The Shape of Code

The Ada language has now almost completely disappeared from developer consciousness and the Ada Standards’ committee has decided this desperate situation calls for desperate actions.

Undefined behavior in programming languages has been getting a lot of publicity over the last few years. It might not be good publicity, but the Ada committee has taken to heart the old adage ‘The only thing worse than being talked about is not being talked about.’

While the Ada standard does not use the phrase undefined behavior, it does contain a very close equivalent. The Ada community has bitten the bullet and decreed that constructs previously denoted by the term bounded error shall henceforth be referred to as undefined behavior. Technically, constructs generating a bounded error have effectively always been undefined behavior, but the original positioning of Ada as a sophisticated language suitable for critical applications required something less in-your-face unsafe sounding.

Will this minor change of wording (ISO rules are very strict on what changes can be made to published standards without going through long winded voting procedures) make any difference? I guess it will enable Ada users to jump on the ‘undefined behavior’ bandwagon (their claims that Ada was better because it did not contain undefined behavior always has the effect of annoying people, rather than generating any interest in changing languages).

I think there is a bigger public perception problem than the terminology used to denote undefined behavior. Ada Lovelace was born 200 years ago and gets lots of publicity as the first programmer; somehow this association with Lovelace has transferred to the language named after her, generating a perception of an old, fuddy-duddy language (being strongly typed has not helped, but this feature does appear to be coming back into fashion).

The following are some numbers from a while ago (both standards have been revised since these measurements were made).

In the Ada 2005 Standard there are:

36 occurrences of bounded error.
53 occurrences of the subsection header Erroneous execution and 116 occurrences of erroneous.
343 occurrences of implementation-defined.
373 occurrences of may, some of which describe optional behavior.
22 occurrences of must some of which that read as-if shall was intended.
38 occurrences of optional.
zero occurrences of processor dependent and processor-dependent.
1,018 occurrences of shall of which 131 have the form shall not.
452 occurrences of should.
8 occurrences of undefined, one referencing to an undefined range, three having the form undefined
range and the rest occurring in annexes (also see bounded error).
• 89 occurrences of unspecified.

In the C99 Standard there are:

zero occurrences of bounded error.
3 occurrences of erroneous.
163 occurrences of implementation-defined.
862 occurrences of may.
1 occurrence of must.
34 occurrences of optional.
zero occurrences of processor dependent and processor-dependent.
596 occurrences of shall of which 71 have the form shall not.
63 occurrences of should.
183 occurrences of undefined.
98 occurrences of unspecified.

In the C++ 2003 Standard (which now contains many more pages) there are:

zero occurrences of bounded error.
5 occurrences of erroneous.
236 occurrences of implementation-defined.
371 occurrences of may.
111 occurrences of must.
30 occurrences of optional.
zero occurrences of processor dependent and processor-dependent.
779 occurrences of shall of which 211 have the form shall not.
38 occurrences of should.
195 occurrences of undefined.
113 occurrences of unspecified.