Re: [boost] Request For a feature - Templated virtual functions

Subject: Re: [boost] Request For a feature - Templated virtual functions
From: Gokulakannan Somasundaram (gokul007_at_[hidden])
Date: 2009-02-12 01:01:57

On Thu, Feb 12, 2009 at 3:56 AM, Ravi <lists_ravi_at_[hidden]> wrote:

> On Wednesday 11 February 2009 13:27:46 Gokulakannan Somasundaram wrote:
> >          As you all know, there are situations, wherein the virtual
> > function usage can be replaced with templates. But virtual functions can
> > never be used with templates, since all the instantiations have to be
> > tracked to form the virtual function table. But we can ask the user to
> > provide a typelist of classes, that would probably be used with the
> virtual
> > template function. I think this would solve this issue for most of the
> > cases.
>
> If the list of arguments/return types is known at compile time, is this not
> the same as overloaded virtual functions? Here's an example:
>
> #include <iostream>
> using namespace std;
> struct Base {
>  virtual void func( int ) { cout << "Base::int overload" << endl; }
>  virtual void func( double ) { cout << "Base::double overload" << endl; }
>  template <typename T> void virtfunc( T x ) { func( x ); } // "virtual"
>  virtual ~Base() {}
> };
> struct Derived : Base {
>  virtual void func( int ) { cout << "Derived::int overload" << endl; }
>  virtual void func( double ) { cout << "Derived::double overload" << endl;
> }
>  virtual ~Derived() {}
> };
>
> int main( int, char *[] ) {
>  Base *p = new Derived;
>  p->virtfunc( 3 );
>  p->virtfunc( 3.14 );
> }
>
> $ g++ -o out -Wall tempvirt.cc
> $ ./out
> Derived::int overload
> Derived::double overload
>
> Regards,
> Ravi
>
>
I think, i am trying to accomplish, what you have done through overloading
through templates. Say if you have 10 such functions, with each
implementation being different in the inheritance hierarchy, then you will
write the same piece of code again and again, with different types. In your
example you have written the functions twice, which is redundant. (same
piece of code repeated twice). More overhead on maintenance,as the number of
overloads increase. Hope i am able convey my point.

Thanks,
Gokul.