon all together and just add the fields 
directly to VertexData, but make Purpose flag-based.

enum Purpose {
   position = 0x00,   // Assuming all verts have a position
   colorOnly = 0x01,
   colorAlpha = 0x02,
   triple = 0x04,
   quad = 0x08
}

enum hasColorOnly(Purpose p) = (p & Purpose.colorOnly) == 
Purpose.colorOnly;
struct VertexData(T, Purpose purpose){
   T x, y, z;
   static if(hasColorOnly!purpose)
      T r, g, b;
   ...
}

Then you can templatize your batches or meshes, or whatever you 
use to represent vertex objects, on the VertexData type.

alias ColorVertexf = VertexData!(float, Purpose.colorOnly);
auto batch = new Batch!(ColorVertexf);