以下代码有效:
#include <iostream>
#include <list>
struct Base {};
struct A : Base {}; struct B : Base {}; struct C : Base {};
struct D : Base {}; struct E : Base {}; struct F : Base {};
template <int KEY, typename... RANGE> struct Map {}; // one-to-many map (mapping KEY to RANGE...)
template <typename...> struct Data {};
using Database = Data< Map<0, A,B,C>, Map<1, D,E,F> >;
template <int N, typename FIRST, typename... REST> // N has meaning in my program, but not shown here.
void insertInMenu (std::list<Base*>& menu) {
menu.push_back(new FIRST);
insertInMenu<N, REST...> (menu);
}
template <int N>
void insertInMenu (std::list<Base*>&) {} // End of recursion.
template <int N>
std::list<Base*> menu() {
std::list<Base*> m;
insertInMenu<0, A,B,C>(m); // A,B,C should be obtained using N and Database.
return m;
}
int main() {
std::list<Base*> m = menu<0>();
std::cout << "m.size() = " << m.size() << std::endl; // 3
}
但是正如我在上面的评论中所指出的,我想使用Database
和值N
来获得范围A,B,C
(或D,E,F
)或其他值。但是我不知道该怎么做?有人可以帮忙吗?行
insertInMenu<0, A,B,C>(m);
需要用类似的东西代替
obtainRange<Database, N>()
因为那些编译时已知的值应该足够获得我想要的范围的信息。
obtainRange<Database, 0>()
应返回A,B,C
和
obtainRange<Database, 1>()
在这种情况下应返回D,E,F
。
template <typename D, int N>
struct obtainRange;
template <int N, typename... Ts, typename... Maps>
struct obtainRange<Data<Map<N, Ts...>, Maps...>, N>
{
using type = std::tuple<Ts...>;
};
template <int N, int M, typename... Ts, typename... Maps>
struct obtainRange<Data<Map<M, Ts...>, Maps...>, N>
: obtainRange<Data<Maps...>, N> {};
template <int N, typename Tuple, std::size_t... Is>
std::list<Base*> menu(std::index_sequence<Is...>)
{
std::list<Base*> m;
insertInMenu<0, typename std::tuple_element<Is, Tuple>::type...>(m);
return m;
}
template <int N>
std::list<Base*> menu()
{
using Tuple = typename obtainRange<Database, N>::type;
return menu<N, Tuple>(std::make_index_sequence<std::tuple_size<Tuple>::value>{});
}
// There is no need to take the length here, btw.
template <int, typename... Args>
void insertInMenu (std::list<Base*>& menu)
{
// Non-recursive push_backs:
std::initializer_list<int>{ (menu.push_back(new Args), 0)... };
}
template <int, typename> struct InsertEnv;
template <int key, int otherKey, typename... Args, typename... Rest>
struct InsertEnv<key, Data<Map<otherKey, Args...>, Rest...>> :
InsertEnv<key, Data<Rest...>> {};
template <int key, typename... Args, typename... Rest>
struct InsertEnv<key, Data<Map<key, Args...>, Rest...>>
{
void operator()(std::list<Base*>& menu)
{
insertInMenu<key, Args...> (menu);
}
std::list<Base*> operator()()
{
return {new Args...};
}
};
template <int N>
void addToMenu (std::list<Base*>& menu)
{
InsertEnv<N, Database>()(menu);
}
template <int N>
std::list<Base*> menu()
{
return InsertEnv<N, Database>()();
}