使用查找表选择具有运行时索引的可变参数类型
Selecting Variadic Type with Runtime Index using Lookup Table
考虑一个可变参数包的类型。可以使用递归索引函数和具有auto
参数的 lambda 来选择和使用由运行时值索引的类型之一,如下所示:
#include <iostream>
#include <functional>
#include <variant>
template<typename T>
struct identity { using type = T; };
template<typename ... Cs>
struct variadic
{
template<size_t N>
using type = std::tuple_element_t<N, std::tuple<Cs ...>>;
template<typename F, size_t I = 0>
static void identify(size_t index, F action)
{
if (index == I)
{
std::invoke(action, identity<type<I>>());
return;
}
if constexpr (I < (sizeof...(Cs) - 1))
{
identify<F, I + 1>(index, action);
return;
}
throw std::runtime_error("failed to identify type");
}
};
struct s_a { void speak() { std::cout << "s_a" << std::endl; } };
struct s_b { void speak() { std::cout << "s_b" << std::endl; } };
struct s_c { void speak() { std::cout << "s_c" << std::endl; } };
struct s_d { void speak() { std::cout << "s_d" << std::endl; } };
struct s_e { void speak() { std::cout << "s_e" << std::endl; } };
struct s_f { void speak() { std::cout << "s_f" << std::endl; } };
void speak(size_t index)
{
variadic<s_a, s_b, s_c, s_d, s_e, s_f>::identify(index, [](auto id)
{
using state_type = typename decltype(id)::type;
state_type().speak();
});
}
这种方法编译时生成一系列 if 语句,这些语句在后台解析为多个分支。我想在某一点之后,这将退化为分支错误预测的障碍。
有没有办法实现相同的行为,而是在引擎盖下生成一个查找表(本质上是一个 vtable(,类似于std::visit
的工作方式?
"简单"的方法是构建一个函数指针数组:
template<typename F>
static void identify(size_t index, F action) {
// handle case when index > sizeof...(Cs)
using FPtr = void(*)(F&);
static constexpr FPtr table[] = {
+[](F& action){ action(identity<type<Cs>>()) }...
};
table[index](action);
}
一些编译器可能不喜欢像这样扩展 lambda,所以你可以创建一个真正的函数:
template <typename T, typename F>
static void _apply(F& action) {
action(identity<type<T>>());
}
template<typename F>
static void identify(size_t index, F action) {
// handle case when index > sizeof...(Cs)
using FPtr = void(*)(F&);
static constexpr FPtr table[] = { _apply<Cs, F>... };
table[index](action);
}
<小时 />但是优化器往往非常不擅长内联这样的函数指针。 variant
实现曾经看起来像上面一样,但正在远离它。
如果你真的想全力以赴,你必须写一个递归开关......迈克尔·帕克对此给出了很好的解释,这相当复杂。
作为参考,这里是迈克尔·帕克展开的开关方法的实现,由Barry链接。
#include <iostream>
#include <functional>
#include <variant>
template<typename T>
struct identity { using type = T; };
template<size_t I, typename ... Cs>
using type = std::tuple_element_t<I, std::tuple<Cs ...>>;
template<bool Valid, typename ... Cs>
struct dispatcher;
template<typename ... Cs>
struct dispatcher<false, Cs ...>
{
template<size_t I, typename F>
constexpr static void case_(F)
{
static_assert(I >= sizeof...(Cs));
__builtin_unreachable();
}
template<size_t B, typename F>
constexpr static void next_(size_t, F)
{
static_assert(B >= sizeof...(Cs));
__builtin_unreachable();
}
};
template<typename ... Cs>
struct dispatcher<true, Cs ...>
{
template<size_t I, typename F>
constexpr static void case_(F action)
{
static_assert(I < sizeof...(Cs));
std::invoke(action, identity<type<I, Cs ...>>());
}
template<size_t B, typename F>
constexpr static void next_(size_t index, F action)
{
constexpr size_t size = sizeof...(Cs);
switch (index)
{
case B + 0: dispatcher<(B + 0 < size), Cs ...>::template case_<B + 0>(action); break;
case B + 1: dispatcher<(B + 1 < size), Cs ...>::template case_<B + 1>(action); break;
case B + 2: dispatcher<(B + 2 < size), Cs ...>::template case_<B + 2>(action); break;
case B + 3: dispatcher<(B + 3 < size), Cs ...>::template case_<B + 3>(action); break;
case B + 4: dispatcher<(B + 4 < size), Cs ...>::template case_<B + 4>(action); break;
case B + 5: dispatcher<(B + 5 < size), Cs ...>::template case_<B + 5>(action); break;
case B + 6: dispatcher<(B + 6 < size), Cs ...>::template case_<B + 6>(action); break;
case B + 7: dispatcher<(B + 7 < size), Cs ...>::template case_<B + 7>(action); break;
case B + 8: dispatcher<(B + 8 < size), Cs ...>::template case_<B + 8>(action); break;
case B + 9: dispatcher<(B + 9 < size), Cs ...>::template case_<B + 9>(action); break;
case B + 10: dispatcher<(B + 10 < size), Cs ...>::template case_<B + 10>(action); break;
case B + 11: dispatcher<(B + 11 < size), Cs ...>::template case_<B + 11>(action); break;
case B + 12: dispatcher<(B + 12 < size), Cs ...>::template case_<B + 12>(action); break;
case B + 13: dispatcher<(B + 13 < size), Cs ...>::template case_<B + 13>(action); break;
case B + 14: dispatcher<(B + 14 < size), Cs ...>::template case_<B + 14>(action); break;
case B + 15: dispatcher<(B + 15 < size), Cs ...>::template case_<B + 15>(action); break;
case B + 16: dispatcher<(B + 16 < size), Cs ...>::template case_<B + 16>(action); break;
case B + 17: dispatcher<(B + 17 < size), Cs ...>::template case_<B + 17>(action); break;
case B + 18: dispatcher<(B + 18 < size), Cs ...>::template case_<B + 18>(action); break;
case B + 19: dispatcher<(B + 19 < size), Cs ...>::template case_<B + 19>(action); break;
case B + 20: dispatcher<(B + 20 < size), Cs ...>::template case_<B + 20>(action); break;
case B + 21: dispatcher<(B + 21 < size), Cs ...>::template case_<B + 21>(action); break;
case B + 22: dispatcher<(B + 22 < size), Cs ...>::template case_<B + 22>(action); break;
case B + 23: dispatcher<(B + 23 < size), Cs ...>::template case_<B + 23>(action); break;
case B + 24: dispatcher<(B + 24 < size), Cs ...>::template case_<B + 24>(action); break;
case B + 25: dispatcher<(B + 25 < size), Cs ...>::template case_<B + 25>(action); break;
case B + 26: dispatcher<(B + 26 < size), Cs ...>::template case_<B + 26>(action); break;
case B + 27: dispatcher<(B + 27 < size), Cs ...>::template case_<B + 27>(action); break;
case B + 28: dispatcher<(B + 28 < size), Cs ...>::template case_<B + 28>(action); break;
case B + 29: dispatcher<(B + 29 < size), Cs ...>::template case_<B + 29>(action); break;
case B + 30: dispatcher<(B + 30 < size), Cs ...>::template case_<B + 30>(action); break;
case B + 31: dispatcher<(B + 31 < size), Cs ...>::template case_<B + 31>(action); break;
default: dispatcher<(B + 32 < size), Cs ...>::template next_<B + 32>(index, action); break;
}
}
};
template<typename ... Cs>
struct variadic
{
template <typename F>
constexpr static void identify(size_t index, F action)
{
constexpr size_t size = sizeof...(Cs);
if (index >= size)
throw std::out_of_range("type index out of bounds");
dispatcher<(0 < size), Cs ...>::template next_<0>(index, action);
}
};
struct s_a { void operator()() { std::cout << "s_a" << std::endl; } };
struct s_b { void operator()() { std::cout << "s_b" << std::endl; } };
struct s_c { void operator()() { std::cout << "s_c" << std::endl; } };
struct s_d { void operator()() { std::cout << "s_d" << std::endl; } };
struct s_e { void operator()() { std::cout << "s_e" << std::endl; } };
struct s_f { void operator()() { std::cout << "s_f" << std::endl; } };
void speak(size_t index)
{
variadic<s_a, s_b, s_c, s_d, s_e, s_f>::identify(index, [](auto id)
{
using state_type = typename decltype(id)::type;
std::invoke(state_type());
});
}
为了完整起见,这里有一个版本的函数指针方法,正如宣传的那样,它并没有很好地优化。
#include <iostream>
#include <functional>
#include <variant>
template<typename T>
struct identity { using type = T; };
template<size_t I, typename ... Cs>
using type = std::tuple_element_t<I, std::tuple<Cs ...>>;
template<typename ... Cs>
struct variadic
{
template <typename T, typename F>
static void _apply(F& action)
{
action(identity<T>());
}
template<typename F>
static void identify(size_t index, F action)
{
constexpr size_t size = sizeof...(Cs);
if (index >= size)
throw std::out_of_range("type index out of bounds");
using func_ptr = void(*)(F&);
static constexpr func_ptr table[] = { _apply<Cs, F>... };
table[index](action);
}
};
struct s_a { void operator()() { std::cout << "s_a" << std::endl; } };
struct s_b { void operator()() { std::cout << "s_b" << std::endl; } };
struct s_c { void operator()() { std::cout << "s_c" << std::endl; } };
struct s_d { void operator()() { std::cout << "s_d" << std::endl; } };
struct s_e { void operator()() { std::cout << "s_e" << std::endl; } };
struct s_f { void operator()() { std::cout << "s_f" << std::endl; } };
void speak(size_t index)
{
variadic<s_a, s_b, s_c, s_d, s_e, s_f>::identify(index, [](auto id)
{
using state_type = typename decltype(id)::type;
std::invoke(state_type());
});
}
展开的开关方法在优化方面看起来是赢家,并且将是我最终可能使用的。
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