As control passes from the point where an exception is thrown to a handler, destructors are invoked for all automatic objects constructed since the try block was entered. The automatic objects are destroyed in the reverse order of the completion of their construction.
An object of any storage duration whose initialization or destruction is terminated by an exception will have destructors executed for all of its fully constructed subobjects (excluding the variant members of a union-like class), that is, for subobjects for which the principal constructor ([class.base.init]) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object's destructor will be invoked. If the object was allocated in a new-expression, the matching deallocation function ([basic.stc.dynamic.deallocation], [expr.new], [class.free]), if any, is called to free the storage occupied by the object.
The process of calling destructors for automatic objects constructed on the path from a try block to the point where an exception is thrown is called “stack unwinding.” If a destructor called during stack unwinding exits with an exception, std::terminate is called ([except.terminate]). [ Note: So destructors should generally catch exceptions and not let them propagate out of the destructor. — end note ]