Using D2, I have a template class that looks something like this:

class C(T) { T t; }

(For simplicity, assume T is required to be a struct of some kind.) I want to have a constructor that passes on some of it's args to a constructor for t. This is easy as long as I don't need it to work for arbitrary constructors.

What I want to do (that doesn't seem to work) is this:

this(Args...)(int foo, float bar, Args args)
{
  t = T(args);

  ...
}

Any ideas on fixes or work arounds?

-- 
... <IXOYE><

On 3/11/10 08:58, BCS wrote:
> Using D2, I have a template class that looks something like this:
>
> class C(T) { T t; }
>
> (For simplicity, assume T is required to be a struct of some kind.) I
> want to have a constructor that passes on some of it's args to a
> constructor for t. This is easy as long as I don't need it to work for
> arbitrary constructors.
>
> What I want to do (that doesn't seem to work) is this:
>
> this(Args...)(int foo, float bar, Args args)
> {
> t = T(args);
>
> ...
> }
>
> Any ideas on fixes or work arounds?
>

D doesn't seem support constructor templates in classes.

BCS wrote:

> Using D2, I have a template class that looks something like this:
> 
> class C(T) { T t; }
> 
> (For simplicity, assume T is required to be a struct of some kind.) I want to have a constructor that passes on some of it's args to a constructor for t. This is easy as long as I don't need it to work for arbitrary constructors.
> 
> What I want to do (that doesn't seem to work) is this:
> 
> this(Args...)(int foo, float bar, Args args)
> {
>    t = T(args);
> 
>    ...
> }
> 
> Any ideas on fixes or work arounds?
> 

Workaround if T has a single constructor, perhaps it can be generalized with some work:

this(int foo, float bar, std.traits.ParameterTypeTuple!(T.__ctor) args)
{
    t = T(args);
}

On Thu, 11 Mar 2010 02:58:52 -0500, BCS <none@anon.com> wrote:

> Using D2, I have a template class that looks something like this:
>
> class C(T) { T t; }
>
> (For simplicity, assume T is required to be a struct of some kind.) I want to have a constructor that passes on some of it's args to a constructor for t. This is easy as long as I don't need it to work for arbitrary constructors.
>
> What I want to do (that doesn't seem to work) is this:
>
> this(Args...)(int foo, float bar, Args args)
> {
>    t = T(args);
>
>    ...
> }
>
> Any ideas on fixes or work arounds?

What about a static function instead of a constructor?

i.e.

static C create(Args...)(int foo, float bar, Args args)
{
   auto c = new C(foo, bar);
   c.t = T(args);
   return c;
}

It's a shame template constructors aren't allowed, they aren't even virtual functions!

-Steve

Hello Lutger,

> Workaround if T has a single constructor, perhaps it can be
> generalized with some work:
> 
> this(int foo, float bar, std.traits.ParameterTypeTuple!(T.__ctor)
> args)
> {
> t = T(args);
> }

Not exactly ideal, but... :)


-- 
... <IXOYE><

Hello Steven,

> What about a static function instead of a constructor?
> 
> i.e.
> 
> static C create(Args...)(int foo, float bar, Args args)
> {
> auto c = new C(foo, bar);
> c.t = T(args);
> return c;
> }

That's my fallback position.

> It's a shame template constructors aren't allowed, they aren't even
> virtual functions!

Ditto.

-- 
... <IXOYE><

On Thu, Mar 11, 2010 at 21:03, BCS <none@anon.com> wrote:

>
>
>> i.e.
>>
>> static C create(Args...)(int foo, float bar, Args args)
>> {
>> auto c = new C(foo, bar);
>> c.t = T(args);
>> return c;
>> }
>>
>
> What about a static function instead of a constructor?
>

Two variations on the same theme:

storing (Args...) as a template parameter in the host class

class Host(Hosted, Args...) {
    Hosted t;
    this(int foo, float bar, Args args)
    {
     t = Hosted(args);
    }
}

and using a helper function to do the type extraction:

Host!(Hosted, Args) host(Hosted, Args...)(int foo, float bar, Args args)
{
    return new Host!(Hosted, Args)(foo, bar, args);
}

Or, give the host a constructor taking a Hosted already created:


class Host2(Hosted) {
    Hosted h;
    this(int foo, float bar, Hosted h)
    {
        this.h = h;
    }
}

Host2!(Hosted) host2(Hosted, Args...)(int foo, float bar, Args args)
{
    auto h = Hosted(args);
    return new Host2!(Hosted)(foo, bar, h);
}

Usage:

struct OneArg { char c;}

struct ManyArgs { int i; float f;}

void main()
{
    auto c0 = host!ManyArgs(1,2.0,3,4.0);
    auto c1 = host!OneArg(1,2.0,'c');
    auto c2 = host2!ManyArgs(1,2.0,3,4.0);
 }