Exception specification of overriding function is more lax than base version

Exception Specification in Overriding Functions

In C++, exception specification allows you to specify the types of exceptions that a function may throw. It is a way to communicate to the caller about the potential exceptions that can be raised during the execution of the function. When a function is overridden in a derived class, the exception specification of the overriding function may be more lax (allowing more exceptions) compared to the base version of the function.

Let’s say we have a base class called Base and a derived class called Derived. The base class has a function foo with an exception specification, and the derived class overrides this function. Here’s an example:

    
      class Base {
        public:
          virtual void foo() throw(ExceptionType1) {
            // code
          }
      };
      
      class Derived : public Base {
        public:
          void foo() throw(ExceptionType1, ExceptionType2) {
            // code
          }
      };
    
  

In this example, the foo function in the base class has an exception specification that allows throwing only ExceptionType1. However, the foo function in the derived class overrides it and has a more lax exception specification that allows throwing both ExceptionType1 and ExceptionType2.

By allowing the derived class to throw additional exceptions, it provides flexibility for the derived class to handle specific exceptions or perform additional error handling. However, this laxity comes with a potential drawback. If a caller expects the base class version of foo and tries to catch only ExceptionType1, and then calls the derived class version, it may not catch ExceptionType2 and result in unexpected behavior.

It’s important to carefully consider the exception specifications when designing and using inheritance hierarchies to ensure proper exception handling and predictable behavior.