design-patterns | code examples discussed as part of design patterns course | Architecture library

The comment from @user207421 is spot on. If a language does not natively support double dispatch, no design pattern can alter the language to make it so. A pattern merely provides an alternative which may solve some of the problems that double dispatch would be applied to in another language.

People learning the Visitor Pattern who already have an understanding of double dispatch may be assisted by explanations such as, "Visitor solves a similar set of problems to those solved by double dispatch". Unfortunately, that explanation is often reduced to, "Visitor implements double dispatch" which is not true.

The fact you've recognized this means you have a solid understanding of both concepts already.

Normally, when people talk about using composition vs. inheritance, they are talking about alternative ways of solving the same problem. In both cases, a "base class" provides an implementation of an interface that you want to reuse in your "derived class"

When you implement this with inheritance, there is an undesirable is-a relationship between the derived class and the base class, with the effect that implementation details of the base class, which should be hidden, can become changes in the derived class class.

When you implement this with composition -- a real composition relationship -- the "derived" only has an is-a relationship with the interface that it wants to implement, and the cost of this is that it must delegate calls to the "base" class.

In the Bridge pattern, which you reference, the goal is a little different. You do want to isolate the containing class from change to the connected implementation, but there is no is-a relationship between the containing class and an interface of the contained class.

The relationship between them may be one of composition, or may be simple aggregation -- that is an implementation detail. Often, the concrete implementation of the contained class will be injected as an interface into the containing class constructor, and in that case the relationship is just aggregation.

You're using the wrong dynamic method. What you want is dynamicMemberLookup. Watch closely. First, the preparation:

No, there is no access to a move from scoring code. That is by design - scoring needs to be independent of moves executed. Any solution has a score, and if two solutions represent the same state of the problem, their scores must equal - therefore the only thing that matters for the purposes of scoring is the state of the solution, not the state of the solver or any other external factor.

Yes whenever you use new keyword to allocate some memory then you must always use delete to free up that memory later when no longer needed. Otherwise you will have a memory leak as in your program. In your case you should use delete inside the destructor in the MallarDuck.cpp .

Other solution would be to use smart pointers like unique_ptr so that you don't have to explicitly free the memory.

unique_ptr`'s syntax would something like:

In general, one needs to be very careful when looking at UML models for design patterns. There are three reasons:

• The reference for design patterns is the GoF. This book was published in 1995, before UML even existed, and used a variation of the OMT notation (an ancestor of UML). In case if doubts, this is the source to trust.
• The Java language design was made public in 1995, JDK 1.0 release in January 1996, and the language not yet widely known. The authors based their designs and code examples on a C++ class model, i.e. on classes and inheritance only, without bothering for multiple inheritance. Most modern languages do not support multiple inheritance. As a consequence, some of the original patterns have to be adapted for Java, C# or Swift, to make a distinction between classes, abstract classes and interfaces, and between inheritance and interface implementation. There are multiple ways to do this. For example, some systematically converted abstract classes to interfaces. Some converted them only when there was a conflict. And some converted according to the interface "spirit".
• Misunderstandings happen. Some people publish such diagrams on the net but misunderstand the pattern, or misunderstand UML, or both together. Also, Gamma et al started the book when they were PhD students and also made some mistakes. You'll find for example some inconcsitencies in their use of the OMT aggregation.
• The worst case is the Builder pattern, where you have not only those problems, but on top of that you have a naming conflict: a very popular Java book introduced a (very different) variant of the builder pattern, with a different intent and another design, which caused total confusion.

Now to your specific example, and based on the content you have provided:

• oodesign is the closest to the original pattern and fully accurate. Btw, I mostly use links to their pages when it comes to patterns.
• refactoring guru is also a good choice. They opted to use interfaces instead of abstract classes, which is fine. But they have redefined the interface's operations and properties (probably in relation to own examples) and require a bidirectional navigation that seems not justified. If you're interested, there is a "Head First: Design patterns" which uses a similar mapping but keeps the original interface.
• wikipedia is not bad either: they have left out the client (which the GoF showed in light gray, to suggest that it's not essential for this pattern); They are more precise and complete on dependencies; But they also redefined the operations and properties differently. And they added an unnecessary aggregation symbol.
• How to make it in Java is confusing, because they kept one abstract class and opted for an interface for the other. And they have hidden the operations and properties. But there's nothing fundamentally wrong at first sight.
• Forget about tutorial points, at least for this UML diagram. It's plain wrong, as @qwerty_so already mentioned in the comments.

The statement of accuracy is for this pattern only and for the current version of the content. This cannot be generalized to all the design patterns in view of my introductory remarks.

Here's an idea

Yes, this works. There's nothing wrong with adding two Bridge relationships to one abstraction (beyond the complexity of juggling three different hierarchies).

Decorator would certainly not work for this purpose, because it maintains a single hierarchy, which is known to the client. The Implementor hierarchy in a Bridge (or hierarchies in this case) are unknown to the client.

I would make a clarification to the linked article, where it says,

You want to extend this [shape] class hierarchy to incorporate colors

I think this oversimplifies the motivation for a Bridge. The Implementors are not just some attributes you choose to add to your Abstraction to enhance it. Your Abstraction requires an Implementor in order to function at all. The method implementations within subclasses of Abstraction generally do little except call methods of the Implementor.

The Abstraction represents your high-level, business API, while the Implementor represents a lower-level, primitive API. They are both abstractions, but at different levels. I don't think this is conveyed adequately by shape & color examples because shape and color seem like abstractions at the same level. Both shape and color would be known to the clients, and neither one strictly depends on the other.

So a Bridge is applied for more specific reasons than the given example, but you certainly can have two.

In essence you don't need to include either header into the other. This fixes the circular includes. In ROS_Topic_Thread.h, friend class Master_Thread; already forward declares Mater_Thread, so you don't need the Master_Thread.h header. In Master_Thread.h, you can also forward declare class ROS_Topic_Thread; before the declaration of the Master_Thread class, since you only use references to ROS_Topic_Thread in the transfer_to_master_buffer method.