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digitalmars.D.learn - How to chain constructor args to a template memeber

reply 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><
Mar 10 2010
next sibling parent 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.
Mar 11 2010
prev sibling next sibling parent reply 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);
}
Mar 11 2010
parent 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><
Mar 11 2010
prev sibling parent reply "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
Mar 11 2010
parent reply 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><
Mar 11 2010
parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
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);
 }
Mar 11 2010