Code complexity metrics and ifdefs

Question

I was wondering how standard code complexity metrics (e.g., LOC, McCabe cyclomatic complexity, Halstead metrics, etc) are computed in the face of ifdefs in C/C++ code.

If ifdefs are ignored, syntax errors (or even type errors) may arise, such as in:

#ifdef FOO
for(i = 0; i < x; i++) {
#else
for(i = 0; i < y; i++) {
#endif
printf(...)
}

If lines 1, 3 and 5 are ignored, the resulting code would have one loop inside the other, and a missing '}' (thus the syntax error).

If #ifdefs are taken into account, then one would need a single resulting measure per variant (in this case, one when FOO is defined, and another one for the case when FOO is not defined). However, such an approach can easily explode in practice.

Looking at http://manpages.ubuntu.com/manpages/natty/man1/pmccabe.1.html, the manpage reports that:

Parsing pmccabe ignores all cpp preprocessor directives - calculating the complexity of the appearance of the code rather than the complexity after the preprocessor mangles the code. This is especially important since simple things like getchar(3) expand into macros which increase complexity.

But, as I said, this approach may lead to code errors to appear, which in turn, may hinder the calculation process or result in an incorrect value.

How do tools overcome this problem (if they overcome at all)?

Regards.

Fred Foo Fred Foo · Accepted Answer · 2013-02-25T16:16:14

If you rewrite that construct as

#ifdef FOO
# define LOOPEND x
#else
# define LOOPEND y
#endif

or something like that, there should be no problem.

If you don't want to do that, and your complexity checker doesn't understand the preprocessor, you can run it on the output of the preprocessor, both with FOO defined and with FOO undefined, then take the max of the values it produces to get a lower bound on the actual complexity. With GCC, the preprocessor is invoked with gcc -E; other compilers will have a similar option.

(Of course, if you have a lot of this CPP wizardry, you're left with an exponential number of possibilities. But that's the price you pay for using the preprocessor; the same applies to testing.)

Code complexity metrics and ifdefs

3 Answers