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bf(C) supports several ways for a program to react to situations breaking
the normal unhampered flow of a program:
itemization(
it() The function may notice the abnormality and issue a message. This is
probably the least disastrous reaction a program may show.
it() The function in which the abnormality is observed may decide to stop
its intended task, returning an i(error code) to its caller. This is a great
example of i(postponing decisions): now the em(calling function) is faced with
a problem. Of course the calling function may act similarly, by passing the
error code up to em(its) caller.
it() The function may decide that things are going out of hand, and may
call ti(exit) to terminate the program completely. A tough way to handle a
problem if only because the destructors of local objects aren't activated.
it() The function may use a combination of the functions ti(setjmp) and
ti(longjmp) to enforce non-local exits. This mechanism implements a kind
of ti(goto) jump, allowing the program to continue at an outer level,
skipping the intermediate levels which would have to be visited if a series of
returns from nested functions would have been used.
)
In bf(C++) all these i(flow-breaking methods) are still
available. However, of the mentioned alternatives, tt(setjmp) and
tt(longjmp) isn't frequently encountered in bf(C++) (or even in bf(C))
programs, due to the fact that the program flow is completely disrupted.
bf(C++) offers emi(exceptions) as the preferred alternative to, e.g.,
tt(setjmp) and tt(longjmp). Exceptions allow bf(C++) programs to perform a
controlled i(non-local return), without the disadvantages of tt(longjmp) and
tt(setjmp).
Exceptions are the proper way to bail out of a situation which cannot be
handled easily by a function itself, but which is not disastrous enough for
a program to terminate completely. Also, exceptions provide a flexible layer
of control between the short-range ti(return) and the crude ti(exit).
In this chapter exceptions are covered. First an example will be given of the
different impact exceptions and the tt(setjmp/longjmp) combination have on
programs. This example is followed by a discussion of the formal aspects
of exceptions. In this part the guarantees our software should be able
to offer when confronted with exceptions are presented. Exceptions and their
guarantees have consequences for constructors and destructors. We'll encounter
these consequences at the end of this chapter.
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