Exception guarantees

Exception guarantees, also known as the Abrahams guarantees after David Abrahams who formalized the guidelines, are a set of contractual guidelines that class library implementors and clients use when reasoning about exception safety in C++ programs.

The rules apply to class implementations (components) in the presence of exceptions; they are as follows:

• The strong guarantee: that the operation has either completed successfully or thrown an exception, leaving the program state exactly as it was before the operation started.
• The basic guarantee: that the invariants of the component are preserved, and no resources are leaked. Often refered to as the weak guarantee, because following an exception the system is left in a safe but unknown state.
• The no-throw guarantee: that the operation will not throw an exception.

Code that doesn't follow at least one of the above rules is called exception unsafe.