Variadic templates and recursive specialization

A typical approach used with variadic templates is to specialize it for one argument with the rest of the arguments being optional (which works recursively). Here’s a rather naïve example which is probably an example of how not to use variadic templates, but it helped me understand variadic templates better.

template<typename... Args> class Test;

// Specialization for 0 arguments
template<> class Test<>
{
};

// Specialization for at least 1 argument

template<typename T1, typename... TRest> class Test<T1, TRest...> 
  : public Test<TRest...>
{
public:
  T1 Data;

  // This will return the base type
  Test<TRest...>& Rest() 
  {
    return *this;
  }
};

void Foo()
{
  Test<int> data;
  data.Data = 24;

  Test<int, int> twovalues;
  twovalues.Data = 10;
  // Rest() returns Test<int>
  twovalues.Rest().Data = 11;

  Test<int, int, char*> threevalues;
  threevalues.Data = 1;
  // Rest() returns Test<int, int>
  threevalues.Rest().Data = 2;
  // Rest().Rest() returns Test<char*>
  threevalues.Rest().Rest().Data = "test data";
}

Please be aware that no one ever writes code like this. There are correct ways to do this that I will write about in this blog. This example was merely for academic purposes.

Update – I’ve written a follow-up blog entry that shows the proper way to use variadic templates.

• Tuple implementation via variadic templates