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Coupling metrics measure the dependencies between a given entity and other entities the program consists of. The goal of these metrics is to estimate the stability of the whole program considered as the collection of entities (modules, classes etc.).

Gnatmetric computes the following coupling metrics:

Two kinds of coupling metrics are computed:

fan-out coupling (efferent coupling)
the number of entities the given entity depends upon. It estimates in what extent the given entity depends on the changes in “external world”
fan-in coupling (afferent coupling)
the number of entities that depend on a given entity. It estimates in what extent the “external world” depends on the changes in a given entity

Object-oriented coupling metrics are metrics that measure the dependencies between a given class (or a group of classes) and the other classes in the program. In this subsection the term “class” is used in its traditional object-oriented programming sense (an instantiable module that contains data and/or method members). A category (of classes) is a group of closely related classes that are reused and/or modified together.

A class K's fan-out coupling is the number of classes that K depends upon. A category's fan-out coupling is the number of classes outside the category that the classes inside the category depend upon.

A class K's fan-in coupling is the number of classes that depend upon K. A category's fan-in coupling is the number of classes outside the category that depend on classes belonging to the category.

Ada's implementation of the object-oriented paradigm does not use the traditional class notion, so the definition of the coupling metrics for Ada maps the class and class category notions onto Ada constructs.

For the coupling metrics, several kinds of modules – a library package, a library generic package, and a library generic package instantiation – that define a tagged type or an interface type are considered to be a class. A category consists of a library package (or a library generic package) that defines a tagged or an interface type, together with all its descendant (generic) packages that define tagged or interface types. That is a category is an Ada hierarchy of library-level program units. So class coupling in case of Ada is called as tagged coupling, and category coupling - as hierarchy coupling.

For any package counted as a class, its body and subunits (if any) are considered together with its spec when counting the dependencies, and coupling metrics are reported for spec units only. For dependencies between classes, the Ada semantic dependencies are considered. For object-oriented coupling metrics, only dependencies on units that are considered as classes, are considered.

For unit and control coupling also not compilation units but program units are counted. That is, for a package, its spec, its body and its subunits (if any) are considered as making up one unit, and the dependencies that are counted are the dependencies of all these compilation units collected together as the dependencies as a (whole) unit. And metrics are reported for spec compilation units only (or for a subprogram body unit in case if there is no separate spec for the given subprogram).

For unit coupling, dependencies between all kinds of program units are considered. For control coupling, for each unit the dependencies of this unit upon units that define subprograms are counted, so control fan-out coupling is reported for all units, but control fan-in coupling - only for the units that define subprograms.

When computing coupling metrics, gnatmetric counts only dependencies between units that are arguments of the gnatmetric call. Coupling metrics are program-wide (or project-wide) metrics, so to get a valid result, you should call gnatmetric for the whole set of sources that make up your program. It can be done by calling gnatmetric from the GNAT driver with -U option (see The GNAT Driver and Project Files for details).

By default, all the coupling metrics are disabled. You can use the following switches to specify the coupling metrics to be computed and reported:

Report all the coupling metrics
Report tagged (class) fan-out coupling
Report tagged (class) fan-in coupling
Report hierarchy (category) fan-out coupling
Report hierarchy (category) fan-in coupling
Report unit fan-out coupling
Report unit fan-in coupling
Report control fan-out coupling
Report control fan-in coupling