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Proper handling of errors and exceptional situations is essential for the continued correct operation of software. The preferred mechanism for reporting errors in a C++ program is exceptions rather than using error codes. A number of core language facilities, including dynamic_cast, operator new(), and typeid, report failures by throwing exceptions. In addition, the C++ standard library makes heavy use of exceptions to report several different kinds of failures. Few C++ programs manage to avoid using some of these facilities. Consequently, the vast majority of C++ programs must be prepared for exceptions to occur and must handle each appropriately. (see See ERR51-CPP. Handle all exceptions.).

Because exceptions introduce code paths into a program, it is important to consider the effects of code taking such paths and to avoid any undesirable effects that might arise otherwise. Some such effects include failure to release an acquired resource, thereby introducing a leak, and failure to reestablish a class invariant after a partial update to an object or even a partial object update while maintaining all invariants. Code that avoids any such undesirable effects is said to be exception safe.

Based on the preceding effects, the following table distinguishes three kinds of exception safety guarantee guarantees from most to least desired:.

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Because all exceptions thrown in an application must be handled, in compliance with ERR50-CPP. Do not abruptly terminate the program, it is critical that thrown exceptions do not leave the program in an indeterminate state where invariants are violated. That is, the program must provide basic exception safety for all invariants , and may choose to provide strong exception safety for some invariants. Whether exception handling is used to control the termination of the program, or to recover from an exceptional situation, a violated invariant leaves the program in a state where graceful continued execution is likely to introduce security vulnerabilities. Thus, code that provides no exception safety guarantee is unsafe and must be considered defective.

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The following noncompliant code example shows a flawed copy assignment operator. The implicit invariants of the class are that the array member is a valid (possibly null) pointer and that the nElems member stores the number of elements in the array pointed to by array. The function deallocates array and assigns the element counter, nElems, before allocating a new block of memory for the copy. As a result, if the new expression throws an exception, the function will have modified the state of both member variables in a way that violates the implicit invariants of the class. Consequently, such an object is in an indeterminate state and any operation on it, including its destruction, results in undefined behavior.

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