CS102: Introduction to Computer Science II

Each public function in a class essentially forms a contract with the user; if you pass it certain arguments, it will perform certain operations and/or return a result. The same contract must hold true in derived classes; otherwise the expected "is-a" relationship between derived and base classes is violated. Since exception specifications are logically part of a function's declaration, they too must remain consistent across an inheritance hierarchy. For example, if a member function in a base class says it will only throw an exception of type A, an override of that function in a derived class must not add any other exception types to the specification list because that would break any programs that adhere to the base class interface. You can, however, specify fewer exceptions or none at all, since that doesn't require the user to do anything differently. You can also specify anything that "is-a" A in place of A in the derived function's specification. Here's an example.

//: C01:Covariance.cpp {-xo}
// Should cause compile error. {-mwcc}{-msc}
#include 
using namespace std;
 
class Base {
public:
  class BaseException {};
  class DerivedException : public BaseException {};
  virtual void f() throw(DerivedException) {
    throw DerivedException();
  }
  virtual void g() throw(BaseException) {
    throw BaseException();
  }
};
 
class Derived : public Base {
public:
  void f() throw(BaseException) {
    throw BaseException();
  }
  virtual void g() throw(DerivedException) {
    throw DerivedException();
  }
}; ///:~

A compiler should flag the override of Derived::f( ) with an error (or at least a warning) since it changes its exception specification in a way that violates the specification of Base::f( ). The specification for Derived::g( ) is acceptable because DerivedException "is-a" BaseException (not the other way around). You can think of Base/Derived and BaseException/DerivedException as parallel class hierarchies; when you are in Derived, you can replace references to BaseException in exception specifications and return values with DerivedException. This behavior is called covariance  (since both sets of classes vary down their respective hierarchies together). (Reminder from Volume 1: parameter types are not covariant ­– you are not allowed to change the signature of an overridden virtual function).