我知道我以前解决过这个问题,但不记得是如何解决的。哦,好吧,这是一个新鲜的外观。
可以使用 将数字序列转换为参数序列std::get
,因此它更基本。所以,假设我需要实现某种自定义工具,数字包生成器似乎是一个不错的选择。
(啊,这太乏味了。我确实偷看了霍华德的回答并了解了forward_as_tuple
,但我的编译器或 ideone.com 上什至还不存在该功能,所以等等。还有很多事情我仍然需要弄清楚,这肯定是有史以来最糟糕的函数式语言之一。)
http://ideone.com/u5noV
#include <tuple>
// Generic pack array (metacontainer)
template< typename T, T ... seq > struct value_sequence {
// Append a value to the array (metafunction)
template< T val > struct append
{ typedef value_sequence< T, seq..., val > type; };
};
// Generate a sequential array (metafunction)
template< size_t N >
struct index_sequence {
typedef typename index_sequence< N - 1 >::type
::template append< N - 1 >::type type;
};
template<>
struct index_sequence< 0 >
{ typedef value_sequence< size_t > type; };
// Generate indexes up to size of given tuple (metafunction)
template< typename T >
struct index_tuple {
typedef typename index_sequence< std::tuple_size< T >::value
>::type type;
};
// The magic function: passes indexes, makes all the function calls
template< typename F, typename G,
typename T, size_t ... N >
void compose_with_indexes_helper( F f, G g, T args,
value_sequence< size_t, N ... > ) {
f( g( std::get< N >( args ), N ) ... );
}
template< typename F, typename G, typename ... T >
void compose_with_indexes( F f, G g, T && ... args ) {
typedef std::tuple< T && ... > tuple_t;
compose_with_indexes_helper
// forwarding seems broken on ideone.com/GCC 4.5.1, work around.
// ( f, g, std::forward_as_tuple( std::forward( args ) ... ) );
( f, g, tuple_t( args ... ), typename index_tuple< tuple_t >::type() );
}