问题
I was playing around with variadic templates (gcc 4.5) and hit this problem :
template <typename... Args>
boost::tuple<Args...>
my_make_tuple(Args... args)
{
return boost::tuple<Args...>(args...);
}
int main (void)
{
boost::tuple<int, char> t = my_make_tuple(8, 'c');
}
GCC error message :
sorry, unimplemented: cannot expand 'Arg ...' into a fixed-length argument list
In function 'int my_make_tuple(Arg ...)'
If I replace every occurrence of boost::tuple
by std::tuple
, it compiles fine.
Is there a problem in boost tuple implementation? Or is this a gcc bug ?
I must stick with Boost.Tuple for now. Do you know any workaround ?
Thanks.
回答1:
It doesn't seem to like expanding Args...
to T1, T2, T3, ..., T9
as Boost has it.
As a workaround, use constructs that don't require this expansion:
#include <boost/tuple/tuple.hpp>
template <typename... Args>
auto my_make_tuple(Args... args) -> decltype(boost::make_tuple(args...))
{
return {args...};
}
int main (void)
{
boost::tuple<int, char> t = my_make_tuple(8, 'c');
}
Another option might be to do the expanding manually, seeing that boost::tuple
supports up to 10 arguments.
#include <boost/tuple/tuple.hpp>
template <unsigned, class, class...> struct nth_argument;
template <unsigned N, class Default, class T, class... Args>
struct nth_argument<N, Default, T, Args...>
{
typedef typename nth_argument<N - 1, Default, Args...>::type type;
};
template <class Default, class T, class... Args>
struct nth_argument<0, Default, T, Args...>
{
typedef T type;
};
template <unsigned N, class Default>
struct nth_argument<N, Default>
{
typedef Default type;
};
template <typename ...Args>
struct tuple_from_var_template
{
typedef boost::tuple<
typename nth_argument<0, boost::tuples::null_type, Args...>::type,
typename nth_argument<1, boost::tuples::null_type, Args...>::type,
typename nth_argument<2, boost::tuples::null_type, Args...>::type,
typename nth_argument<3, boost::tuples::null_type, Args...>::type,
typename nth_argument<4, boost::tuples::null_type, Args...>::type,
typename nth_argument<5, boost::tuples::null_type, Args...>::type,
typename nth_argument<6, boost::tuples::null_type, Args...>::type,
typename nth_argument<7, boost::tuples::null_type, Args...>::type,
typename nth_argument<8, boost::tuples::null_type, Args...>::type,
typename nth_argument<9, boost::tuples::null_type, Args...>::type
> type;
};
template <typename... Args>
typename tuple_from_var_template<Args...>::type my_make_tuple(Args... args)
{
return typename tuple_from_var_template<Args...>::type(args...);
}
int main (void)
{
boost::tuple<int, char> t = my_make_tuple(8, 'c');
}
来源:https://stackoverflow.com/questions/2709315/is-boost-tuple-compatible-with-c0x-variadic-templates