我想“生成”函数指针的跳转表。所指向的函数有两种类型的模板。应该为两个类型列表中的每个可能的对实例化一个不同的函数。理想情况下，我们可以有这样的东西：
#include <tuple>
template <typename X, typename Y>
void foo()
{}
template <typename... Xs, typename... Ys>
void bar(const std::tuple<Xs...>&, const std::tuple<Ys...>&)
{
using fun_ptr_type = void (*) (void);
static constexpr fun_ptr_type jump_table[sizeof...(Xs) * sizeof...(Ys)]
= {&foo<Xs, Ys>...};
}
int main ()
{
using tuple0 = std::tuple<int, char, double>;
using tuple1 = std::tuple<float, unsigned long>;
bar(tuple0{}, tuple1{});
}
正如预期的那样，当元组长度不同时它会失败：
foo.cc:15:20: error: pack expansion contains parameter packs 'Xs' and 'Ys' that have different lengths (3 vs. 2)
= {&foo<Xs, Ys>...};
~~ ~~ ^
foo.cc:23:3: note: in instantiation of function template specialization 'bar<int, char, double, float, unsigned long>' requested here
bar(tuple0{}, tuple1{});
^
1 error generated.
为了实现这种功能，我已经尝试并成功使用了间接（第一个跳转表，其中包含指向另一个跳转表的函数的指针），但我发现它很笨拙。
所以，我的问题是：有解决方法吗？
您的示例代码是错误的，即使它可以编译（即当 sizeof...(Xs) == sizeof...(Ys) 时）。 假设你有 N 元元组，那么 Jump_table 有 N*N 个元素，但只有前 N 个元素是用函数 ptrs 初始化的。
首先，您需要内连接 2 个类型列表：
template<class A, class B>
struct Pair;
template<class... Ts>
struct List {};
template<class T, class... Ts>
using mul = List<Pair<T, Ts>...>;
template<class...>
struct cat;
template<class T>
struct cat<T>
{
using type = T;
};
template<class... As, class... Bs>
struct cat<List<As...>, List<Bs...>>
{
using type = List<As..., Bs...>;
};
template<class A, class B, class... Ts>
struct cat<A, B, Ts...>
{
using type = typename cat<typename cat<A, B>::type, Ts...>::type;
};
template<class A, class B>
struct join;
template<class... As, class... Bs>
struct join<List<As...>, List<Bs...>>
{
using type = typename cat<mul<As, Bs...>...>::type;
};
例如，
join<List<int[1], int[2]>, List<float[1], float[2], float[3]>>::type
给你
List<Pair<int[1], float[1]>, Pair<int[1], float[2]>, Pair<int[1], float[3]>, Pair<int[2], float[1]>, Pair<int[2], float[2]>, Pair<int[2], float[3]>
回到你的例子：
template <typename X, typename Y>
void foo()
{}
template<class T, std::size_t N>
struct jump_table
{
template<class... As, class... Bs>
constexpr jump_table(List<Pair<As, Bs>...>)
: table{&foo<As, Bs>...}
{}
T table[N];
};
template <typename... Xs, typename... Ys>
void bar(const std::tuple<Xs...>&, const std::tuple<Ys...>&)
{
using fun_ptr_type = void (*) (void);
static constexpr jump_table<fun_ptr_type, sizeof...(Xs) * sizeof...(Ys)> table
= {typename join<List<Xs...>, List<Ys...>>::type()};
}
int main ()
{
using tuple0 = std::tuple<int, char, double>;
using tuple1 = std::tuple<float, unsigned long>;
bar(tuple0{}, tuple1{});
}
这应该符合您的预期。
这里的其他答案对于当前的问题来说似乎太复杂了。我的做法如下：
#include <array>
#include <tuple>
template <typename X, typename Y> void foo() {}
using fun_ptr_type = void (*) (void);
// Build one level of the table.
template <typename X, typename ...Ys>
constexpr std::array<fun_ptr_type, sizeof...(Ys)>
jump_table_inner = {{&foo<X, Ys>...}};
// Type doesn't matter, we're just declaring a primary template that we're
// about to partially specialize.
template <typename X, typename Y> void *jump_table;
// Build the complete table.
template <typename ...Xs, typename ...Ys>
constexpr std::array<std::array<fun_ptr_type, sizeof...(Ys)>, sizeof...(Xs)>
jump_table<std::tuple<Xs...>, std::tuple<Ys...>> = {jump_table_inner<Xs, Ys...>...};
int main () {
using tuple0 = std::tuple<int, char, double>;
using tuple1 = std::tuple<float, unsigned long>;
// Call function for (int, float).
jump_table<tuple0, tuple1>[0][0]();
}
Clang 3.5 的 C++14 模式接受了这一点。
我的产品扩展
context( f<Xs, Ys>... ) /* not what we want */context2( g<Xs, Ys...>... )gYsXgXs在我们的例子中，我们将拥有一个函数指针数组的数组，而不是函数指针的平面数组。 与您尝试的解决方案不同，尽管这些实际上是我们关心的&foo<X, Y>
函数指针，并且扁平化很简单。
#include <cassert>
#include <utility>
#include <array>
template<typename X, typename Y>
void foo() {}
using foo_type = void(*)();
template<typename... T>
struct list {
static constexpr auto size = sizeof...(T);
};
template<typename X, typename Y, typename Indices = std::make_index_sequence<X::size * Y::size>>
struct dispatch;
template<
template<typename...> class XList, typename... Xs
, template<typename...> class YList, typename... Ys
, std::size_t... Indices
>
struct dispatch<XList<Xs...>, YList<Ys...>, std::index_sequence<Indices...>> {
private:
static constexpr auto stride = sizeof...(Ys);
using inner_type = std::array<foo_type, stride>;
using multi_type = inner_type[sizeof...(Xs)];
template<typename X, typename... Yss>
static constexpr inner_type inner()
{ return {{ &foo<X, Yss>... }}; }
static constexpr multi_type multi_value = {
inner<Xs, Ys...>()...
};
public:
static constexpr auto size = sizeof...(Xs) * sizeof...(Ys);
static constexpr foo_type value[size] = {
multi_value[Indices / stride][Indices % stride]...
};
};
template<
template<typename...> class XList, typename... Xs
, template<typename...> class YList, typename... Ys
, std::size_t... Indices
>
constexpr foo_type dispatch<XList<Xs...>, YList<Ys...>, std::index_sequence<Indices...>>::value[size];
int main()
{
using dispatch_t = dispatch<
list<int, char, double>,
list<float, unsigned long>
>;
constexpr auto&& table = dispatch_t::value;
static_assert( dispatch_t::size == 6, "" );
static_assert( table[0] == &foo<int, float>, "" );
static_assert( table[1] == &foo<int, unsigned long>, "" );
static_assert( table[2] == &foo<char, float>, "" );
static_assert( table[3] == &foo<char, unsigned long>, "" );
static_assert( table[4] == &foo<double, float>, "" );
static_assert( table[5] == &foo<double, unsigned long>, "" );
}
Coliru 演示.