Design-Pattern | java中二十三种设计模式讲解，配合demo实现

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.

Ok i think the comments are right, the way i see the solution is the following :

No Setter, the only way to interact with the object will be with function with a purpose. For exemple, modify HP, add a DoDamage function that will return false if the ship is destoyed and will internally modify the hp (and maybe the damage too, etc..)

Getter can be public, but of course, only "const &"

Clone method is a good idea for future development (prototype pattern)

As far as I know, the book AntiPatterns by Brown et al, 1998 may have been the first to popularise the term. It defines an anti-pattern like this:

"An AntiPattern is a literary form that describes a commonly occurring solution to a problem that generates decidedly negative consequences."

I think that it's worthwhile to pay attention to the phrase decidedly negative consequences. Most solutions come with some disadvantages, but in order for a 'solution' to be an anti-pattern, the disadvantages must clearly outweigh any advantages produced.

I usually look at it another way. If I can solve the problem in a different way that generates at least as many advantages, but fewer disadvantages, then the first 'solution' might be an anti-pattern.

As described in the GoF book, the Singleton design pattern addresses a particular problem: Sometimes you want to ensure that only one instance of a class can exist.

This can be appropriate for performance reasons. If the object holds a large amount of data, making sure that only one exists could improve performance.

Even if the object doesn't hold a large amount of memory, you may want to make a small immutable object a Singleton if you otherwise expect that client code might instantiate millions of identical objects.

Particularly when the Singleton object represents immutable data it's hard to argue that it has decidedly negative consequences. It's basically just a constant, like string.Empty. The Singleton pattern is a good solution to that kind of problem.

On the other hand, many people use mutable Singletons as a replacement for techniques like Dependency Injection or partial function application. When used in this way, a Singleton becomes a global variable, which most people nowadays agree should be avoided.

Since Singleton has valid uses, I don't think it can be called an anti-pattern. On the other hand, I'd consider a mutable Singleton a code smell bordering on anti-pattern.

The two are compatible patterns that address different problems, Sagas handle workflow processes where as event sourcing addresses how state is stored. Sagas provide a mechanism for handling a multi-step process and rolling back should steps fail (like a workflow). Where as Event Sourcing is the process of encoding the state of an entity by recording all its past changes.

Lets say we are booking a holiday, we need to book flights, a hotel and hire a car. each of these processes is handled by a different microservice.

We could create a microservice named BookingSaga which would be responsible for keeping track of the state of each booking. When we make a booking the BookingSaga service would

• book the hotel
• book the flight
• book the car

these can reply in any order, but if any one fails the BookingSaga would then begin a rollback and cancel any that had already been booked.

https://microservices.io/patterns/data/saga.html

Event sourcing keeps track of the state of some entity by recording the changes that have happened to it.

• Object A Name changed to "dave"
• Object A Age Changed to 3
• Object A Named changed to "sue"

So we can see that Object A has a name of "sue" and an Age of 3 at the end of all the events. https://microservices.io/patterns/data/event-sourcing.html

As the error says, you are trying to use an object as a react component (childProp.title, and childProp.content). It seems you want to use their children as "content".

A fundamental trade-off between these choices in a broad sense has a name: "the expression problem". You should find plenty on Google under that name, both in general and in the context of Rust.

In the context of the question, the "problem" is to write the code in such a way that both adding a new state and adding a new operation on states does not involve modifying existing implementations.

When using a trait object, it is easy to add a state, but not an operation. To add a state, one defines a new type and implements the trait. To add an operation, naively, one adds a method to the trait but has to intrusively update the trait implementations for all states.

When using an enum for state, it is easy to add a new operation, but not a new state. To add an operation, one defines a new function. To add a new state, naively, one must intrusively modify all the existing operations to handle the new state.

If I explained this well enough, hopefully it should be clear that both will have a place. They are in a way dual to one another.

With this lens, an enum would be a better fit when the operations on the enum are expected to change more than the alternatives. For example, suppose you were trying to represent an abstract syntax tree for C++, which changes every three years. The set of types of AST nodes may not change frequently relative to the set of operations you may want to perform on AST nodes.

With that said, there are solutions to the more difficult options in both cases, but they remain somewhat more difficult. And what code must be modified may not be the primary concern.

It is used for passing data into views. The with method returns an instance of the view object so that you can continue chaining methods before returning the view. All of the syntax below archives the same thing:

The answer is the user who has the ShoppingCart (ShoppingCart cart = new ShoppingCart();)

I don't know exactly you mean

We need the interface PaymentStrategy because we need use Polymorphism to implement many way to pay (paypal or credit card), let's change the source a bit and you can see clearer:

There isn't a real generic answer to this question. It all depends on the structure of your database, span of the transactions across entities, distributed transactions, how much time/transactions are allowed to pass before your can revert the change, etc.

Memento Pattern is one of the possible approaches, however it needs to be modified due to the nature of the relational databases as follows:

1. You need to have transaction log table/list that will hold the information of the entities and attributes (tables and columns) that ware affected by the transaction with their primary key, the old and new values (values before the transaction had occurred, and values after the transaction) as well as datetimestamp. This is same with the command (memento) pattern.
2. Next you need a mechanism to identify the non-explicit updates that ware triggered by the stored procedures in the database as a consequence of the transaction. This is important, since a change in a table can trigger changes in other tables which ware not explicitly captured by the command.
3. Mechanism for rollback will need to determine if the transaction is eligible for roll-back by building a list of subsequent transactions on the same entities and determine if this transaction is eligible for roll-back, or some subsequent transaction would need to be rolled-back as well before this transaction can be rolled-back.
4. In case of a roll-back is allowed after longer period of time, or a near-realtime consumption of the data, there should also be a list of transaction observers, processes that need to be informed that the transaction is no longer valid since they already read the new data and took a decision based on it. Example would be a process generating a cumulative report. When transaction is rolled-back, the rollback will invalidate the report, so the report needs to be generated again.

For a short term roll-back, mainly used for distributed transactions, you can check the Microservices Saga Pattern, and use it as a starting point to build your solution.

Another approach is to keep incremental updates or also known as history tables. Where each update of the row will be an insert in the history table with new version. Similar to previous case, you need to decide how far back you can go in the history when you try to rollback the committed transaction.

Finally, when you work with business data such as invoice, inventory, etc. you also need to check what are the regulations related with the cancelation of committed transactions. As example, in the accounting systems, it's not allowed to delete data, rather a new row with the compensation is added (ex. removing product from shipment list will not delete the product, but add a row with -quantity to cancel the effect of the original row and keep audit track of the change at the same time.