May I use a constant number to choose a class at compile time, possibly using templates?

Question

Let\'s say I have a constant value (possibly of some enum type). Let\'s say I have many classes A, B, D, etc.

Can I have something like this?

C<1>          


        

Answer 1

This is a fairly simple metafunction:

template 
struct C {
  typedef typename std::conditional::type type;
};

You would use this as C<1>::type foo;.

If your compiler supports C++11 template aliases, you can simplify to:

template 
using C = typename std::conditional::type;

and have your preferred C<1> foo; syntax.

In pure C++03, implement std::conditional as:

template 
struct conditional {
  typedef A type;
};

template 
struct conditional {
  typedef B type;
};