Force type of C++ template

Question

I've a basic template class, but I'd like to restrain the type of the specialisation to a set of classes or types. e.g.:

template <typename T>
class MyClass
{
.../...
private:
    T* _p;
};

MyClass<std::string> a; // OK
MYCLass<short> b;       // OK
MyClass<double> c;      // not OK

Those are just examples, the allowed types may vary.

Is that even possible? If it is, how to do so?

Thanks.

Answer 1

Another version is to leave it undefined for the forbidden types

template<typename T>
struct Allowed; // undefined for bad types!

template<> struct Allowed<std::string> { };
template<> struct Allowed<short> { };

template<typename T>
struct MyClass : private Allowed<T> { 
  // ...
};

MyClass<double> m; // nono

Answer 2

Yust a quick idea, I'm sure there are better approaches:

template <typename T> struct protector {
static const int result = 1;
};

template <> struct protector<double> {
static const int result = -1;
};

template <typename T> 
class MyClass
{
   private:
     char isfine[protector<T>::result];
};

It might be better, however, to put a fat comment over your code to keep users from instantiating with the wrong types :-)

Answer 3

Take a look at the Boost Concept Check Library: http://www.boost.org/doc/libs/1_42_0/libs/concept_check/concept_check.htm