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digitalmars.D.learn - No constructor for a templated class?

reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
OK, I must be tired, I don't know.
While switching from a template struct to a templated class, I got the
following problem:

class A(T)
{
    T _t;
    this(U)(U u) if (is(U == T))
    {
        _t = u;
    }
}

void main()
{
    auto aa = new A!(double)(1.0); // line 12
}

Error (12): no constructor for A.

What am I doing wrong?


Philippe
Jul 25 2010
next sibling parent reply div0 <div0 sourceforge.net> writes:
On 25/07/2010 13:55, Philippe Sigaud wrote:

 OK, I must be tired, I don't know.



Nope you're not tired.
Templated constructor functions are not currently supported in D. :(
Jul 25 2010
next sibling parent Jacob Carlborg <doob me.com> writes:
On 2010-07-25 23:30, Philippe Sigaud wrote:

 On Sun, Jul 25, 2010 at 16:08, div0 <div0 sourceforge.net
 <mailto:div0 sourceforge.net>> wrote:

     On 25/07/2010 13:55, Philippe Sigaud wrote:

         OK, I must be tired, I don't know.


     Nope you're not tired.
     Templated constructor functions are not currently supported in D. :(


 Ouch.

 I did that many many times, but for structs. Case in point, I
 encountered this while deciding to switch from struct to classes,
 thinking inheritance would suppress lot of code duplication.  Aw, man,
 first time in months I use classes in D, and it's killed in the womb.
 OK, there are solutions for for what I envision, but that will be more
 painful.

 Thanks, div0



Perhaps a factory function like opCall or something similar ?

class Foo
{
     int x;

     static Foo opCall (T) (T y)
     {
         Foo foo = new Foo;
         foo.x = y;
         return foo;
     }
}

auto foo = Foo(3);

-- 
/Jacob Carlborg
Jul 26 2010
prev sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
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On Mon, Jul 26, 2010 at 13:15, Jacob Carlborg <doob me.com> wrote:


 Perhaps a factory function like opCall or something similar ?

 class Foo
 {
    int x;

    static Foo opCall (T) (T y)
    {
        Foo foo = new Foo;
        foo.x = y;
        return foo;
    }
 }

 auto foo = Foo(3);




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<div class=3D"gmail_quote">On Mon, Jul 26, 2010 at 13:15, Jacob Carlborg <s=
pan dir=3D"ltr">&lt;<a href=3D"mailto:doob me.com">doob me.com</a>&gt;</spa=
n> wrote:<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;">
Perhaps a factory function like opCall or something similar ?<br>
<br>
class Foo<br>
{<br>
 =A0 =A0int x;<br>
<br>
 =A0 =A0static Foo opCall (T) (T y)<br>
 =A0 =A0{<br>
 =A0 =A0 =A0 =A0Foo foo =3D new Foo;<br>
 =A0 =A0 =A0 =A0foo.x =3D y;<br>
 =A0 =A0 =A0 =A0return foo;<br>
 =A0 =A0}<br>
}<br>
<br>
auto foo =3D Foo(3);<br>
<br></blockquote><div><br>Ah yes, I forgot about static opCall. Thanks Jaco=
b, I&#39;ll try this. <br><br>=A0<br></div></div><br>

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Jul 26 2010
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
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On Sun, Jul 25, 2010 at 16:08, div0 <div0 sourceforge.net> wrote:


 On 25/07/2010 13:55, Philippe Sigaud wrote:


 OK, I must be tired, I don't know.



 Nope you're not tired.
 Templated constructor functions are not currently supported in D. :(



Ouch.

I did that many many times, but for structs. Case in point, I encountered
this while deciding to switch from struct to classes, thinking inheritance
would suppress lot of code duplication.  Aw, man, first time in months I use
classes in D, and it's killed in the womb.
OK, there are solutions for for what I envision, but that will be more
painful.

Thanks, div0

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<br><br><div class=3D"gmail_quote">On Sun, Jul 25, 2010 at 16:08, div0 <spa=
n dir=3D"ltr">&lt;<a href=3D"mailto:div0 sourceforge.net">div0 sourceforge.=
net</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"mar=
gin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-=
left: 1ex;">
<div class=3D"im">On 25/07/2010 13:55, Philippe Sigaud wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; borde=
r-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
OK, I must be tired, I don&#39;t know.<br>
</blockquote>
<br></div>
Nope you&#39;re not tired.<br>
Templated constructor functions are not currently supported in D. :(<br>
</blockquote></div><br>Ouch.<br><br>I did that many many times, but for str=
ucts. Case in point, I encountered this while deciding to switch from struc=
t to classes, thinking inheritance would suppress lot of code duplication.=
=A0 Aw, man, first time in months I use classes in D, and it&#39;s killed i=
n the womb.<br>
OK, there are solutions for for what I envision, but that will be more pain=
ful. <br><br>Thanks, div0<br>

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Jul 25 2010
prev sibling next sibling parent Mafi <mafi example.org> writes:
Am 25.07.2010 14:55, schrieb Philippe Sigaud:

 OK, I must be tired, I don't know.
 While switching from a template struct to a templated class, I got the
 following problem:

 class A(T)
 {
      T _t;
      this(U)(U u) if (is(U == T))
      {
          _t = u;
      }
 }



Probably this case is reduced, but anyways:
Having a template(U) where U must be == T (another template parameter) 
is complettly useless. Maybe I'm missing something but isn't this 
equivalent to the above:
class A(T)
  {
       T _t;
       this(T u)
       {
           _t = u;
       }
  }
Jul 25 2010
prev sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
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On Mon, Jul 26, 2010 at 00:00, Mafi <mafi example.org> wrote:


 Am 25.07.2010 14:55, schrieb Philippe Sigaud:

  OK, I must be tired, I don't know.

 While switching from a template struct to a templated class, I got the
 following problem:

 class A(T)
 {
     T _t;
     this(U)(U u) if (is(U == T))
     {
         _t = u;
     }
 }



 Probably this case is reduced, but anyways:
 Having a template(U) where U must be == T (another template parameter) is
 complettly useless. Maybe I'm missing something but isn't this equivalent to
 the above:

 class A(T)
  {
      T _t;
      this(T u)
      {
          _t = u;
      }
  }



most complicated template constraint I've ever build. It's five lines long
and invoke somthing like four other templates.

It worked for struct A. That's switching to class A that killed everything
:-(

I've a templated Graph struct, templated on Node and Edge (which may very
well be templated types themselves, as the user wish). A very handy
constructor, invoked by a factory function,  is one taking any number of
nodes and edges, checking their compatibility, deducing common properties at
compile-time and then constructing the graph.

Philippe

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<div class=3D"gmail_quote">On Mon, Jul 26, 2010 at 00:00, Mafi <span dir=3D=
"ltr">&lt;<a href=3D"mailto:mafi example.org">mafi example.org</a>&gt;</spa=
n> wrote:<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;">
Am 25.07.2010 14:55, schrieb Philippe Sigaud:<div class=3D"im"><br>
<blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; borde=
r-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
OK, I must be tired, I don&#39;t know.<br>
While switching from a template struct to a templated class, I got the<br>
following problem:<br>
<br>
class A(T)<br>
{<br>
 =A0 =A0 T _t;<br>
 =A0 =A0 this(U)(U u) if (is(U =3D=3D T))<br>
 =A0 =A0 {<br>
 =A0 =A0 =A0 =A0 _t =3D u;<br>
 =A0 =A0 }<br>
}<br>
</blockquote>
<br></div>
Probably this case is reduced, but anyways:<br>
Having a template(U) where U must be =3D=3D T (another template parameter) =
is complettly useless. Maybe I&#39;m missing something but isn&#39;t this e=
quivalent to the above:<div class=3D"im"><br>
class A(T)<br>
=A0{<br>
 =A0 =A0 =A0T _t;<br></div>
 =A0 =A0 =A0this(T u)<br>
 =A0 =A0 =A0{<br>
 =A0 =A0 =A0 =A0 =A0_t =3D u;<br>
 =A0 =A0 =A0}<br>
=A0}<br>
<br>
</blockquote></div><br>Yes, to original case is much more complicated. In f=
act, it may well be the most complicated template constraint I&#39;ve ever =
build. It&#39;s five lines long and invoke somthing like four other templat=
es.<br>
<br>It worked for struct A. That&#39;s switching to class A that killed eve=
rything :-(<br><br>I&#39;ve a templated Graph struct, templated on Node and=
 Edge (which may very well be templated types themselves, as the user wish)=
. A very handy constructor, invoked by a factory function,=A0 is one taking=
 any number of nodes and edges, checking their compatibility, deducing comm=
on properties at compile-time and then constructing the graph.<br>
<br>Philippe<br><br>

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Jul 25 2010