Added missing test case docs

docs/test_cases/t00018.md

@@ -0,0 +1,136 @@
+# t00018 - Pimpl pattern
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t00018_class:
+    type: class
+    glob:
+      - ../../tests/t00018/**.h
+      - ../../tests/t00018/**.cc
+    using_namespace:
+      - clanguml::t00018
+    include:
+      namespaces:
+        - clanguml::t00018
+
+```
+## Source code
+File t00018_impl.h
+```cpp
+#pragma once
+
+#include "t00018.h"
+
+namespace clanguml {
+namespace t00018 {
+namespace impl {
+
+class widget {
+    int n;
+
+public:
+    void draw(const clanguml::t00018::widget &w) const;
+    void draw(const clanguml::t00018::widget &w);
+    widget(int n);
+};
+}
+}
+}
+
+```
+File t00018.h
+```cpp
+#pragma once
+
+#include <experimental/propagate_const>
+#include <iostream>
+#include <memory>
+
+namespace clanguml {
+namespace t00018 {
+
+namespace impl {
+class widget;
+}
+
+// Pimpl example based on https://en.cppreference.com/w/cpp/language/pimpl
+class widget {
+    std::unique_ptr<impl::widget> pImpl;
+
+public:
+    void draw() const;
+    void draw();
+    bool shown() const { return true; }
+    widget(int);
+    ~widget();
+
+    widget(widget &&);
+
+    widget(const widget &) = delete;
+    widget &operator=(widget &&);
+    widget &operator=(const widget &) = delete;
+};
+}
+}
+
+```
+File t00018.cc
+```cpp
+#include "t00018.h"
+#include "t00018_impl.h"
+
+namespace clanguml {
+namespace t00018 {
+
+void widget::draw() const { pImpl->draw(*this); }
+
+void widget::draw() { pImpl->draw(*this); }
+
+widget::widget(int n)
+    : pImpl{std::make_unique<impl::widget>(n)}
+{
+}
+
+widget::widget(widget &&) = default;
+
+widget::~widget() = default;
+
+widget &widget::operator=(widget &&) = default;
+}
+}
+
+```
+File t00018_impl.cc
+```cpp
+#include "t00018_impl.h"
+#include "t00018.h"
+
+namespace clanguml {
+namespace t00018 {
+namespace impl {
+
+widget::widget(int n)
+    : n(n)
+{
+}
+
+void widget::draw(const clanguml::t00018::widget &w) const
+{
+    if (w.shown())
+        std::cout << "drawing a const widget " << n << '\n';
+}
+
+void widget::draw(const clanguml::t00018::widget &w)
+{
+    if (w.shown())
+        std::cout << "drawing a non-const widget " << n << '\n';
+}
+}
+}
+}
+
+```
+## Generated UML diagrams
+![t00018_class](./t00018_class.png "Pimpl pattern")