假设我有两个可变参数模板；typename... T, typename... U，我将如何找到他们的；
- 级联
- 最大公共子序列
- 最大公共子序列的逆
所以据我了解，串联很简单；(t..., u...)，但是如何找到两者的最大公共子序列呢？- 甚至可能吗？
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1 回答
12
这是一个计算对元组类型对的集合操作的解决方案。我假设元组可以用来保存变量参数包，所以一旦你有了Ts...and Us...，你就可以：
typename tuple_intersect<std::tuple<Ts...>, std::tuple<Us...>>::type
这给了你一个元组，和Vs...的交集在哪里。如果您需要再次提取为参数包，只需提供元组作为接受 a 的函数的输入：Ts...Us...Vs...tuple<Ts...>
template<typename... Vs>
void func(std::tuple<Vs...>)
{
// Here, you have Vs... (= Us... & Ts...) as an argument pack
}
框架：
以下是以下所有主要元函数共有的一些简单元函数：
template<typename T, typename... Ts>
struct is_member_of_type_seq { static const bool value = false; };
template<typename T, typename U, typename... Ts>
struct is_member_of_type_seq<T, U, Ts...>
{
static const bool value = std::conditional<
std::is_same<T, U>::value,
std::true_type,
is_member_of_type_seq<T, Ts...>
>::type::value;
};
template<typename, typename>
struct append_to_type_seq { };
template<typename T, typename... Ts>
struct append_to_type_seq<T, std::tuple<Ts...>>
{
using type = std::tuple<Ts..., T>;
};
template<typename, typename>
struct prepend_to_type_seq { };
template<typename T, typename... Ts>
struct prepend_to_type_seq<T, std::tuple<Ts...>>
{
using type = std::tuple<T, Ts...>;
};
1 - 串联
这个很简单：
template<typename, typename>
struct concat_type_seq { };
template<typename... Ts, typename... Us>
struct concat_type_seq<std::tuple<Ts...>, std::tuple<Us...>>
{
using type = std::tuple<Ts..., Us...>;
};
和一些测试：
static_assert(
std::is_same<
concat_type_seq<
std::tuple<char, int, bool>,
std::tuple<double, double, int>
>::type,
std::tuple<char, int, bool, double, double, int>
>::value,
"Error"
);
2 -最长公共子序列
这个稍微复杂一点：
namespace detail
{
// Meta-function that returns, given two sequences S1 and S2, the longest
// subsequence of S1 in S2 that starts with the first element of S1 and
// begins at the first element of S2 (in other words, it returns the
// subsequence S2[0]..S2[N] such that S1[i] = S2[i] for each 0 <= i <= N.
template<typename, typename>
struct match_seq_in_seq_from_start
{
using type = std::tuple<>;
};
template<typename T, typename U, typename... Ts, typename... Us>
struct match_seq_in_seq_from_start<std::tuple<T, Ts...>, std::tuple<U, Us...>>
{
using type = typename std::conditional<
std::is_same<T, U>::value,
typename prepend_to_type_seq<
T,
typename match_seq_in_seq_from_start<
std::tuple<Ts...>,
std::tuple<Us...>
>::type
>::type,
std::tuple<>
>::type;
};
// Some testing...
static_assert(
std::is_same<
match_seq_in_seq_from_start<
std::tuple<int, double, char>,
std::tuple<int, double, long>
// ^^^^^^^^^^^
>::type,
std::tuple<int, double>
>::value,
"Error!"
);
// Meta-function that returns the same as the meta-function above,
// but starting from the first element of S2 which is identical to
// the first element of S1.
template<typename, typename>
struct match_first_seq_in_seq
{
using type = std::tuple<>;
};
template<typename T, typename U, typename... Ts, typename... Us>
struct match_first_seq_in_seq<std::tuple<T, Ts...>, std::tuple<U, Us...>>
{
using type = typename std::conditional<
std::is_same<T, U>::value,
typename match_seq_in_seq_from_start<
std::tuple<T, Ts...>,
std::tuple<U, Us...>
>::type,
typename match_first_seq_in_seq<
std::tuple<T, Ts...>,
std::tuple<Us...>
>::type
>::type;
};
// Some testing...
static_assert(
std::is_same<
match_first_seq_in_seq<
std::tuple<int, double, char>,
std::tuple<bool, char, int, double, long, int, double, char>
// ^^^^^^^^^^^
>::type,
std::tuple<int, double>
>::value,
"Error!"
);
// Meta-function that returns, given two sequences S1 and S2, the longest
// subsequence of S1 in S2 that starts with the first element of S1.
template<typename T, typename U>
struct match_seq_in_seq
{
using type = std::tuple<>;
};
template<typename U, typename... Ts, typename... Us>
struct match_seq_in_seq<std::tuple<Ts...>, std::tuple<U, Us...>>
{
using type1 = typename match_first_seq_in_seq<
std::tuple<Ts...>,
std::tuple<U, Us...>
>::type;
using type2 = typename match_seq_in_seq<
std::tuple<Ts...>,
std::tuple<Us...>
>::type;
using type = typename std::conditional<
(std::tuple_size<type1>::value > std::tuple_size<type2>::value),
type1,
type2
>::type;
};
// Some testing...
static_assert(
std::is_same<
match_seq_in_seq<
std::tuple<int, double, char>,
std::tuple<char, int, double, long, int, double, char>
// ^^^^^^^^^^^^^^^^^
>::type,
std::tuple<int, double, char>
>::value,
"Error!"
);
}
// Meta-function that returns, given two sequences S1 and S2, the longest
// subsequence of S1 in S2 (longest common subsequence).
template<typename T, typename U>
struct max_common_subseq
{
using type = std::tuple<>;
};
template<typename T, typename... Ts, typename... Us>
struct max_common_subseq<std::tuple<T, Ts...>, std::tuple<Us...>>
{
using type1 = typename detail::match_seq_in_seq<
std::tuple<T, Ts...>,
std::tuple<Us...>
>::type;
using type2 = typename max_common_subseq<
std::tuple<Ts...>,
std::tuple<Us...>
>::type;
using type = typename std::conditional<
(std::tuple_size<type1>::value > std::tuple_size<type2>::value),
type1,
type2
>::type;
};
和一些测试：
// Some testing...
static_assert(
std::is_same<
max_common_subseq<
std::tuple<int, double, char>,
std::tuple<char, int, char, double, char, long, int, bool, double>
>::type,
std::tuple<double, char>
>::value,
"Error!"
);
// Some more testing...
static_assert(
std::is_same<
max_common_subseq<
std::tuple<int, double, char, long, long, bool>,
// ^^^^^^^^^^^^^^^^
std::tuple<char, long, long, double, double, char>
// ^^^^^^^^^^^^^^^^
>::type,
std::tuple<char, long, long>
>::value,
"Error!"
);
3 -反转
这是反转类型序列的特征（返回具有反转类型列表的元组）：
template<typename... Ts>
struct revert_type_seq
{
using type = std::tuple<>;
};
template<typename T, typename... Ts>
struct revert_type_seq<T, Ts...>
{
using type = typename append_to_type_seq<
T,
typename revert_type_seq<Ts...>::type
>::type;
};
和一些测试：
// Some testing...
static_assert(
std::is_same<
revert_type_seq<char, int, bool>::type,
std::tuple<bool, int, char>
>::value,
"Error"
);
4 -交叉口
这个没有被要求，但作为奖励提供：
template<typename, typename>
struct intersect_type_seq
{
using type = std::tuple<>;
};
template<typename T, typename... Ts, typename... Us>
struct intersect_type_seq<std::tuple<T, Ts...>, std::tuple<Us...>>
{
using type = typename std::conditional<
!is_member_of_type_seq<T, Us...>::value,
typename intersect_type_seq<
std::tuple<Ts...>,
std::tuple<Us...>>
::type,
typename prepend_to_type_seq<
T,
typename intersect_type_seq<
std::tuple<Ts...>,
std::tuple<Us...>
>::type
>::type
>::type;
};
和一些测试：
// Some testing...
static_assert(
std::is_same<
intersect_type_seq<
std::tuple<char, int, bool, double>,
std::tuple<bool, long, double, float>
>::type,
std::tuple<bool, double>
>::value,
"Error!"
);
于 2013-03-07T15:47:07.613 回答