"Value class instance" pattern?

[bearophile]

Benjamin Thaut:

> I also wanted to mention the "ListAvailableCtors" template 
> which is a nice addition in case there is no constructor 
> available to be called with the given arguments. It will 
> generate a list of all aviable ctors with the types of its 
> arguments, and thus greatly improve the error message given 
> when no appropriate constructor can be found:
>
> string ListAvailableCtors(T)()
> {
>   string result = "";
>   foreach(t; __traits(getOverloads, T, "__ctor"))
>     result ~= typeof(t).stringof ~ "\n";
>   return result;
> }
>
> In my original code it was used during construction like this:
>
> static if(is(typeof(result.__ctor(args))))
> {
>   result.__ctor(args);
> }
> else
> {
>   static assert(args.length == 0 && !is(typeof(T.__ctor)), 
> "Don't know
>   how to initialize an object of type " ~ T.stringof ~ " with
>   arguments:\n" ~ ARGS.stringof ~ "\nAvailable ctors:\n" ~
>   ListAvailableCtors!T() );
> }

In my version of your code I have just added a template 
constraint, this is simpler, and it generates an error at the 
calling point:

     this(Targs...)(Targs args)
     if (__traits(compiles, _instance.__ctor(args))) {
         classInstanceBuf[] = typeid(T).init[];
         _instance.__ctor(args);
     }

Isn't this enough?

Bye,
bearophile