Tools.h++ Manual

104011 Tandem Computers Incorporated 18-7

Note – Note that the line between an internal and external error can sometimes

be fuzzy. For example, the rules could say “don’t give me an invalid date” and

then the programmer would be responsible for detecting a bad date before

using a Tools.h++ routines. Of course, this is a lot of work and probably the

reason why you bought the library in the ﬁrst case: the

RWDate

object is

probably in a much better position than you to detect invalid dates.

In theory, the response to an external error is either to throw an exception or to

provide a test for object validity. It should never abort the program. However,

in practice, exceptions have not been widely adopted by compilers and so

Tools.h++ provides an opportunity to either test for a status value or to recover

in an error handler.

Here’s an example:

18.3 Exception architecture

When an exception is thrown a throw operand is passed. The type of the throw

operand determines which handlers can catch it. Tools.h++ uses the following

hierarchy for throw operands:

RWDate date;

while (1)

{

cout << “Give a date: “;

cin >> date;

if (date.isValid()) break;

cout << “You entered a bad date; try again\n”;

}

xmsg

RWxmsg

RWInternalErr

RWBoundsErr

RWExternalErr

RWFileErr

RWStreamErr

xalloc

RWxalloc