golang interface to struct


// will implement all the methods defined within our `Store` interface. (T) asserts that the dynamic type of x is identical to the type T. Types CommandID and Field are not identical as described in Type identity. Interface values have an underlying type. In the following, Ill be referring to both functions and methods. An interface{} is a collection of type implementable method signature. Note how the Sync error method from the Core interface is never redefined. // the recyclable interface. @wangjun No, the composition is the only way to do it. The type operator accepts expressions as arguments. The following characteristics are worth highlighting: That last point is worth exploring a bit. The implication is that our own interfaces only work against our own code base. However, the underlying type does have value nil. Ill assume that you have at least a passing familiarity with the Go programming language and interfaces. Also, since all weapons can be equipped/unequipped, we can define a separate interface equippable that describes this behavior and embed it into both meleeWeapon and rangedWeapon: If we hadnt embedded equippable in this way, we would have to perform a runtime interface conversion each and every time we wanted to equip or unequip a weapon, which is tedious and repetitive work. Show that involves a character cloning his colleagues and making them into videogame characters? Worse, each repository would need to implement all three methods and return an error for the two methods that arent implemented. I could effectively define a client package that would handle all the implementation details needed to talk to this external API, in my user service I could define another interface such as APIClient that would define all the methods this client package would need to implement. // University is defined in package schools. Only your own code can do that. Because weve assigned *myError to the variable err, which is an interface type. Whoever is not aware of what an empty interface is should read this excellent article: https://research.swtch.com/interfaces. Example: Code snippet 20 Example in the Playground. Comments and implementation details from the original source code has been left out for brevity, but feel free to visit the public repository on GitHub if you are curious about how the methods are implemented. An interface and a type are structurally equivalent if they both define a set of methods of the same name, and where methods from each share the same number of parameters and return values, of the same data type. If that didnt make sense, then just read on. For conversion of interface{} to a struct, we will use the library https://github.com/mitchellh/mapstructure . Was there a Russian safe haven city for politicians and scientists? External users of your package can reference the interface but cant embed it in their own types. In the first line of main, we create a new acmeToaster. In this article, we are going to be covering the concept of accepting interfaces and returning structs and how this can help improve your code to make it more testable as well as maintainable. 2021 Weve changed the return type of doSomething from *myError to error. Call any method defined as part of its interface. They are more like a contract that dictates that we can use whatever value wed like, as long as that value satisfies the interface. The fact that there isnt an easy way to cover all your bases, though, is something that has been a source of debate in the Go community for a while. We need to realize that the method receiver is not part of the interface contract and is therefore a good candidate for handling the varying part. This is perfectly valid. Its therefore preferable to always use the smallest possible interface that still has a meaningful use case. To learn more, see our tips on writing great answers. Why can't I assign a *Struct to an *Interface? Its not possible to satisfy School because it contains an unexported method, students. Thanks for contributing an answer to Stack Overflow! Welcome to Encodingcompiler Q&A, where you can ask questions and receive answers from expert members of the community. For example, if I needed to talk to a downstream API, I could follow this same approach as I have done above. Example: Code snippet 08 Example in the Playground. In the example below, cat and dog both satisfy the speaker interface, although cat does it via a pointer receiver and dog does it via a value receiver. Generally speaking, doFoo will not know whether it received a function parameter of type itemA or itemB, nor should it care. Sure, if we always know what were passing along then we can assume a type and force a type conversion. Gos runtime will check if the underlying type satisfies the interface we wish to convert to, and will let us know if the conversion was possible. Lets look at a (simplified) example in Java[7]: Car must explicitly declare that it implements Vehicle. Interfaces enable polymorphism[4], i.e. Interfaces are not concrete values. Despite this difference in implementation, function doSpeak is able to call speak on both arguments, without discrimination. Example: Code snippet 18 Example in the Playground. I will change my struct definition. When I was first introduced to interfaces in Java, they seemed immediately useful: they allowed polymorphism without requiring each type to inherit from the same base class, as in C++. Let's look at some examples of using this type of consent function in setup and address them below: Note that from the blogpost above, we need to make sure we have saved the Data area in the expected format. By the way, is there another way to implement inheriting, except compose? A defined type is always different from any other type. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Another interesting use case comes from Ubers logging package Zap, where interfaces are used to extend existing functionality by wrapping a Core struct in a struct that embeds the Core interface and then implements the necessary methods to provide new functionality for adding hooks (function callbacks) to the core data type. // New - our constructor function that takes in a pointer to a database. In the second line, we pass acmeToaster to doToast. However, in line two we call attackOrDefend which takes an argument of type interface{}. Well call the new interface Hydrater because its argument is a database row and it must be implemented by populating/hydrating the individual fields with their respective columns from that row: And take a look at the implementation for entity User: Code snippet 33 Example in the Playground using a custom Row and Scan method. Building Production Ready Services in Go - 2nd Edition - if you would like to see this in a full Go app, then feel free to subscribe and check out the course! Basically, were now returning the interface value nil instead of a nil *myError.

Find centralized, trusted content and collaborate around the technologies you use most. Lets try embedding interface School: This works as long as we dont attempt to redefine method students. This becomes important when we check for nil values. The empty interface matches anything and non-empty interfaces can be converted into both specific types and named interfaces, but also anonymous interfaces and ad-hoc interfaces with a different structure. Generally, we can do three things with an interface argument: Weve already covered the first item. An interface represents a set of methods that can be called on any given type that satisfies the interface. but it not working.I have tried to use unsafe.Pointer but failed also. This is how the above example would look in Go: If Car was defined in a third-party package (outside of our immediate control), Vehicle could still be used in our own code base anywhere where a Car might be accepted. // we can then call our method and then run assertions that the business logic, // defined within this method is working as we expect it to, "sad path - errors can be handled properly", Working with Websockets and Socket.IO in Go - Tutorial, Go WebAssembly Tutorial - Building a Calculator Tutorial, Go Encryption and Decryption using AES - Tutorial, Building a Solid Continuous Integration Pipeline with TravisCI for Your Go Projects, Creating Real-Time Chat and Activity Systems With Getstream.io, Getting Starting With Go Generics - Tutorial, Writing a Frontend Web Framework with WebAssembly And Go, Go Sorting With the sort Package - Tutorial, Containerizing your Go Applications with Docker - Tutorial, Working with Temporary Files and Directories in Go 1.11, Building a Network Command Line Interface in Go, Building a Basic REST API in Go using Fiber, An Introduction to Go Closures - Tutorial, An Introduction to Benchmarking Your Go Programs, Getting Started with Redis and Go - Tutorial, Golang Integer String Conversion Tutorial, Checking if a string contains a sub-string in Go, Type Casting an Interface to a String in Go, Getting the Size of an Array or Slice in Go, Improving Your Go Tests and Mocks With Testify, Improving Your Go Development Workflow With Git Hooks, Building a Real-time YouTube Subscriber Monitor in Go, Building Production Ready Services in Go - 2nd Edition, Building a Production-Ready REST API in Go. Announcing the Stacks Editor Beta release! a type doesnt need to declare that it must satisfy the interface, then how do we know if the type satisfies the interface? // func (u University) students() []string { // Can't redefine. This can be particularly useful to avoid unnecessary runtime conversions for types that share common behavior. TutorialEdge is a rapidly growing site focused on delivering high quality, in-depth courses on Go. With this new approach, our user package no longer imports or necessarily cares about the db package used in the first example. Using the above approach you can then employ either mocks or fakes in your unit tests to exercise your user package with actually hitting these downstream APIs which could prevent you burning API credits or hitting rate limits. If a creature's best food source was 4,000 feet above it, and only rarely fell from that height, how would it evolve to eat that food? This stands in contrast to explicit interfaces that must be referenced by name from the type that implements it[5]. We could use tools such as golang/mock in order to generate mocked implementations of our Store interface and then use these mocks within our test. Then how do you check if the item in the box (the value of the underlying type) is nil? Lets imagine we get a requirement from our companies' product team that we need to move to a different type of backing store for any arbitrary reason. Consequently, both of these types can be embedded in player (and interchanged). If we call it, its the Sync method of the embedded type that gets called due to method promotion. By setting up these expectations, we can very quickly exercise both the happy paths and sad paths within our UpdateUserPreferences method and we do not need a Postgres instance running at all for this. It should be noted that the abstractions weve covered in the above code snippets dont necessarily come for free. In line 5, we make an interface type object, Person. Thus, it makes no sense to use an interface as a pointer. Lets say that we introduced a new type that we also wanted to get the status of: But sword is not a character, and it makes little sense to provide sword with a sleep method, which would be a requirement for it to work with the getStatus function. References: [10] [11]. This form of embedding is particularly useful for composing larger structs from independent and interchangeable parts. Thanks for your answer and edition. Lets look at two scenarios: Code snippet 05 Example in the Playground. Have a look at Type assertions chapter in Go reference. Why? The interface conversion inside maybeDoToast is a runtime check that may fail. If we wanted to test our UpdateUserPreferences method we would then have to ensure that a locally running Postgres instance is available and that we have set up all the required environment variables we need in order to run our tests. We have now modified our package in an incredibly subtle way, however this approach unlocks our ability to do things like unit-test this package and ensure that, no matter what our store returns, we are handling it appropriately. They can be composed and used without fully implementing their methods. In Go, the compiler is quick to complain if a variable is unused, but if we assign to a variable named _ (underscore), the compiler wont complain as that special variable is used specifically for discarding unused values. Learn more about Collectives on Stack Overflow, How APIs can take the pain out of legacy system headaches (Ep. They are more flexible and more powerful than their counterparts in other languages. We come around a situation sometimes in programming where an empty interface might a struct internally and we have to get the concrete struct out of it. In essence, we force implementations to embed types exported by our own package and any call to such a method will always be handled internally, by our own packages implementation. if we have a car, do A, if we have a bus, do B etc. Take interface School as an example: Type University aims to satisfy interface School, but fails to do so. // containing r. Here, v is still just a vehicle. Note > I follow this same mantra in my new course By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. In Go, there is no such restriction. The trick is to not make any assumptions about them and check them every step of the way. When writing Go applications, one of the key things I like to keep in mind is how can I make this particular function as testable as possible?. Therefore, we can call recycle() on it! Other solution is to introduce a Field interface with Set and Get methods. How should I do for converting interface{} to Field(CommandID, Language) in InitField() function? Here is a contrived example from a fantasy role playing game: Code snippet 14 Example in the Playground. An interface is like a box. To support and avoid inconsistent types we can proceed using the type syntax below: In this case, the ok value is true if the statement is true. Why do you accept an empty interface in InitField if it can only work with SipField in the first place? In the second case, it means that there is an underlying type, but that the value of that type is nil (such as a pointer or an uninitialized slice). The benefit of this approach becomes more and more apparent as the underlying code you are trying to test becomes more and more complex. Its the underlying type that always changes, based on which value we provide in place of the interface. There will always be exceptional circumstances and edge cases that make this approach impossible. Some examples are json.Unmarshal in the encoding package and Row.Scan in the database package. This also means that we lose type safety. It works today, but if we introduce a new vehicle (say, a truck) into the code base tomorrow, well need to manually find and update these volatile sections and make sure that we handle truck accurately in each case, before the code returns to a reliable operational state. At least not while it knows nothing else about the parameter item. We lose type safety, and we would have to manually convert the first return value into the correct entity, whether its User, Address, PurchaseOrder or other. We kind of have to turn the concept on its head in order to handle the part of the interface methods that varies between implementations. Scientifically plausible way to sink a landmass. Another benefit of this approach is the ability to easily define and swap our concrete implementations of our Store interface. The empty interface says nothing and should be avoided except in rare situations where we really dont know what we are receiving. Well, to beging with, we would need to create a new *db.Database struct. Its a way of reusing smaller interfaces to create larger ones via composition. New videos are added at the end of every week and a roughly 10% of the site's revenue goes towards tackling climate change through tree planting and carbon capture initiatives. Here is an example from our role playing game: Code snippet 21 Example in the Playground. Code snippet 11 Example in the PlayGround. // return []string{"George", "Ben", "Louise", "Calvin"}. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. But programs change and what works today may crash tomorrow. In this example, the persona interface is embedded in player. This is where it gets complicated. This approach allows us to implement a second package that handles all the implementation details when it comes to talking to this new database type. This is already pretty neat, but we can also check dynamically for the availability of certain methods. All this code is focused on now is the business logic surrounding being able to UpdateUserPreferences. To achieve this, the repository method can accept a named interface value that can be implemented by any type. How to print struct variables in console? For writing mocks, though, it can be quite useful. Interestingly, it turns out that a type can embed an interface and thus satisfy an interface even if only a subset of those methods are actually implemented. In this example, well be defining a user package in our application that will require some form of database in order to fetch users and then, after a little bit of business logic, it will then be able to persist any changes made. And to fully grasp their usefulness, it may be useful to realize that Go applies interfaces a bit differently from what were used to from other languages. Martin Kock When a value is represented as an interface, Gos runtime is still aware of the underlying type. cannot convert data (type interface {}) to type string: need type assertion. While the exciting part - the actual conversion of a regular car into a BatMobile - has been left out, this example should demonstrate the power and flexibility of Gos structurally typed interfaces. Lets take a look at how we can improve on the above code snippet and make it loosely coupled and easier to test.

Empty interfaces are sometimes used when working with multiple different types that have nothing in common, i.e. Heres a simple example of type conversion: Code snippet 17 Example in the Playground. For more complex applications, being able to exercise all of the various code-paths within our application can be a bit of a nightmare depending on the way that we architect certain components. itemA and itemB may both implement other methods beyond what is mandated by the interface, but doFoo can not see them nor invoke them. // RegisterHooks wraps a Core and runs a collection of user-defined callback. Whilst the abstractions we have covered in the above article are incredibly useful and allow us to test our code, if you need to squeaze every last ounce of performance out of your code, you may find that youll need to cut out these abstractions in certain places. But in order for the runtime to be able to perform a method call on an interface value, the actual value must be reachable from the interface itself. Check this language proposal, for example, which goes all the way back to 2017 and is still being debated. I hope you had fun reading this and maybe even learned something along the way. How to convert an int value to string in Go? How to convert interface to struct in Go? In line three, though, we pass acmeToaster to maybeDoToast, which takes an empty interface argument. // v.recycle() // <- This is not possible! In lines 11-14, we add Joe Bloggs as a Person to the interface object. As it turns out, this still works, but the method call is treated as having a nil pointer receiver, which panics when we try to call one of the unimplemented methods: Code snippet 22 Example in the Playground_. In line three of main, we call attackOrDefend with an invalid argument - except that technically its not really invalid since in Go anything satisfies the empty interface. See this post from Ardan Labs for more details on how this works. A type is said to satisfy an interface if, and only if, it implements all the methods described by that interface. Go supports composing interfaces, i.e. As a closing remark on the issue, Ill quote Ian Lance Taylor, a member of the Go core team: Go makes a clear and necessary distinction between an interface that is nil and an interface that holds a value of some type where that value is nil. Scientific writing: attributing actions to inanimate objects. At this point onwards, the section of code where were branching off our logic based on the type of vehicle - i.e. They can also be satisfied by several different types that each satisfy only part of the interface but when composed satisfies it fully. In Go, interfaces take the center stage. In our the constructor func we have defined, youll notice that we take in a pointer to a db.Database struct which we hope will implement the methods that our user package will need in order to function. The reason is that the interface value itself can be nil, or the underlying type can have a nil value.

As well see, its not necessary to wrap all the interface methods. Forcing it to require both methods is probably not necessary, and the required extra method sleep would make getStatus both harder to test (more methods to mock) and harder to call with a valid type. So, in this article, we have covered why the mantra of accepting interfaces, and returning structs can be incredibly useful for Go developers wanting to write testable and highly maintainable services in Go. RegisterHooks then creates a slice of functions (using variadic function arguments[12]) and stores them in hooked, a struct that embeds the Core interface and redefines some of the Core methods in order to make use of the hook functions (not shown).
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