Item 15: Understand the costs of exception handling

To handle exceptions at runtime, programs must do a fair amount of bookkeeping. At each point during execution, they must be able to identify the objects that require destruction if an exception is thrown; they must make note of each entry to and exit from a try block; and for each try block, they must keep track of the associated catch clauses and the types of exceptions those clauses can handle. This bookkeeping is not free. Nor are the runtime comparisons necessary to ensure that exception specifications are satisfied. Nor is the work expended to destroy the appropriate objects and find the correct catch clause when an exception is thrown. No, exception handling has costs, and you pay at least some of them even if you never use the keywords try, throw, or catch.

Let us begin with the things you pay for even if you never use any exception-handling features. You pay for the space used by the data structures needed to keep track of which objects are fully constructed (see Item 10), and you pay for the time needed to keep these data structures up to date. These costs are typically quite modest. Nevertheless, programs compiled without support for exceptions are typically both faster and smaller than their counterparts compiled with support for exceptions.