add Command pattern

README.md

@@ -23,8 +23,8 @@ objects).
  - [Flyweight], storage costs of objects
  - [Proxy], how an object is accessed (its location)
 - Behavioral Patterns
- - [Chain of Responsibility], todo
- - [Command], todo
+ - [Chain of Responsibility], object that can fulfill a request
+ - [Command], when and how a request is fulfilled
  - [Interpreter], todo
  - [Iterator], todo
  - [Mediator], todo

command/Command.cpp

@@ -0,0 +1,99 @@
+/*
+ * C++ Design Patterns: Commnand
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT licence
+ * (for more details see LICENCE)
+ *
+ */
+
+#include <iostream>
+
+/*
+ * Receiver
+ * knows how to perform the operations associated 
+ * with carrying out a request
+ */
+class Receiver {
+public:
+  void action() {
+    std::cout << "Receiver: execute action" << std::endl;
+  }
+  // ...
+};
+
+/*
+ * Command
+ * declares an interface for all commands
+ */
+class Command {
+public:
+  virtual ~Command() {}
+  virtual void execute() = 0;
+  // ...
+  
+protected:
+  Command() {}
+};
+
+/*
+ * Concrete Command
+ * implements execute by invoking the corresponding 
+ * operation(s) on Receiver 
+ */
+class ConcreteCommand : public Command {
+public:
+  ConcreteCommand(Receiver *r)
+    : receiver(r) {}
+
+  ~ConcreteCommand() {
+    if (receiver) {
+      delete receiver;
+    }
+  }
+
+  void execute() {
+    receiver->action();
+  }
+  // ...
+  
+private:
+  Receiver *receiver;
+  // ...
+};
+
+/*
+ * Invoker
+ * asks the command to carry out the request
+ */
+class Invoker {
+public:
+  void set(Command *c) {
+    command = c;
+  }
+
+  void confirm() {
+    if (command) {
+      command->execute();
+    }
+  }
+  // ...
+
+private:
+  Command *command;
+  // ...
+};
+
+
+int main()
+{
+  Receiver receiver = Receiver();
+  ConcreteCommand command = ConcreteCommand(&receiver);
+
+  Invoker invoker = Invoker();
+  invoker.set(&command);
+  invoker.confirm();
+
+  return 0;
+}

command/README.md

@@ -0,0 +1,13 @@
+## Command
+
+Command pattern encapsulates a request as an object, thereby letting you parameterize
+clients with different requests, queue or log requests, and supports undoable
+operations. The pattern has a behavioral purpose and deals with relationships between objects. 
+
+### When to use
+
+* want to parameterize objects by an action to perform
+* want to specify, queue, and execute requests at different times
+* support undo
+* support logging changes so that they can be reapplied in case of a system crash
+* structure a system around high-level operations built on primitives operations