Add PlantUML script for generating a UML class diagram

.gitignore

@@ -1,32 +1,16 @@
-# Prerequisites
-*.d
+# Generated documentation
+doc/html
+*.png
+
+# Build directory
+build
 
 # Compiled Object files
-*.slo
-*.lo
 *.o
-*.obj
-
-# Precompiled Headers
-*.gch
-*.pch
-
-# Compiled Dynamic libraries
-*.so
-*.dylib
-*.dll
 
 # Fortran module files
 *.mod
 *.smod
 
-# Compiled Static libraries
-*.lai
-*.la
-*.a
-*.lib
-
-# Executables
-*.exe
-*.out
-*.app
+# Executable program
+a.out

README.md

@@ -42,13 +42,20 @@ into submodules.
 As compared to the original code, this repository also adds
 1. A [Fortran Package Manager] build system,
 2. Tests based on the [Vegetables] unit-testing software,
-3. Documentation generated by [FORD] and deployed to the web via GitHub Actions, and
+3. Documentation generated by [`ford`] and deployed to the web via GitHub Actions, and
 4. Quality control via continuous integration testing using GitHub Actions.
 
 Documentation
 -------------
 See [Reference Counter's GitHub Pages site] for HTML documentation generated with [`ford`].
 
+See the [doc/] subdirectory for a [PlantUML] script that generates the Unified Modeling Langauge (UML) class diagram below of the three derived types in reference-counter.  
+
+![class_diagram](https://user-images.githubusercontent.com/13108868/165135689-4d2e85fe-6946-472f-a154-aaabebf6d4f5.png)
+
+The above image was created with the PlantuML package in the [Atom] editor.
+
+
 Compiler Status
 ---------------
 Correct execution of the Reference Counter library code requires comprehensive
@@ -110,5 +117,8 @@ fpm test --compiler flang --flag -cpp
 [3]: https://doi.org/10.1109/MCSE.2012.33
 [Fortran Package Manager]: https://github.com/fortran-lang/fpm
 [Vegetables]: https://gitlab.com/everythingfunctional/vegetables
-[FORD]: https://github.com/Fortran-FOSS-Programmers/ford
+[`ford`]: https://github.com/Fortran-FOSS-Programmers/ford
 [Reference Counter's GitHub Pages site]: https://sourceryinstitute.github.io/reference-counter
+[Atom]: https://atom.io
+[PlantUML]: https://plantuml.com
+[doc/]: ./doc

doc/class_diagram.puml

@@ -0,0 +1,28 @@
+@startuml
+
+title Reference Counter Class Diagram
+
+hide empty members
+
+abstract class resource_t {
+  {abstract} free()
+}
+
+class counter_t {
+  - count_ : integer, pointer
+  - object_ : resource_t
+  grab()
+  release()
+}
+
+abstract class reference_t {
+  - counter_ : counter_t
+  release_handle()
+  start_ref_counter()
+}
+
+reference_t -up-|> resource_t
+reference_t *- counter_t
+counter_t *- resource_t
+
+@enduml