C++ templates: Select different type based on value of template parameter

Question

How do I accomplish the following in C++, and what is doing such things called?

template 
class NuclearPowerplantControllerFactoryProviderFactory {         


        

Answer 1

@Kerrek has answered the question sufficiently, but that can be more generic as follows:

template<bool b, typename T, typename U>
struct select
{
    typedef T type;
};
template<typename T, typename U>
struct select<false, T, U>
{
    typedef U type;
};

And use as:

template <bool S>
class NuclearPowerplantControllerFactoryProviderFactory 
{
  typedef typename select<S, int, unsigned int>::type data_t;
  //use data_t as data type
};

If S is true, the first type argument in select will be selected, or else second type argument will be selected. It is generic because you specify both types in select<>, and based on the value of the boolean, select<b,T,U>::type returns either first type, or second type.

Answer 2

By specialization:

template <bool> class Foo;

template <> class Foo<true>
{
  typedef int data_t;
};

template <> class Foo<false>
{
  typedef unsigned int data_t;
};

You can choose to make one of the two cases the primary template and the other one the specialization, but I prefer this more symmetric version, given that bool can only have two values.

---

_{If this is the first time you see this, you might also like to think about partial specialization:}

template <typename T> struct remove_pointer     { typedef T type; };
template <typename U> struct remove_pointer<U*> { typedef U type; };

---

As @Nawaz says, the easiest way is probably to #include <type_traits> and say:

typedef typename std::conditional<S, int, unsigned int>::type data_t;