How to call function with variable arguments at runtime?

[Marc Schütz]

On Tuesday, 10 October 2017 at 02:58:45 UTC, Mr. Jonse wrote:
> I need to store a hetrogeneous array of delegates. How can I do 
> this but still call the function with the appropriate number of 
> parameters at run time?
>
> I have the parameters as Variant[] params and a 
> function/delegate pointer(void* for now).
>
> Normally I'd push the parameters on the stack and use a call, 
> but I'm sure D has some ability to do this, like apply(foo, 
> args) would be the same as foo(args[0], ..., args[1]).
>
> I'm not concerned about type correctness, it should always be 
> consistent between what I call and what is stored.
>
> Thanks.

Like so?

import std.variant;

void foo(int a, string b, float c) {
     import std.stdio;
     writefln("a = %s, b = %s, c = %s", a, b, c);
}

auto apply(alias fn)(Variant[] values) {
     import std.traits : ParameterTypeTuple;
     import std.conv : emplace;
     alias Types = ParameterTypeTuple!fn;
     assert(values.length == Types.length);
     Types args = void;
     foreach(i, ref arg; args) {
         // using emplace instead of assignment here to be fully 
correct
         emplace!(typeof(arg))(&arg, values[i].get!(typeof(arg)));
     }
     return fn(args);
}

void main() {
     Variant[] values = [Variant(1), Variant("Hello world"), 
Variant(3.14159f)];
     apply!foo(values);
}