遍历模板 int

Question

我有一个功能：

template<class Real, int N>
constexpr std::array<Real, N> get_array();

而且我想在很多类型和很多整数上测试它。在伪代码中：

auto types = {float, double, long double};
for(int i = 0; i < 25; ++i) {
   for (type : types) {
        auto arr = get_array<type, i>();
        // test arr
   }
}

显然这不能编译。有没有办法修补循环以使其循环，以便我可以遍历数组？

Answer 1

既然你已经 Boost.Hana 标记了，我们就可以使用它了：

auto types = hana::tuple_t<float, double, long double>;
hana::for_each(types, [](auto type){
    hana::for_each(std::make_index_sequence<25>(), [=](auto idx){
        // here type is an object that denotes the type and
        // idx is an integral constant that denotes the next value

        get_array<typename decltype(type)::type, idx>();
    });
});

Answer 2

如下（不加boost）呢？

#include <array>
#include <iostream>

template <typename T, std::size_t Dim>
constexpr std::array<T, Dim> get_array ()
 { return {}; }

// fake test
template <typename T, std::size_t Dim>
constexpr bool checkArray (std::array<T, Dim> const &)
 { return true; }

template <typename T, std::size_t ... Is>
constexpr bool checkSequence (std::index_sequence<Is...> const &)
 { return (... && checkArray(get_array<T, Is>())); }

template <typename ... Ts>
constexpr bool checkTypes ()
 { return (... && checkSequence<Ts>(std::make_index_sequence<25u>{})); }

int main ()
 {
   constexpr auto value = checkTypes<float, double, long double>();

   std::cout << value << std::endl;
 }

Answer 3

接受的答案很好，但如果您不想污染您的调用堆栈，最好在到达实际的运行时间之前完成所有可能的处理.

此处的示例使用 hana::cartesian_product which is probably overkill for ad hoc use cases, but you could use nested hana::unpacks 来获得相同的效果。

这是一个工作示例：

https://godbolt.org/z/zKwpqe

#include <array>
#include <boost/hana/assert.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/cartesian_product.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/for_each.hpp>
#include <boost/hana/range.hpp>
#include <boost/hana/transform.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>

namespace hana = boost::hana;

template <typename Real, int N>
constexpr std::array<Real, N> get_array() {
  return {};
}

int main() {
  auto types   = hana::tuple_t<float, double, long, double>;
  auto lengths = hana::to_tuple(hana::range_c<int, 0, 4>);
  auto tls = hana::cartesian_product(hana::make_tuple(types, lengths));
  auto get_array_fns = hana::transform(tls, [](auto pair) {
    return [] {
      return get_array<typename decltype(+hana::at_c<0>(pair))::type,
                       decltype(+hana::at_c<1>(pair))::value>();
      };
  });

  hana::for_each(get_array_fns, [](auto get_array) {
    auto arr = get_array();
    // test arr 
  });

  auto result_types = hana::unpack(get_array_fns, [](auto ...get_array) {
    return hana::tuple_t<decltype(get_array())...>;
  });

  BOOST_HANA_CONSTANT_CHECK(hana::equal(
    decltype(result_types){},
    hana::tuple_t<std::array<float,  0>,
                  std::array<float,  1>,
                  std::array<float,  2>,
                  std::array<float,  3>,
                  std::array<double, 0>,
                  std::array<double, 1>,
                  std::array<double, 2>,
                  std::array<double, 3>,
                  std::array<long,   0>,
                  std::array<long,   1>,
                  std::array<long,   2>,
                  std::array<long,   3>,
                  std::array<double, 0>,
                  std::array<double, 1>,
                  std::array<double, 2>,
                  std::array<double, 3>>));
}

遍历模板 int

Iterate over template int

c++

templates

template-meta-programming

boost-hana

c++17