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I want to write a templated constructor with a non-type template argument,
like this.

class X
{
public:
X() : val(0) {}

template <int I>
X(X const&, X const&) : val(I) {}

private:
int val;
};

This compiles but I have no idea how to 'call' this constructor. Is it
possible?

(Essentially I have a single algorithm to construct an X from two other X's
and I would like to customise that algorithm at compile time using an
integer parameter).

john

John Harrison wrote in news: in
comp.lang.c++:

> I want to write a templated constructor with a non-type template
> argument, like this.
>
> class X
> {
> public:
> X() : val(0) {}
>
> template <int I>
> X(X const&, X const&) : val(I) {}
>
> private:
> int val;
> };
>
> This compiles but I have no idea how to 'call' this constructor. Is it
> possible?
>

No.

> (Essentially I have a single algorithm to construct an X from two
> other X's and I would like to customise that algorithm at compile time
> using an integer parameter).
>

Wrap the integer up in a type, say boost::mpl::int_<> if you have
boost (*)

eg:

template < int I >
struct int_
{
};

your ctor becomes:

tempalte < int I >
X( X const&, X const&, int_< I > const & /* unused */)
: val( I )
{
}


usage:

int main()
{
X a, b, x( a , b, int_< 1 >() );
}

HTH.

*) boost::mpl -> http://www.boost.org/libs/mpl/doc/

Rob.
--
http://www.victim-prime.dsl.pipex.com/

"Rob Williscroft" <> wrote in message
news:Xns94E38752969E7ukcoREMOVEfreenetrtw@130.133. 1.4...
> John Harrison wrote in news: in
> comp.lang.c++:
>
> > I want to write a templated constructor with a non-type template
> > argument, like this.
> >
> > class X
> > {
> > public:
> > X() : val(0) {}
> >
> > template <int I>
> > X(X const&, X const&) : val(I) {}
> >
> > private:
> > int val;
> > };
> >
> > This compiles but I have no idea how to 'call' this constructor. Is it
> > possible?
> >
>
> No.
>
> > (Essentially I have a single algorithm to construct an X from two
> > other X's and I would like to customise that algorithm at compile time
> > using an integer parameter).
> >
>
> Wrap the integer up in a type, say boost::mpl::int_<> if you have
> boost (*)
>
> eg:
>
> template < int I >
> struct int_
> {
> };
>
> your ctor becomes:
>
> tempalte < int I >
> X( X const&, X const&, int_< I > const & /* unused */)
> : val( I )
> {
> }
>
>
> usage:
>
> int main()
> {
> X a, b, x( a , b, int_< 1 >() );
> }
>
> HTH.
>

OK, that's clever. Thanks.

john