What are Signatures anyway?
————————— Roughly, signatures are type abstractions or interfaces of classes. They are related to ML’s signatures, categories in Axiom, definition modules in Modula-2, interface modules in Modula-3, and types in POOL-I. The main language constructs added are signatures and signature pointers. For example, the signature declaration signature S { int foo (void); int bar (int); }; defines a new abstract type `S’ with member functions `int foo (void)’ and `int bar (int).’ Signature types cannot be instantiated since they don’t provide any implementation. Only signature pointers and signature references can be defined. For example, C obj; S * p = &obj; defines a signature pointer `p’ and initializes it to point to an object of class type `C,’ where `C’ is required to contain the public member functions `int foo (void)’ and `int bar (int).’ The member function call int i = p->foo (); executes then `obj.foo ().’ Class `C’ is called an implementation of the abstract type `S.’
