Getting Started

Client errors

The following is an example of a client error in the use of the String(C++) component:

       #include <String.h>
       main() {
           String x = "hello world";
           char c = x[24];
The client has made an error by attempting to extract the 24th character from a String having only 11 characters (numbered 0 through 10). How should this error be treated?

One possibility would be for the square brackets operator to check its argument and take some well-defined action if the check indicates a bad subscript:

       char String::operator[](unsigned i) {
           if (i >= length of this String ) {
               do something to signal an error
However, a programmer who knows that the operator is being used properly will surely object: ``Why should I have to pay for checking something I already know to be correct?'' For this reason, our components never check for client errors. For the String component, this fact is documented in the form of a precondition on the use of the square brackets operator. The following excerpt is from the String(C++) manpage:
       char operator[](unsigned i);
           Returns character number i.  Precondition:
           i must be less than the length of the String.

Failure to satisfy a precondition leads to undefined behavior. A core dump or an infinite loop might result; even worse, a problem might not turn up until much later in a way that would be difficult to trace.

There are generally two ways to avoid client errors:

Finally, note that while our components do have some preconditions, there are very few of them. In this regard, there is a strong resemblance to the standard C libraries. Instead, the vast majority of operations are total functions, having well-defined behavior for all possible inputs. An example using the Map(C++) component illustrates this. A Map is a set of key-value pairs such that no two pairs have the same key. A Map type has two parameters: the type of the keys and the type of the values. Maps are also known as associative arrays.
       #include <Map.h>
       // Map.h contains template class definitions for
       // instantiating two classes: Map<int,int> and
       // Mapiter<int,int>
       main() {
           Map<int,int> m;
           m[24] = 1;
Initially, the Map m has no elements. Attempting to assign to the 24th element, as in the above example, might be construed as analogous to attempting to access to the 24th character of a short String. However we decided that the effect of the square brackets operator should be well-defined for all possible keys. The effect is as follows: if a given key is referenced and a pair with that key does not exist in the Map, then a new pair will be created with this key. Thus a condition with potentially disastrous (and completely undefined) behavior is transformed into a condition with well-defined (though not necessarily intuitive) behavior.
Next topic: Internal errors
Previous topic: Treatment of errors

© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 -- 02 June 2005