Anthony Williams wrote:

 > > > template<typename T>
 > > > class MyClass
 > > > {
 > > >   template<typename U>
 > > >   friend void func(MyClass<T> mc,U data);
 > > > };
 > > >
 > > > How can I define this function outside the class template?
 > > >
 > > > I tried:
 > > >
 > > > template<typename T>
 > > > template<typename U>
 > > > void func(MyClass<T> mc,U data)
 > > > {}
 > >
 > > This is ill-formed. If that has worked, your compiler is broken.
 >
 > This is the syntax for a member template - see 14.5.2 of the
standard. If
 > func had been a member function rather than a friend, then
 >
 > template<typename T>
 > template<typename U>
 > void MyClass<T>::func(MyClass<T> mc,U data)
 > {}
 >
 > should work.

Right. Unfortunately, here we don't have a member but a friend.

>
> It works if it is declared and defined as part of the friend declaration in
> the class template, and does exactly what I expect:
>
> template<typename T>
> class MyClass
> {
>   template<typename U>
>   friend void func(MyClass<T> mc,U data)
>   {
>   // function body here
>   }
> };
>

OK, got it now.
If you define it outside the class template, you have to define this
function for each T seperately, since T is not a template parameter for
the function, i.e. you write

template <typename U>
void func(MyClass<int>, U data ){ /*...*/}

template <typename U>
void func(MyClass<char>, U data ){ /*...*/}

That means you better define it inside the class.

Sorry for confusion
Thomas

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Anthony Williams wrote:
>
> I have a class template, and I want to declare a friend function that
> performs
> some operation on this class. The problem is, I want this friend
> function to be
> a template
>
> template<typename T>
> class MyClass
> {
>   template<typename U>
>   friend void func(MyClass<T> mc,U data);

That probably does not do what you want. Here you declare a function
template with *one* template parameter, and with two arguments. The
first argument type is MyClass<T> and the second one is defined through
the template parameter.

> };
>
> How can I define this function outside the class template?
>
> I tried:
>
> template<typename T>
> template<typename U>
> void func(MyClass<T> mc,U data)
> {}

This is ill-formed. If that has worked, your compiler is broken.

>
> and
>
> template<typename T,typename U>
> void func(MyClass<T> mc,U data)
> {}
>

This declares an unrelated function template with two template
parameters.
What you probably want is a function template with two template
parameters

template<typename A,typename B>
void func(MyClass<A> mc,B data)
{}

that is a friend of myclass<T> when A and T are the same type. That is
not possible in c++, a friend can be either the full function template
or some specialization of it.

Hope this helps,
Thomas

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"Thomas Kunert" <kunert@physik.tu-dresden.de> wrote in message
news:39C0CE36.31C08FFB@physik.tu-dresden.de...
> Anthony Williams wrote:
> >
> > I have a class template, and I want to declare a friend function that
> > performs
> > some operation on this class. The problem is, I want this friend
> > function to be
> > a template
> >
> > template<typename T>
> > class MyClass
> > {
> >   template<typename U>
> >   friend void func(MyClass<T> mc,U data);
>
> That probably does not do what you want. Here you declare a function
> template with *one* template parameter, and with two arguments. The
> first argument type is MyClass<T> and the second one is defined through
> the template parameter.

This is indeed what I want, and if I define the function inline as part of
the friend declaration, then everything works OK

>
> > };
> >
> > How can I define this function outside the class template?
> >
> > I tried:
> >
> > template<typename T>
> > template<typename U>
> > void func(MyClass<T> mc,U data)
> > {}
>
> This is ill-formed. If that has worked, your compiler is broken.

This is the syntax for a member template - see 14.5.2 of the standard. If
func had been a member function rather than a friend, then

template<typename T>
template<typename U>
void MyClass<T>::func(MyClass<T> mc,U data)
{}

should work.
>
> >
> > and
> >
> > template<typename T,typename U>
> > void func(MyClass<T> mc,U data)
> > {}
> >
>
> This declares an unrelated function template with two template
> parameters.
> What you probably want is a function template with two template
> parameters
>
> template<typename A,typename B>
> void func(MyClass<A> mc,B data)
> {}

Which is exactly my second example, but with T==A and U==B

> that is a friend of myclass<T> when A and T are the same type. That is
> not possible in c++, a friend can be either the full function template
> or some specialization of it.

It works if it is declared and defined as part of the friend declaration in
the class template, and does exactly what I expect:

template<typename T>
class MyClass
{
  template<typename U>
  friend void func(MyClass<T> mc,U data)
  {
  // function body here
  }
};

Anthony
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