Template class with conditional typenames

You can use std::conditional, from <type_traits>.

If you want the T2 be optix::float2 when T == float and otherwise optix::double2, use std::conditional. This is availble since c++11 and will resolve the type T2 at compile time.

#include <type_traits>  // std::conditional, std::is_same

template <class T>
class MyClass
{
    using T2 = typename std::conditional<std::is_same<T, float>::value,
                                          optix::float2, optix::double2>::type;
    T2 my_T2_variable;

    // ... other code
};

(See demo)

---

As @HikmatFarhat pointed out, std::conditional will not catch the user mistakes. It checks only the first condition, and for the false case gives the type optix::double2.

Another option is series of SFINAE ed functions, and decltype to those for the T2 as follows:

#include <type_traits>  // std::is_same, std::enable_if

template <class T> // uses if T == float and return `optix::float2`
auto typeReturn() -> typename std::enable_if<std::is_same<float, T>::value, optix::float2>::type { return {}; }

template <class T> // uses if T == double and return `optix::double2`
auto typeReturn() -> typename std::enable_if<std::is_same<double, T>::value, optix::double2>::type { return {}; }

template <class T>
class MyClass
{
    using T2 = decltype(typeReturn<T>()); // chooses the right function!

    T2 my_T2_variable;

    // ... other codes
};

(See demo)

Typically you'd do this by creating a trait type whose specializations define the additional types. For example:

// Base template is undefined.
template <typename T>
struct optix_traits;

template <>
struct optix_traits<float> {
    using dim2 = optix::float2;
    // etc
};

template <>
struct optix_traits<double> {
    using dim2 = optix::double2;
    // etc
};

Then you can alias from these types to a name in your type, if desired:

template <typename T>
class MyClass {
public:
    using T2 = typename optix_traits<T>::dim2;
};

Implement a meta-function using template specialization that maps standard C++ types to OptiX types with the desired "rank":

template <typename T, std::size_t N> struct optix_type;

template <> struct optix_type<float, 2> { using type = optix::float2; };
template <> struct optix_type<float, 3> { using type = optix::float3; };
template <> struct optix_type<double, 2> { using type = optix::double2; };
// ...

template <typename T, std::size_t N>
using optix_type_t = typename optix_type<T, N>::type;

You can then use this within your class(es) to easily get the right types:

template <class T>
class MyClass {
  using T2 = optix_type_t<T, 2>;
  MyClass() {
    T2 my_T2_variable;
    optix_type_t<T, 3> my_T3_variable;
  }
  void SomeFunction() { T2 another_T2_variable; };
};