Add the class relationship examples

Author: urbytes21

CMakeLists.txt

@@ -122,6 +122,7 @@ set(APP_SOURCES
     "src/patterns/creational/AbstractFactory.cpp"
     "src/patterns/creational/Builder.cpp"
     "src/patterns/creational/Prototype.cpp"
+    "src/core/datatypes/class/Relationship.cpp"
     ## Exceptions
     "src/core/exception/BasicHandle.cpp"
     "src/core/exception/ThrowNoexcept.cpp"

src/core/datatypes/class/README.md

@@ -15,4 +15,253 @@ operator OP (type_of_X, type_of_Y)
 - It give access to only once specific function.
 ### 1.3. Class
 - Friend Class : I trust this class to see my private data
-- It give access to all member functions of another class.
+- It give access to all member functions of another class.
+
+## 2. Class Relationship
+### 2.1. Composition
+- **Part-of, strong ownership**
+- The member is part of the class
+- The member can only belong to one class at a time
+- The member has its existence managed by the class
+- The member does not know about the existence of the class
+
+- UML:
+```cpp
++------------------------+
+|          Car           |
++------------------------+
+| - engine : Engine      |
++------------------------+
+            ♦
+            │
+            │
++------------------------+
+|         Engine         |
++------------------------+
+| + Engine()             |
+| + ~Engine()            |
++------------------------+
+
+class Engine
+{
+public:
+    Engine() { std::cout << "Engine created\n"; }
+    ~Engine() { std::cout << "Engine destroyed\n"; }
+};
+
+class Car
+{
+private:
+    Engine engine; // composition
+
+public:
+    Car() { std::cout << "Car created\n"; }
+    ~Car() { std::cout << "Car destroyed\n"; }
+};
+```
+
+## 2.2. Aggregations
+- **Has-a, weak ownership**
+- The member is part of the class
+- The member can belong to more than one class at a time
+- The member does not have its existence managed by the class
+- The member does not know about the existence of the class
+
+- UML:
+```cpp
++-----------------------------+
+|        Department           |
++-----------------------------+
+| - teacher : Teacher*        |
++-----------------------------+
+            ◇
+            │
+            │
++-----------------------------+
+|           Teacher           |
++-----------------------------+
+| - name : std::string        |
++-----------------------------+
+
+class Teacher
+{
+public:
+    std::string name;
+    Teacher(const std::string& n) : name(n) {}
+};
+
+class Department
+{
+private:
+    Teacher* teacher; // aggregation
+
+public:
+    Department(Teacher* t) : teacher(t) {}
+};
+```
+
+## 2.3. Associations 
+- **Uses-a, loose relationship**
+- The associated member is otherwise unrelated to the class
+- The associated member can belong to more than one class at a time
+- The associated member does not have its existence managed by the class
+- The associated member may or may not know about the existence of the class
+- UML
+```cpp
++---------------------+                 +---------------------+
+|       Doctor        |-----------------|       Patient       |
++---------------------+                 +---------------------+
+| + treat(p:Patient&) |                 | - name : string     |
++---------------------+                 +---------------------+
+
+class Patient
+{
+public:
+    std::string name;
+};
+
+class Doctor
+{
+public:
+    void treat(Patient& p)
+    {
+        std::cout << "Treating " << p.name << "\n";
+    }
+};
+```
+
+## 2.4. Dependency
+- **Denpends-on**
+-  One class uses another class to perform a task. 
+-  It is temporarily created, used, and then destroyed, or passed into a member function from an external source.
+- UML
+```cpp
++---------------------+
+|         Car         |
++---------------------+
+| + start()           |
++---------------------+
+           - - - - - - - - - - - - - - - - - - >
+                           +---------------------+
+                           |       Logger        |
+                           +---------------------+
+                           | + log(msg:string)   |
+                           +---------------------+
+class Logger
+{
+public:
+    void log(const std::string& msg)
+    {
+        std::cout << msg << std::endl;
+    }
+};
+
+class Car
+{
+public:
+    void start()
+    {
+        Logger logger;  // dependency
+        logger.log("Car started");
+    }
+};
+
+```
+## 2.5. Container
+-  The class one class provides a container to hold multiple objects of another type
+- UML
+```cpp
+#include <vector>
+
++-----------------------------+
+|          Library1           |
++-----------------------------+
+| - books : vector<string>    |
++-----------------------------+
+              ♦
+              │
+              │
++-----------------------------+
+|          string             |
++-----------------------------+
+class Library1
+{
+private:
+    std::vector<std::string> books; // copy values
+};
+
++-----------------------------+
+|          Library2           |
++-----------------------------+
+| - teachers : vector<T*>     |
++-----------------------------+
+              ◇
+              │
+              │
++-----------------------------+
+|           Teacher           |
++-----------------------------+
+class Library2
+{
+private:
+    std::vector<Teacher*> teachers; // store pointers
+};
+
+```
+
+## 2.6. Inheritance
+- **Is-a**
+```cpp
++----------------------+
+|        Animal        |
++----------------------+
+| + eat()              |
++----------------------+
+           △
+           │
+           │
++----------------------+
+|         Dog          |
++----------------------+
+| + bark()             |
++----------------------+
+class Animal
+{
+public:
+    void eat() { std::cout << "Eating\n"; }
+};
+
+class Dog : public Animal
+{
+public:
+    void bark() { std::cout << "Woof\n"; }
+};
+```
+
+## 2.7 Embedded a.k.a Nested/Inner Class
+- Type-level containment
+```cpp
++----------------------+
+|        Car           |
++----------------------+
+|                      |
++----------------------+
+           (+)
+           │
+           │
++----------------------+
+|         Engine       |
++----------------------+
+| + start()            |
++----------------------+
+
+class Car
+{
+public:
+    class Engine
+    {
+    public:
+        void start();
+    };
+};
+```