rvalues -> ref (yup... again!)

[Rubn]

On Saturday, 24 March 2018 at 23:03:36 UTC, John Colvin wrote:
> Here is a small proof of concept I made to demonstrate how easy 
> it seems to be to use `auto ref` to call a C++ virtual const& 
> function without incurring any more copies than would happen 
> with the same calls from C++. I'm sure it could be improved a 
> lot, but does the basic concept match what you would need?
>
> // D source file:
>
> /// mix this in to your extern(C++) class with a list of the 
> functions where you
> /// want to be able to pass rvalues to ref parameters.
> auto rValueRefCalls(Funcs ...)()
> {
>     string ret;
>     foreach (foo; Funcs)
>         ret ~= `extern(D) void ` ~ __traits(identifier, foo) ~ 
> `(Args ...)(auto ref Args args)
>     if (__traits(compiles, (&this.` ~ __traits(identifier, foo) 
> ~ `)(args)))
>     {
>         (&this.foo)(args);
>     }`;
>
>     return ret;
> }
>
> extern(C++)
> {
>     class A
>     {
>         void foo(const ref int v);
>         mixin(rValueRefCalls!foo);
>     }
>
>     A makeA();
> }
>
> void main()
> {
>     int x = 3;
>     auto a = makeA();
>     a.foo(x);
>     a.foo(3);
> }
>
>
> // C++ source file:
>
> #include<cstdio>
>
> class A
> {
> public:
>     virtual void foo(const int& v)
>     {
>         printf("%d\n", v);
>     }
> };
>
> A *makeA()
> {
>     return new A;
> }

That isn't going to scale to the number of potential functions 
that are going to need this. It's going to cause slow compile 
speeds and create a bloated binary.