Item29: Strive for exception-safe code.(Effective C++)

• Exception-safe functions leak no resources and allow no data structures to become corrupted, even when exceptions are thrown. Such functions offer the basic, strong, or nothrow guarantees.

• The strong guarantee can often be implemented via copy-and-swap, but the strong guarantee is not practical for all functions.

• A function can usually offer a guarantee no stronger than the weakest guarantee of the functions it calls.