• BCS <none anon.com> Mar 10 2010
• Jacob Carlborg <doob me.com> Mar 11 2010
• Lutger <lutger.blijdestijn gmail.com> Mar 11 2010
• BCS <none anon.com> Mar 11 2010
• "Steven Schveighoffer" <schveiguy yahoo.com> Mar 11 2010
• BCS <none anon.com> Mar 11 2010
• Philippe Sigaud <philippe.sigaud gmail.com> Mar 11 2010

BCS <none anon.com> writes:

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><

Jacob Carlborg <doob me.com> writes:

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.

Lutger <lutger.blijdestijn gmail.com> writes:

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);
}

BCS <none anon.com> writes:

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><

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

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

BCS <none anon.com> writes:

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><

Philippe Sigaud <philippe.sigaud gmail.com> writes:

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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);
 }

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<div class=3D"gmail_quote">On Thu, Mar 11, 2010 at 21:03, BCS <span dir=3D"=
ltr">&lt;<a href=3D"mailto:none anon.com">none anon.com</a>&gt;</span> wrot=
e:<br><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex;=
 border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<br><div class=3D"im"><blockquote class=3D"gmail_quote" style=3D"margin: 0p=
t 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1=
ex;">
<br>
i.e.<br>
<br>
static C create(Args...)(int foo, float bar, Args args)<br>
{<br>
auto c =3D new C(foo, bar);<br>
c.t =3D T(args);<br>
return c;<br>
}<br>
</blockquote>
<br></div>What about a static function instead of a constructor?<br></block=
quote><div><br>Two variations on the same theme:<br><br>storing (Args...) a=
s a template parameter in the host class<br><br>class Host(Hosted, Args...)=
 {<br>
=A0=A0=A0 Hosted t;<br>=A0=A0=A0 this(int foo, float bar, Args args)<br>=A0=
=A0=A0 {<br>=A0=A0=A0=A0 t =3D Hosted(args);<br>=A0=A0=A0 }<br>}<br><br>and=
 using a helper function to do the type extraction:<br><br>Host!(Hosted, Ar=
gs) host(Hosted, Args...)(int foo, float bar, Args args)<br>
{<br>=A0=A0=A0 return new Host!(Hosted, Args)(foo, bar, args);<br>}<br><br>=
Or, give the host a constructor taking a Hosted already created:<br><br><br=
class Host2(Hosted) {<br>=A0=A0=A0 Hosted h;<br>=A0=A0=A0 this(int foo, fl=


=A0=A0=A0 {<br>=A0=A0=A0=A0=A0=A0=A0 this.h =3D h;<br>=A0=A0=A0 }<br>}<br><=
br>Host2!(Hosted) host2(Hosted, Args...)(int foo, float bar, Args args)<br>=
{<br>=A0=A0=A0 auto h =3D Hosted(args);<br>=A0=A0=A0 return new Host2!(Host=
ed)(foo, bar, h);<br>}<br><br>Usage:<br>
<br>struct OneArg { char c;}<br><br>struct ManyArgs {=A0int i; float f;}<br=
<br>void main()<br>{<br>=A0=A0=A0 auto c0 =3D host!ManyArgs(1,2.0,3,4.0);<=


 =3D host2!ManyArgs(1,2.0,3,4.0);<br>
=A0}</div></div>

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