Hey, template gurus :)

given this:

struct M(alias I : S!E, alias S, E...)
{
	R!E r;
	this(S)(S initStruct)    // line 4
	{
		r = R!E(initStruct);
	}
}
struct R(E...)
{
	this(S)(S initStruct)    // line 12
	{
		// do some cool stuff
	}
}
void main()
{
	FS!(Etype1) fs;
	auto m = M!(typeof(fs))(fs);    // line 21.
	auto a = M!(fs); // does not work
	auto b = M(fs); // does not work	
}
struct FS(T...){}
struct Etype1{}

Everything works as expected, especially line 21. The question is about syntactic sugar: What I have to change, to use auto deduction and to create the M struct like in line 22 or 23?

By the way, I'm aware, that the type matching in lines 4 and 12 is lost, in the way it is written here. However, it is meant to exist, if this helps in some way...

Thanks a lot in advance
Alex

On 10/6/17 5:08 PM, Alex wrote:
> Hey, template gurus :)
> 
> given this:
> 
> struct M(alias I : S!E, alias S, E...)
> {
>      R!E r;
>      this(S)(S initStruct)    // line 4
>      {
>          r = R!E(initStruct);
>      }
> }
> struct R(E...)
> {
>      this(S)(S initStruct)    // line 12
>      {
>          // do some cool stuff
>      }
> }
> void main()
> {
>      FS!(Etype1) fs;
>      auto m = M!(typeof(fs))(fs);    // line 21.
>      auto a = M!(fs); // does not work
>      auto b = M(fs); // does not work
> }
> struct FS(T...){}
> struct Etype1{}
> 
> Everything works as expected, especially line 21. The question is about syntactic sugar: What I have to change, to use auto deduction and to create the M struct like in line 22 or 23?
> 
> By the way, I'm aware, that the type matching in lines 4 and 12 is lost, in the way it is written here. However, it is meant to exist, if this helps in some way...
> 
> Thanks a lot in advance
> Alex

What you need is IFTI or "Implicit Function Template Instantiation"

Note the "Function" part of it, in that it's only valid for functions.

So you need a factory function:

auto m(T)(T x)
{
  return M!(T)(x);
}

...

auto b = m(fs); // ok

There is an enhancement request to make constructors have the same mechanism. Not sure if or when it would be implemented.

-Steve

On Friday, 6 October 2017 at 21:42:40 UTC, Steven Schveighoffer wrote:
>
> What you need is IFTI or "Implicit Function Template Instantiation"
>
> Note the "Function" part of it, in that it's only valid for functions.
>
> So you need a factory function:
>
> auto m(T)(T x)
> {
>   return M!(T)(x);
> }
>
> ...
>
> auto b = m(fs); // ok
>
> There is an enhancement request to make constructors have the same mechanism. Not sure if or when it would be implemented.
>
> -Steve

Ha! Yes :)
Thanks a lot :)
That's what is meant by convenience methods inside the library, sometimes...