Matching Exceptions With Handlers

A handler type T matches a throw type E if any of the following is true:

• T is the same as E.

• T is const or volatile of E.

• E is const or volatile of T.

• T is ref of E or E is ref of T.

• T is a public base class of E.

• T and E are both pointer types, and E can be converted to T by a standard pointer conversion.

Throwing exceptions of reference or pointer types can result in a dangling pointer if the object pointed or referred to is destroyed before exception handling is complete. When an object is thrown, a copy of the object is always made through the copy constructor, and the copy is passed to the catch block. It is therefore safe to throw a local or temporary object.

While handlers of type (X) and (X&) both match an exception of type X, the semantics are different. Using a handler with type (X) invokes the object's copy constructor (again). If the thrown object is of a type derived from the handler type, the object is truncated. Catching a class object by reference therefore usually executes faster.

Handlers for a try block are tried in the order of their appearance. Handlers for a derived class (or a pointer to a reference to a derived class) must precede handlers for the base class to ensure that the handler for the derived class can be invoked.