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FORTRAN90 is not an object-oriented language like Java or C++, and as such does not implement those languages' characteristic properties of inheritance. But FLASH takes advantage of the Unix directory structure to implement an inheritance hierarchy of its own. Every child directory in a unit's hierarchy inherits all the source code of its parent, thus eliminating duplication of common code. During setup, source files in child directories override same-named files in the parent or ancestor directories.
Similarly, when the setup tool parses the source tree, it treats each child or subdirectory as inheriting all of the Config and Makefile files in its parent's directory. While source files at a given level of the directory hierarchy override files with the same name at higher levels, Makefiles and configuration files are cumulative. Since functions can have multiple implementations, selection for a specific application follows a few simple rules applied in order described in Architecture Tips.
However, we must take care that this special use of the directory structure for inheritance does not interfere with its traditional use for organization. We avoid any problems by means of a careful naming convention that allows clear distinction between organizational and namespace directories.
To briefly summarize the convention, which is described in detail online in Architecture Tips, the top level directory of a unit shares its name with that of the unit, and as such always begins with a capital letter. Note, however, that the unit directory may not always exist at the top level of the source tree. A class of units may also be grouped together and placed under an organizational directory for ease of navigation; organizational directories are given in lower case letters. For example the grid management unit, called Grid, is the only one in its class, and therefore its path is source/Grid, whereas the hydrodynamics unit, Hydro, is one of several physics units, and its top level path is source/physics/Hydro. This method for distinguishing between organizational directories and namespace directories is applied throughout the entire source tree.