Template class with conditional typenames

Question

I would like to have a template class (e.g. float/double type), but I am using Nvidia CUDA and OptiX and have multiple other types (e.g. float2, double2, float3,...) that depend on the chosen template type.

Something like this:

#include <optixu/optixu_vector_types.h>
#include <type_traits>

template <class T>
class MyClass 
{
   MyClass()
   {
      if (std::is_same<T, float>::value) 
      {
         typedef optix::float2 T2;
      }
      else if (std::is_same<T, double>::value)
      {
         typedef optix::double2 T2;
      }

      T2 my_T2_variable;
   }

   void SomeFunction() 
   { 
      T2 another_T2_variable; 
   };
};

My solution for now is to have multiple template arguments MyClass<T,T2,T3> my_object;, but this seems to have too much overhead and clutter. Is there a way to achieve the same with a single template argument as desired above?

Answer 1

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;
};

Answer 2

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)