c++17通过生成预先声明的类型列表的笛卡尔乘积来生成std::变体

c++17 making std::variant by generating cartesian product of pre-declared lists of types

本文关键字：std 变体 笛卡尔 列表 声明 c++17 类型      更新时间：2023-10-16

假设我有一个由三个模板类型参数组成的类。

template<typename Transformer, typename Criteria, typename Strategy>
struct ConfiguredPipeline {};

并有以下类稍后在实例化ConfiguredPipeline:时使用

template<typename...>
struct CriteriaList
{
};
using SupportedCriteria = CriteriaList<ChiSquared, Cosine>;
template<typename...>
struct StrategiesList
{
};
using SupportedStrategies = StrategiesList<Voting>;
template<typename...>
struct TransformerList
{
};
using SupportedTransformer = TransformerList<AAGrouper11, AAGrouper15>;

我如何能够生成相当于的std::variant

using PipelineVariant = std::variant< 
ConfiguredPipeline< ChiSquared , Voting , AAGrouper11 > ,
ConfiguredPipeline< ChiSquared , Voting , AAGrouper15 > ,
ConfiguredPipeline< Cosine , Voting , AAGrouper11 > ,
ConfiguredPipeline< Cosine , Voting , AAGrouper15 >>;

通过简单地调用如下函数：

using PipelineVariant = makeVariant< ConfiguredPipeline , SupportedCriteria , SupportedStrategies, SupportedTransformaers >;

我想decltype()、std::declval()和std::tuple_cat()可以帮你很多忙。

给定以下两个重载模板声明(注意：声明，未定义，遵循std::declval()示例(函数，以在单个std::tuple中生成类型的笛卡尔乘积并折叠(由于std::tuple_cat()(

template <template <typename...> class C, typename ... Ts>
constexpr std::tuple<C<Ts...>> tupleExpand (std::tuple<Ts...> const &);
template <template <typename...> class C, typename ... Ts,
template <typename...> class C0, typename ... Ls,
typename ... Cs>
constexpr auto tupleExpand (std::tuple<Ts...> const &, C0<Ls...> const &,
Cs const & ... cs)
-> decltype(std::tuple_cat(
tupleExpand<C>(std::declval<std::tuple<Ts..., Ls>>(), cs...)...));

以及以下简单模板函数(再次：仅声明(，用于将std::tuple类型转换为相应的std::variant类型列表中的类型列表

template <typename ... Ts>
constexpr std::variant<Ts...> tupleToVariant (std::tuple<Ts...> const &);

您可以按照以下编写MakeVariant类

template <template <typename...> class C, typename ... Ts>
struct MakeVariant
{
using type = decltype(tupleToVariant(std::declval<
decltype(tupleExpand<C>(std::declval<std::tuple<>>(),
std::declval<Ts>()...))>()));
};

和一个简单的CCD_ 12辅助

template <template <typename...> class C, typename ... Ts>
using MakeVariantType = typename MakeVariant<C, Ts...>::type;

您可以定义您的PipelineVariant类型如下

using PipelineVariant = MakeVariantType<ConfiguredPipeline,
SupportedCriteria,
SupportedStrategies,
SupportedTransformers>;

以下是的完整编译示例

#include <tuple>
#include <variant>
template <typename, typename, typename>
struct ConfiguredPipeline
{ };
struct ChiSquared {};
struct Cosine     {};
template <typename...>
struct CriteriaList
{ };
using SupportedCriteria = CriteriaList<ChiSquared, Cosine>;
struct Voting {};
template <typename...>
struct StrategiesList
{ };
using SupportedStrategies = StrategiesList<Voting>;
struct AAGrouper11 { };
struct AAGrouper15 { };
template <typename...>
struct TransformerList
{ };
using SupportedTransformers = TransformerList<AAGrouper11, AAGrouper15>;
template <template <typename...> class C, typename ... Ts>
constexpr std::tuple<C<Ts...>> tupleExpand (std::tuple<Ts...> const &);
template <template <typename...> class C, typename ... Ts,
template <typename...> class C0, typename ... Ls,
typename ... Cs>
constexpr auto tupleExpand (std::tuple<Ts...> const &, C0<Ls...> const &,
Cs const & ... cs)
-> decltype(std::tuple_cat(
tupleExpand<C>(std::declval<std::tuple<Ts..., Ls>>(), cs...)...));
template <typename ... Ts>
constexpr std::variant<Ts...> tupleToVariant (std::tuple<Ts...> const &);
template <template <typename...> class C, typename ... Ts>
struct MakeVariant
{
using type = decltype(tupleToVariant(std::declval<
decltype(tupleExpand<C>(std::declval<std::tuple<>>(),
std::declval<Ts>()...))>()));
};
template <template <typename...> class C, typename ... Ts>
using MakeVariantType = typename MakeVariant<C, Ts...>::type;
using PipelineVariant = MakeVariantType<ConfiguredPipeline,
SupportedCriteria,
SupportedStrategies,
SupportedTransformers>;
int main () 
{
static_assert(std::is_same<PipelineVariant, 
std::variant<ConfiguredPipeline<ChiSquared, Voting, AAGrouper11>,
ConfiguredPipeline<ChiSquared, Voting, AAGrouper15>,
ConfiguredPipeline<Cosine, Voting, AAGrouper11>,
ConfiguredPipeline<Cosine, Voting, AAGrouper15>>>::value);
}
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