Command in C++ Design Patterns

Table Of Content

• Classes and Methods
• Command Pattern Class Diagram
• Command Pattern Examples in C++
• Code Examples
• Design Patterns by Tutorials
• UML class diagram
• UML class and sequence diagram

Create a class file named OpenCommand.cs, then copy and paste the following code. Second, it has a constructor, which initializes the Receiver Object. Third, it has the Execute method, which calls the Open method of the Receiver Object, i.e., the Document object Open Method.

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Classes and Methods

If the Invoker calls the Execute method on the Open Command Object, then the Open Method of the Document Object will be executed. Similarly, if the Invoker calls the Execute method on the Save Command Object, then the Save Method of the Document Object will be executed. Concrete command classes implement the Command interface. Each class encapsulates a specific operation related to devices.

Command Pattern Class Diagram

For recreating the deleted items, you have to reset the flag again. While relatively easy to implement initially, adding this mechanism to an existing code base may require some effort as you must update most of your queries. A class is a blueprint that describes the shape and behavior of an object. An object is an instance of a class created during runtime. If we want to know which objects exist in our application, we could store them in a list to keep track of them.

Command Pattern Examples in C++

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Then, we create an instance of the Invoke, i.e., MenuOptions, bypassing the command objects as parameters. The Command Pattern finds its stride in scenarios where the sender and receiver of a request should dance to their own tunes without stepping on each other’s toes. This pattern proves invaluable in systems requiring command queuing, undo/redo functionalities, or the implementation of high-level operations built on a foundation of lower-level actions. A concrete command class must be written and passed to the invoker.

Code Examples

Hence, this pattern allows you to model the concept of executing an action. After we discussed the disadvantages of the poorly constructed code, let’s have a look at a refactored code leveraging the Command Pattern. It remains in a queue until the chef is ready to serve it. The order contains all the relevant information required to cook the meal.

CommandPatternDemo, our demo class, will use Broker class to demonstrate command pattern. CommandAn interface declared with an execute() method that individual commands would implement. It can also have an unexecute() method for unexecuting the last executed operation. For the sake of simplicity, we leave unexecute() out of the picture and focus just on the execute() method.

Only difference is that in command there is one request handler, and in chain of responsibility there can be many handlers for single request object. Command Pattern is one of the Behavioral Design Pattern. Command design pattern is used to implement loose coupling in a request-response model.

UML class diagram

To do this, the Command class must maintain the state for reversing its effects and implement an undo method that restores the object to its previous state. In command pattern, the invoker is decoupled from the action performed by the receiver. The invoker invokes a command, and the command executes the appropriate action of the receiver. Thus, the invoker can invoke commands without knowing the details of the action to be performed. In addition, this decoupling means that changes to the receiver’s action don’t directly affect the invocation of the action. Now we have receiver and command implementations ready, so we can move to implement the invoker class.

Its elegance lies in providing a flexible framework for orchestrating complex actions without the need for tight coupling. This C++14 implementation is based on the pre C++98 implementation in the book.

Create a class file named Document.cs and copy and paste the following code. Here, we created the receiver class with three methods, i.e., Open, Save, and Close. The Receiver class contains the business logic to perform the actual actions. The following methods will be executed whenever the Invoker calls the Execute Method on the command object.

This real-world code demonstrates the Command pattern used in a simple calculator with unlimited number of undo's and redo's. Sequences of Command objects can be assembled into composite (or macro)commands. Consider we have a universal remote with four buttons On, Off, Up, and Down. This remote can be configured for any device such as a light, speaker, air conditioner, and the buttons can be configured for the respective device's use-cases. In the following examples, we first explore a poorly designed scenario that lacks separation of concerns and violates key principles of good software design. Subsequently, we showcase a refactored version using the Command Pattern, addressing the identified issues.