Question

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

template <bool S>
class NuclearPowerplantControllerFactoryProviderFactory {
  // if S == true
  typedef int data_t;
  // if S == false
  typedef unsigned int data_t;
};

Answer 1

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;