问题描述
有一个类型别名
using MyVariantType = std::variant<int,double,std::string,bool>;
和别名模板,
template <typename T>
using MyFunctionType = std::function<bool(T)>
如何从 MyVariantType
和 MyFunctionType
动态创建以下类型别名?
using MyFunctionVariantType = std::variant<MyFunctionType<int>,MyFunctionType<double>,MyFunctionType<std::string>,MyFunctionType<bool>>
解决方法
此代码段将从变体中获取每种类型,并将创建 MyFunctionType
的变体。它与模板特化一起工作以找出变体的类型:
#include <variant>
#include <functional>
using MyVariantType = std::variant<int,double>;
template <typename T>
using MyFunctionType = std::function<bool(T)>;
/// Helper struct to create the FunctionType from the Varaint Type
template <typename T>
struct CreateFunctionVariant;
template <typename... Ts>
struct CreateFunctionVariant<std::variant<Ts...>>
{
using Type = std::variant<MyFunctionType<Ts>...>;
};
using MyFunctionVariantType = CreateFunctionVariant<MyVariantType>::Type;
/// Make sure it actually produces the right type
static_assert(std::is_same_v<MyFunctionVariantType,std::variant<MyFunctionType<int>,MyFunctionType<double>>>);