15 Exception handling [except]

15.5 Special functions [except.special]

The functions std::terminate() ([except.terminate]) and std::unexpected() ([except.unexpected]) are used by the exception handling mechanism for coping with errors related to the exception handling mechanism itself. The function std::current_exception() ([propagation]) and the class std::nested_exception ([except.nested]) can be used by a program to capture the currently handled exception.

15.5.1 The std::terminate() function [except.terminate]

In some situations exception handling must be abandoned for less subtle error handling techniques. [ Note: These situations are:

 — end note ]

In such cases, std::terminate() is called ([exception.terminate]). In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called. In the situation where the search for a handler ([except.handle]) encounters the outermost block of a function with a noexcept-specification that does not allow the exception ([except.spec]), it is implementation-defined whether the stack is unwound, unwound partially, or not unwound at all before std::terminate() is called. In all other situations, the stack shall not be unwound before std::terminate() is called. An implementation is not permitted to finish stack unwinding prematurely based on a determination that the unwind process will eventually cause a call to std::terminate().

15.5.2 The std::unexpected() function [except.unexpected]

If a function with a dynamic-exception-specification throws an exception that is not listed in the dynamic-exception-specification, the function std::unexpected() is called ([exception.unexpected]) immediately after completing the stack unwinding for the former function.

Note: By default, std::unexpected() calls std::terminate(), but a program can install its own handler function ([set.unexpected]). In either case, the constraints in the following paragraph apply.  — end note ]

The std::unexpected() function shall not return, but it can throw (or re-throw) an exception. If it throws a new exception which is allowed by the exception specification which previously was violated, then the search for another handler will continue at the call of the function whose exception specification was violated. If it throws or rethrows an exception that the dynamic-exception-specification does not allow then the following happens: If the dynamic-exception-specification does not include the class std::bad_exception ([bad.exception]) then the function std::terminate() is called, otherwise the thrown exception is replaced by an implementation-defined object of the type std::bad_exception and the search for another handler will continue at the call of the function whose dynamic-exception-specification was violated.

Thus, a dynamic-exception-specification guarantees that only the listed exceptions will be thrown. If the dynamic-exception-specification includes the type std::bad_exception then any exception not on the list may be replaced by std::bad_exception within the function std::unexpected().

15.5.3 The std::uncaught_exception() function [except.uncaught]

The function std::uncaught_exception() returns true after completing the initialization of the exception object ([except.throw]) until completing the activation of a handler for the exception ([except.handle], [uncaught]). This includes stack unwinding. If the exception is rethrown ([except.throw]), std::uncaught_exception() returns true from the point of rethrow until the rethrown exception is caught again.