I do not understand the reason for this metric/rule:
A function should not be called from more than 5 different functions. All calls within the same function are counted as 1. The rule is limited to translation unit scope.
It appears to me completely unintuitive, because this contradicts code reuse and the approach of split code into often used functions instead of duplicated code.
Can someone explain the rationale?
The first thing to say is that Metric-based quality approaches are by their nature a little subjective and approximate. There are no absolutes in following a metric approach to delivering good quality code.
There are two factors to consider in software complexity. One is the internal complexity, expressed by decision complexity within each function (best exemplified by the Cyclomatic Complexity measure) and dependency complexity between functions within the container (Translation Unit or Class). The other is interface complexity, measuring the level of dependency, including cyclic ones, between collaborating and hierarchical components or classes. In the C/C++ world, this is across multiple TUs. In Structure101 terms, the internal form of complexity is called “Fat” and the external form called “Tangles”.
Back to your question, this Hersteller Initiative Software ‘CALLING’ metric is targeting internal complexity (Fat). Their argument appears to be that if you have more than 5 points of reference to a single function, there may be too much implementation logic in that C++ class or C implementation file, and therefore perhaps time to break into separate modules or components. It seems like a peculiarly stinted view of software design and structure, and the list of exceptions may be as long as the areas where such a judgement might apply.