Question

I haven’t used the advanced features of C++ for a while and am refreshing my C++ knowledge..
Having said that, the concept of traits and policy based programming was something that I never really managed to get my head around.

I want to change that. I am writing a generic container. I want to enforce a policy that the container will store only classes that derive from a particular base class. This is because the container returns an invalid object (instead of throwing) when an attempt is made to access an item outside the vector bounds.

template <class T>   
class GenericContainer
{
private:
    typedef std::vector<T> TypeVect;
    void addElement(const T& elem);

    TypeVect m_elems;

public:
    unsigned int size() const;
    T& elementAt(const unsigned int pos);
    const T elementAt(const unsigned int pos) const;
};

How would I use traits to restrict this generic container to contain only subclasses of class ‘ContainerItem’ say?

Answer 1

You can use a little IsDerivedFrom template which can only be instantiated in case a given type ‘D’ inherits another type ‘B’ (this implementation was taken from a nice Guru Of The Week article):

template<typename D, typename B>
class IsDerivedFrom
{
  static void Constraints(D* p)
  {
    B* pb = p; // this line only works if 'D' inherits 'B'
    pb = p; // suppress warnings about unused variables
  } 

protected:
  IsDerivedFrom() { void(*p)(D*) = Constraints; }
}; 

// Force it to fail in the case where B is void
template<typename D>
class IsDerivedFrom<D, void>
{
  IsDerivedFrom() { char* p = (int*)0; /* error */ }
};

You can now simply instantiate the IsDerivedFrom template using inheritance:

template <class T>   
class GenericContainer : public IsDerivedFrom<T, ContainerItem>
{
    ...
};

This code only compiles if T inherits ContainerItem.