Question

as a follow up to this question, how can the code be changed so that i can use it in the constructor of a class? im writing a new class where the input needs to be an number of some kind, but nothing else. the code, however, is like declaring the type in front of a function. since constructors dont exactly have types, i need it to not declare a type for the function itself

my new class:

class C{
     public:
         C();
         C(T value);// specifically looking for this


         T f(T value); // what the code currently does

};

the code in the link creates a function that [accepts and] returns an integer type T. i need it to not return anything at all, so that it can be used with a constructor

Answer 1

I think you want to restrict types for constructor template. If so, then you can do this:

#include <type_traits>
//#include <tr1/type_traits> // for C++03, use std::tr1::

class C
{
  public:

     template<typename T>
     C(T value, typename enable_if<std::is_arithmetic<T>::value,T>::type *p=0)
     {

     }
};

This constructor template can accept only those T for which is_arithmetic<T>::value is true. The implementation of enable_if is exactly same, as given in the other answer.

---

Alternatively, or if you don’t have type_traits, then you can use typelist along with enable_if. I think this is a better solution, as you can specifically define the supported typelist.

typedef typelist<int> t1;
typedef typelist<short, t1> t2;
typedef typelist<char, t2> t3;
typedef typelist<unsigned char, t3> t4;
//and so on

typedef t4 supported_types;//supported_types: int, short, char, unsigned char

class C
{
  public:

     template<typename T>
     C(T value, typename enable_if<exits<T,supported_types>::value,T>::type *p=0)
     {

     }
};

This constructor template can accept only those T for which exists<T,supported_types>::value is true. The exists metafunction checks whether T is existing in the typelist supported_types or no. You can add more types to this typelist.

And the implementation of typelist, and exists is here (see my solution):

• Restrict parameter type in template instantiation