我有一个功能:
template<class Real, int N>
constexpr std::array<Real, N> get_array();
,我想对许多类型和许多整数进行测试。用伪代码:
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之循环,以便我可以遍历数组?
答案 0 :(得分:2)
既然您已经标记了Boost.Hana,我们就可以使用它:
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>();
});
});
答案 1 :(得分:2)
可接受的答案是很好的,但是如果您不想污染您的调用堆栈,最好在获得实际运行时内容之前进行所有可能的处理。
此处的示例使用hana::cartesian_product
,这对于临时用例来说可能是多余的,但是您可以使用嵌套的hana::unpack
来获得相同的效果。
这是一个有效的示例:
#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>>));
}
答案 2 :(得分:1)
怎么办(不加电)?
#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;
}