I want to be able to refer to different types using literals as ids.
template<auto>
using type = void;
template<>
using type<0> = int;
template<>
using type<1> = char;
template<>
using type<2> = string;
int main()
{
type<0> var0;
type<1> var1;
type<2> var2;
}
This results in the compiler giving me errors, since type alias specialization is not yet supported in C . (The technology required to implement such a feature simply does not exist)
CodePudding user response:
This will do it, and will give you the syntax you need. Note I explicitly use std::size_t to avoid specialization on other types then numbers.
#include <string>
#include <type_traits>
//-------------------------------------------------------------------
// hide all the boiler plate in a namespace
// use structs for partial specializations
namespace details
{
template<std::size_t N>
struct type_s { using type = void; };
template<> struct type_s<0> { using type = int; };
template<> struct type_s<1> { using type = char; };
template<> struct type_s<2> { using type = std::string; };
}
//-------------------------------------------------------------------
// now you can use a full template for alias
template<std::size_t N>
using type_t = typename details::type_s<N>::type;
//-------------------------------------------------------------------
int main()
{
type_t<0> var0{ 42 };
type_t<1> var1{ 'A' };
type_t<2> var2{ "Hello World!" };
static_assert(std::is_same_v<type_t<0>, int>);
static_assert(std::is_same_v<type_t<1>, char>);
static_assert(std::is_same_v<type_t<2>, std::string>);
static_assert(std::is_same_v<decltype(var0), int>);
static_assert(std::is_same_v<decltype(var1), char>);
static_assert(std::is_same_v<decltype(var2), std::string>);
}
CodePudding user response:
Partial specialization is not allowed in alias templates, however, you can use std::conditional
instead:
#include <type_traits>
#include <string>
// ...
template <auto X>
using type = std::conditional_t<X == 0, int,
std::conditional_t<X == 1, char,
std::conditional_t<X == 2, std::string,
void>>>;
CodePudding user response:
Luckily, I found a workaround which uses class specialization which is supported in C :
template<auto>
struct TypeAliasWrapper;
template<>
struct TypeAliasWrapper<0>{using type = int;};
template<>
struct TypeAliasWrapper<1>{using type = char;};
template<>
struct TypeAliasWrapper<2>{using type = string;};
int main()
{
TypeAliasWrapper<0>::type var0;
TypeAliasWrapper<1>::type var1;
TypeAliasWrapper<2>::type var2;
}