Are conditional typedef's possible in C++?

Question

this question is related to c++

there is a library which declares a class named Solver < TS,FS >. Solver is a member of another class Domain (written by me)

now there are many Domains which have a member "int region"

what i want to do is depending on the value of region, I want to make the solver accept different arguments for TS and FS. I was thinking of something along the line

template<int region>
struct Decider
{
  if(region==1)
  {
     typedef TSA TS;
     typedef FSA FS;
  }
  else
  if(region==2)
  {
     typedef TSB TS;
     typedef FSB FS;
  }
}

and later use it as

Decider<region>::TS
Decider<region>::FS

However, here due to the scope of the if, i guess the struct is useless. However, I am not able to think of a better method to do this. Any suggestions?

All the different TS and FS have the same interface. So I don't have to worry about the inside code.

Answer 1

You can specialize a template for any region value.

template<int region>
struct Decider;

template<>
struct Decider<1>
{
     typedef TSA TS;
     typedef FSA FS;
};

template<>
struct Decider<2>
{
     typedef TSB TS;
     typedef FSB FS;
};

Answer 2

You need to use template specialisation.

template <int region>
struct Decider;

template <>
struct Decider<1>
{
    typedef TSA TS;
    typedef FSA FS;
};

template <>
struct Decider<2>
{
    typedef TSB TS;
    typedef FSB FS;
};

C++ will choose which version to use based on the region supplied.

You can, of course, extend this as you see fit for other region numbers.

Answer 3

If you need to parameterize Decider based on some compile time constant, you can use template specialization (see other answers).

If you need to parameterize Decider based on a runtime value of region, you have to defer the parameterization to runtime as well. Usually this is done through some sort of creation function or factory idiom.

Answer 4

Note for anyone coming across this now:

It is also possible to do this with the boost libraries using the type_trait boost::conditional.

typedef boost::conditional<condition, type_if_true, type_if_false> MyTypeDef;

condition still needs to be a compile time expression that evaluates to true or false. This also makes it so you don't need to specialize your entire class for just a few lines of differences.