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Subject: RE: Forward iteration in variadic templates
Date: Mon, 14 Jul 2008 14:52:15 -0600
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Thread-Topic: Forward iteration in variadic templates
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From: "Travis Vitek" <Travis.Vitek@roguewave.com>
To: <dev@stdcxx.apache.org>
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=20

Eric Lemings wrote:
>
>> Eric Lemings wrote:
>>=20
>> Consider the following function:
>>=20
>> template <class... Types>
>> void print (const Types&... values);
>>=20
>> How would you define print() to iterate forward from 0..N,=20
>> where N is sizeof... (Types), printing each argument X

I'm unsure about what you mean by iterate here. AFAIK, you can't iterate
over the elements in a parameter pack directly, everything is done with
recursion that terminates with one or more special cases.

>
>...and its corresponding index...
>
>> in the=20
>> argument list (e.g. std::cout << X)?
>>=20
>> Thanks,
>> Brad.
>>=20
>

I don't know if this is what you had in mind, but it does what you ask.

$ cat q.cpp && g++ -std=3Dgnu++0x q.cpp && ./a.out
#include <iostream>

template <class... _TTypes>
void print_impl (unsigned index, const _TTypes&... values);

template <class _TypeT>
void print_impl (unsigned index, const _TypeT& value)
{
    std::cout << index << ':' << value << '\n';
}

template <class _TypeT, class... _TTypes>
void print_impl (unsigned index, const _TypeT& value, const _TTypes&...
values)
{
    print_impl (index, value);
    print_impl (index + 1, values...);
}

template <class... _TTypes>
void print (const _TTypes&... values)
{
    print_impl (0, values...);
}

int main ()
{
  print (1, 'a', 3.14f, "hello");
  return 0;
}
0:1
1:a
2:3.14
3:hello