Array initialization with Struct templates
Mike Parker via Digitalmars-d-learn
digitalmars-d-learn at puremagic.com
Sun Aug 30 23:55:19 PDT 2015
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);
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