Expression template

[rikki cattermole via Digitalmars-d-learn]

On 23/07/2016 11:05 PM, Etranger wrote:
[snip]

> ***************** start of code ******************
> import std.stdio;
> import std.traits;
> import std.conv;
>
> struct VecExpression(alias mixins) {
>   mixin (mixins);
>   VecSum!(typeof(this), VecExpression!(RHS)) opBinary(string op, alias
> RHS)(ref VecExpression!(RHS) rhs)
> {
>   static if (op == "+") return VecSum!(typeof(this),
> VecExpression!(RHS))(&this, &rhs);
> }
> }
>
>
> mixin template Vec_impl(int n) {
>   double[n] elems;
>
>   @disable this();
>
>   this(double[n] e){
>     elems=e;
>   }
>
>   double opIndex(int i) {
>     return elems[i];
>   }
> }
>
> alias Vec(alias n) = VecExpression!("mixin
> Vec_impl!("~to!string(n)~");"); //Vec(int n) does not work
>
> mixin template VecSum_impl(E1, E2) {
>   E1 * e1;
>   E2 * e2;
>
>   @disable this();
>
>   this(E1 * ee1, E2 * ee2){
>     e1=ee1;
>     e2=ee2;
>   }
>
>   double opIndex(int i) {
>     return (*e1)[i]+(*e2)[i];
>   }
> }
>
> alias VecSum(E1, E2) = VecExpression!("mixin
> VecSum_impl!("~fullyQualifiedName!E1~","~fullyQualifiedName!E2~");");
>
> void main()
> {
>   Vec!(3) v1 = Vec!(3)([5., 2., 3.]), v2 = Vec!(3)([1., 4., 3.]), v3 =
> Vec!(3)([3., 2., 1.]);
>   auto res = v1+v2;
>   for(int i=0; i < 3; ++i){
>     writefln("%f + %f = %f", v1[i], v2[i],  res[i]);
>   }
>   auto res2 = res+v3;
>   for(int i=0; i < 3; ++i){
>     writefln("%f + %f = %f", res[i], v3[i],  res2[i]);
>   }
>   writeln(res);
>   writeln(res2);
>
>   // VecExpression!(Vec_impl!(3)) ve; // Error: template instance
> Vec_impl!3 mixin templates are not regular templates
> }
>
> ***************** end of code ******************
>
> My questions:
>
> 1- Is there a cleaner way to do it ? I had to use struct because I want
> every thing to happen at compile time and on the stack (without gc). And
> I had to use string mixins because template mixin does not work the way
> I tried to use it ( see the error last line).
>
> 2- Is there a safer way to do it (without using pointers) ?
>
> 3- Do you think I'll hit a wall with this approach ?
>
> 4- Do you known any D libs that uses expression template for linear
> algebra ?
>
> I thank you in advance for your help and wish you a nice weekend (and
> apologize for my bad english) :)

My goodness that code is awful.

I have a fair idea what you are attempting to do here.
So I'm going to point you directly to gl3n. Its meant for game dev so 
won't provide you a 100% solution. But it should give you ideas of how 
to do it.

https://github.com/Dav1dde/gl3n/blob/master/gl3n/linalg.d#L49

If you have any questions and want somebody to talk with you instead of 
write, reply cycle. Please hop on to Freenode #d channel.