问题描述
我在玩可变模板(gcc 4.5),遇到这个问题:
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错误讯息:
抱歉,未实现:无法将Arg ...展开成固定长度的参数列表
在函数'int my_make_tuple(Arg ...)'
如果我替换每次出现的 boost :: tuple
std :: tuple
,它编译得很好。
boost元组实现有问题吗?或者是一个gcc bug?
现在我必须坚持使用Boost.Tuple。你知道任何解决方法吗?
谢谢。
C $ c> Args ... 到 T1,T2,T3,...,T9
b
$ b
作为解决方法,使用不需要此扩展的构造:
#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');
}
另一种选择是手动扩展,看到 boost :: tuple
最多支持10个参数。
#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,默认>
{
typedef默认类型;
};
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< :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
>类型;
};
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');
}
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.
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');
}
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