模板元组-在每个元素上调用一个函数 [英] Template tuple - calling a function on each element
问题描述
我的问题在代码中:
template<typename... Ts>
struct TupleOfVectors {
std::tuple<std::vector<Ts>...> tuple;
void do_something_to_each_vec() {
//Question: I want to do this:
// "for each (N)": do_something_to_vec<N>()
//How?
}
template<size_t N>
void do_something_to_vec() {
auto &vec = std::get<N>(tuple);
//do something to vec
}
};
推荐答案
您可以使用某些索引机制轻松地做到这一点。给定一个元函数 gen_seq
用于生成编译时整数序列(由 seq
类模板封装):
You can quite easily do that with some indices machinery. Given a meta-function gen_seq
for generating compile-time integer sequences (encapsulated by the seq
class template):
namespace detail
{
template<int... Is>
struct seq { };
template<int N, int... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> { };
template<int... Is>
struct gen_seq<0, Is...> : seq<Is...> { };
}
以及以下功能模板:
#include <tuple>
namespace detail
{
template<typename T, typename F, int... Is>
void for_each(T&& t, F f, seq<Is...>)
{
auto l = { (f(std::get<Is>(t)), 0)... };
}
}
template<typename... Ts, typename F>
void for_each_in_tuple(std::tuple<Ts...> const& t, F f)
{
detail::for_each(t, f, detail::gen_seq<sizeof...(Ts)>());
}
您可以使用上面的 for_each_in_tuple
函数这样:
You can use the for_each_in_tuple
function above this way:
#include <string>
#include <iostream>
struct my_functor
{
template<typename T>
void operator () (T&& t)
{
std::cout << t << std::endl;
}
};
int main()
{
std::tuple<int, double, std::string> t(42, 3.14, "Hello World!");
for_each_in_tuple(t, my_functor());
}
这里是 实时示例 。
在您的具体情况下,可以使用以下方法:
In your concrete situation, this is how you could use it:
template<typename... Ts>
struct TupleOfVectors
{
std::tuple<std::vector<Ts>...> t;
void do_something_to_each_vec()
{
for_each_in_tuple(t, tuple_vector_functor());
}
struct tuple_vector_functor
{
template<typename T>
void operator () (T const &v)
{
// Do something on the argument vector...
}
};
};
再一次,这是一个 实时示例 。
如果您使用的是 C ++ 14 或更高版本,则可以替换 seq
和 gen_seq
上面带有 std :: integer_sequence
像这样:
If you're using C++14 or later, you can replace the seq
and gen_seq
classes above with std::integer_sequence
like so:
namespace detail
{
template<typename T, typename F, int... Is>
void
for_each(T&& t, F f, std::integer_sequence<int, Is...>)
{
auto l = { (f(std::get<Is>(t)), 0)... };
}
} // namespace detail
template<typename... Ts, typename F>
void
for_each_in_tuple(std::tuple<Ts...> const& t, F f)
{
detail::for_each(t, f, std::make_integer_sequence<int, sizeof...(Ts)>());
}
如果您使用的是 C ++ 17 或更高版本,为此(来自下面的评论):
If you're using C++17 or later you can do this (from this comment below):
std::apply([](auto ...x){std::make_tuple(some_function(x)...);} , the_tuple);
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