C++ Template Specialization with Constant Value

Question

Is there a straightforward way for defining a partial specialization of a C++ template class given a numerical constant for one of the template parameters? I'm trying to create special constructors for only certain kinds of template combinations:

template <typename A, size_t B> class Example {     public:         Example() { };          A value[B]; };  template <typename A, 2> class Example {     public:         Example(b1, b2) { value[0] = b1; value[1] = b2; }; }; 

This example won't compile, returning an error Expected identifier before numeric constant in the second definition.

I've had a look through a number of examples here and elsewhere, but most seem to revolve around specializing with a type and not with a constant.

Edit:

Looking for a way to write a conditionally used constructor, something functionally like this:

template <typename A, size_t B> class Example {     public:         // Default constructor         Example() { };          // Specialized constructor for two values         Example<A,2>(A b1, A b2) { value[0] = b1; value[1] = b2; };          A foo() {           A r;            for (size_t i = 0; i < b; ++b)             r += value[i];            return r;         }          // Hypothetical specialized implementation         A foo<A, 2>() {           return value[0] + value[1];         }          A value[B]; }; 

Answer 1

You need to put the specialization in the correct place:

template <typename A> class Example<A,2> 

If you want to create a subclass:

template <typename A> class ExampleSpecialization : public Example<A,2> 

The behavior for specializing on typedefs is similar to the behavior for specializing on an integer parameter.