Termination Model of Exception Handling

If an exception occurs as the result of a statement in a try block, the try block expires (i.e., terminates immediately). Next, the program searches for the first catch handler that can process the type of exception that occurred. The program locates the matching catch by comparing the thrown exception’s type to each catch’s exception-parameter type until the program finds a match. A match occurs if the types are identical or if the thrown exception’s type is a derived class of the exception-parameter type. When a match occurs, the code in the matching catch handler executes. When a catch handler finishes processing by reaching its closing right brace (}), the exception is considered handled and the local variables defined within the catch handler (including the catch parameter) go out of scope. Program control does not return to the point at which the exception occurred (known as the throw point), because the try block has expired. Rather, control resumes with the first statement (line 43) after the last catch handler following the try block. This is known as the termination model of exception handling. Some languages use the resumption model of exception handling, in which, after an exception is handled, control resumes just after the throw point. As with any other block of code, when a try block terminates, local variables defined in the block go out of scope.

If the try block completes its execution successfully (i.e., no exceptions occur in the try block), then the program ignores the catch handlers and program control continues with the first statement after the last catch following that try block.

If an exception that occurs in a try block has no matching catch handler, or if an exception occurs in a statement that is not in a try block, the function that contains the statement terminates immediately, and the program attempts to locate an enclosing try block in the calling function. This process is called stack unwinding and is discussed in Section 17.4.