Re: Variadic templates
Boris Rasin ha scritto:
On Nov 17, 10:45 pm, Alberto Ganesh Barbati <AlbertoBarb...@libero.it>
All you need is an helper function like this:
template <class T, class... Args>
/* whatever */ invoke(T f, tuple<Args...>&& args);
which calls f with the supplied arguments. It should not be too
difficult to make it a perfect forwarder.
Ganesh
Seeing that it should not be too difficult, would you mind actually
showing how to call a function with any number of parameters given
tuple<> object as parameter container (as in your sample)? No need to
make it a perfect forwarding, just anything that would actually work.
You can't do that for an arbitrary number of parameters, due to the
limitation of the pack expansion syntax you already noticed. However, it
is definitely possible to do it the "old way", that is to provide
several specializations for 1, 2, 3 up to N parameters some N. For
example for N = 3:
template <typename T, typename A0>
/* some result_of or Callable<> expression here */
invoke(T&& f, tuple<A0>&& args)
{
return f(std::forward<A0>(get<0>(args)));
}
template <typename T, typename A0, typename A1>
/* some result_of or Callable<> expression here */
invoke(T&& f, tuple<A0, A1>&& args)
{
return f(
std::forward<A0>(get<0>(args)),
std::forward<A1>(get<1>(args)));
}
template <typename T, typename A0, typename A1, typename A2>
/* some result_of or Callable<> expression here */
invoke(T&& f, tuple<A0, A1, A2>&& args)
{
return f(
std::forward<A0>(get<0>(args)),
std::forward<A1>(get<1>(args)),
std::forward<A2>(get<2>(args)));
}
Using preprocessor metaprogramming it is not difficult to have N
arbitrarily high. See, for example
http://www.boost.org/doc/libs/1_37_0/libs/preprocessor/doc/index.html
HTH,
Ganesh
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