C++ Templates vs. Aggregation

Question

Consider the following piece of code:

class B {
  private:
    // some data members
  public:
    friend bool operator==(const B&,const B&);
    friend ostream& operator<<(ostream&,const B&);
    // some other methods
};

template <typename T=B>
class A {
  private:
    // some data members
    vector<vector<T> > vvlist;
  public:
    // some other methods
 };

My requirement is that the type T that is passed as type parameter must provide definitions for the operator== and the operator<< methods. I do not want to enforce any other restrictions on T.

How can I do this?

One way that I can think of is to Create an Abstract class say "Z" that declares these two methods.

and then write

vector<vector<Z> > vvlist;

and NOT have class A as a template.

Is there a better way to do this?

Thanks! Ajay

Answer 1

You can write a private method in A that would test required stuff on T in compile time.

void TestReq(T x, T y)
{
  if (x==y)
    cout << x;
}

This way even plain integers would pass and work.

Answer 2

It seems like you are looking for a concept check library. See what Boost has to offer: Boost Concept Check Library. That link also has a good explanation what concepts are. Quote:

A concept is a set of requirements (valid expressions, associated types, semantic invariants, complexity guarantees, etc.) that a type must fulfill to be correctly used as arguments in a call to a generic algorithm

In your question, the concept is "type T must provide operator== and operator<<".

Answer 3

It happens automatically.

If your code calls the operators == and <<, then the code simply won't compile if the class is passed a type that doesn't define these operators.

It is essentially duck-typing. If it looks like a duck, and quacks like a duck, then it is a duck. It doesn't matter whether it implements an IDuck interface, as long as it exposes the functionality you try to use.