HP Integrity NonStop J-Series - Manual (Page 256)

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©Copyright 1996 Rogue Wave Software
The Debug Version of Tools.h++
You can build the Tools.h++ library in a debug mode, and gain a very powerful tool for
uncovering and correcting internal errors in your code.
To build a debug version of the library, you must compile the entire library with the
preprocessor flag RWDEBUG defined. You must compile the entire library and application
with a single setting of the flag - either defined or not defined. The resultant library and
program will be slightly larger and slightly slower. See the appropriate makefile for additional
directions.
The flag RWDEBUG activates a set of PRECONDITION and POSTCONDITION clauses at the
beginning and end of critical functions. These pre- and postconditions are implemented with
asserts. A failure will cause the program to halt after first printing out the offending condition,
along with the file and line number where it occurred.
RWPRECONDITION and RWPOSTCONDITION
In his landmark book Object-oriented Software Construction, Bertrand Meyer suggests regarding
functions as a contract between a caller and a callee. If the caller agrees to abide by a set of
preconditions, the callee guarantees to return results that satisfy a set of postconditions. The
following comparison shows the usefulness of Meyer's paradigm in Tools.h++. Let's look first at
a bounds error in C:
char buff[20];
char j = buff[20]; // Bounds error!
Such a bounds error is extremely tough to detect in C, but easy in C++, as shown below:
RWCString buff(20);
char j = buff[20]; // Detectable bounds error
The bounds error is easy to detect in C++ because the operator[] can be overloaded to perform an
explicit bounds check. In other words, when the flag RWDEBUG is set, operator[] also executes
the PRECONDITION clause, as below:
char& RWCString::operator[](size_t I){
RWPRECONDITION(i < length() );
return rep[i];
}