Re: How to simulate variadic templates?

Maxim Yegorushkin <>
Fri, 6 Jun 2008 10:27:45 CST
On Jun 5, 6:27 pm, Peter Holtwick <> wrote:

With a current problem, I found the variadic templates in C++0x:

I would need a template with variable number of parameters, but I need
them now! Is there a way to simulate this?

The background:
In an own implementation of a ref-count-pointer, there should be no
raw pointers at all. Therefore I create the object using a "New<>"

     template< class T > class Ptr;

template< class T >
Ptr<T> New()
{ /* ... */ }

template< class T, class T_ARG1 >
Ptr<T> New( T_ARG1 arg1 )
{ /* ... */ }

So I need a template for each number of arguments. Any way around
this? Maybe something like

Ptr<Obj> pObj = New<Obj> ( ARG( arg1, ARG( arg2, ARG( arg3 ) ) ) );

You can pack constructor arguments into one object - boost::tuple, or
something similar. This way you solve the argument forwarding problem
by having to forward one argument only. Constructors must be able to
unpack the tuple, however. Something like that:

class some
     void init(int, int, int);

     some(int a, int b, int c) // normal ctor
     { this->init(a, b, c); }

     some(tuple<int, int, int> args) // one-argument unpacking ctor
     { this->init(args.get<0>(), args.get<1>(), args.get<2>()); }

// the factory function, only ever accepts one argument
template<class T, class Args>
Ptr<T> New(Args const& args)
     return Ptr<T>(new T(args));

// usage
Ptr<some> p = New<some>(make_tuple(1, 2, 3));

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