Added visitor pattern test case

docs/test_cases/t00020.md

@@ -0,0 +1,100 @@
+# t00020 - Abstract factory pattern
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t00020_class:
+    type: class
+    glob:
+      - ../../tests/t00020/t00020.cc
+    using_namespace:
+      - clanguml::t00020
+    include:
+      namespaces:
+        - clanguml::t00020
+    plantuml:
+      after:
+        - '@A(ProductA1) <.. @A(Factory1)'
+        - '@A(ProductB1) <.. @A(Factory1)'
+        - '@A(ProductA2) <.. @A(Factory2)'
+        - '@A(ProductB2) <.. @A(Factory2)'
+
+```
+## Source code
+File t00020.cc
+```cpp
+#include <memory>
+
+namespace clanguml {
+namespace t00020 {
+
+class ProductA {
+public:
+    virtual ~ProductA() = default;
+    virtual bool sell(int price) const = 0;
+};
+
+class ProductA1 : public ProductA {
+public:
+    bool sell(int price) const override { return price > 1000; }
+};
+
+class ProductA2 : public ProductA {
+public:
+    bool sell(int price) const override { return price > 2000; }
+};
+
+class ProductB {
+public:
+    virtual ~ProductB() = default;
+    virtual bool buy(int price) const = 0;
+};
+
+class ProductB1 : public ProductB {
+public:
+    bool buy(int price) const override { return price < 1000; }
+};
+
+class ProductB2 : public ProductB {
+public:
+    bool buy(int price) const override { return price < 2000; }
+};
+
+class AbstractFactory {
+public:
+    virtual std::unique_ptr<ProductA> make_a() const = 0;
+    virtual std::unique_ptr<ProductB> make_b() const = 0;
+};
+
+class Factory1 : public AbstractFactory {
+public:
+    std::unique_ptr<ProductA> make_a() const override
+    {
+        return std::make_unique<ProductA1>();
+    }
+
+    std::unique_ptr<ProductB> make_b() const override
+    {
+        return std::make_unique<ProductB1>();
+    }
+};
+
+class Factory2 : public AbstractFactory {
+public:
+    std::unique_ptr<ProductA> make_a() const override
+    {
+        return std::make_unique<ProductA2>();
+    }
+
+    std::unique_ptr<ProductB> make_b() const override
+    {
+        return std::make_unique<ProductB2>();
+    }
+};
+}
+}
+
+```
+## Generated UML diagrams
+![t00020_class](./t00020_class.png "Abstract factory pattern")