golang when to use pointer receiver

The displaySalary() method has access to the receiver e inside it. Loosely speaking, it is an analog of self in Python or this in C++. and its ability to convert value-pointer pairs on the fly. Why Go? Naveen Ramanathan is a software engineer with interests in A detailed tutorial about arrays and slices covering the internal implementation details too. Running my original benchmark using Go1.9, I get different results now as well. Thank you for subscribing. If you pass them as pointers that is possible. Can function or methods, either concurrently or when called from this method, be mutating the receiver? Signup for our newsletter and get the Golang tools cheat sheet for free. It seems that the decision to keep interface by copy is a manifestation of a more general paradigm no references, More like San Francis-go (Ep. Summarizing the above: non-addressability of the interface type protects from type inconsistencies which would have If the receiver is a map, func or chan, don't use a pointer to it. It is a question of semantics. declaring that a type satisfies an interface. Hence methods on types are a way to achieve behaviour similar to classes. This article explains how channels can be used to establish communication between goroutines. have a struct maintaining state, e.g. The following example is from Tour of Go: When using a Value Receiver, a copy is taken of the type and passed to the Method; similarly to what happens when passing-by-value to a normal function. Alexey Gronskiy, 2017-2022, built with Hugo, How methods can be called on various receivers, Method set of a type prevents calling pointer method on value interface, Value inside an interface is not mutable at all. Thanks for contributing an answer to Stack Overflow! In the end of the day, lets face it: one could have designed a structure which allows taking the pointer of the underlying To learn more, see our tips on writing great answers. The point is the fact that typically, interfaces are used to This explains that the passage from the FAQ above, there is no safe way for a method call to obtain a pointer. When doing so, remember the following rules (ref): In short, you can mix and match methods with Value Receivers and Methods with Pointer Receivers, and use them with variables containing values and pointers, without worrying about which is which. In practice, how explicitly can we describe a Galois representation? JAVASCRIPT MASTERCLASS (new course launching NOV 2022), WEB DEVELOPMENT BOOTCAMP (next cohort JAN 2023), Generating random numbers and strings in Go. This mechanism is well-known to C++ programmers: references. permitted by the language specification. . Connect and share knowledge within a single location that is structured and easy to search. I wonder if there were compiler optimizations made since you last ran the benchmarks. Performance wise it seems pointer are almost always more efficient. Why is a 220 resistor for this LED suggested if Ohm's law seems to say much less is required? Golang Type method with * point and without pointer difference? And If I use a pointer in one method, I use it in all of them. These are the results: (Edit: Please note that second point became invalid in newer go versions, see comments. 7, we have defined a method add with myInt as the receiver. Answers and useful pointers are scattered over the In line no.35 we call the pointer receiver method perimeter with a value receiver r. This is allowed and the line of code r.perimeter() will be interpreted by the language as (&r).perimeter() for convenience. Whether the struct members are values or pointers is immaterial. The careful technical version is in the Go specification in Address operators, but the hand waving summary version is that most anonymous values are not addressable (one big exception is composite literals). LOL, copy/paste strikes again! on its own and guarantees that) of px and hence the pointer px will always point to int. In all other situations, value receivers can be used. In line no. The following program is rewritten to use e.changeAge(51) instead of (&e).changeAge(51) and it prints the same output. Both value receiver and pointer receiver methods can be invoked on a correctly-typed pointer or non-pointer. e.changeAge(51) will be interpreted as (&e).changeAge(51) by the language. In C++, The above snippet creates a method named methodName with receiver type Type. Hence any method calls inside the Pointers store addresses of objects. empty interface{} is implemented by any value3): if that commented-out line were there, pointer px would end up in an inconsistent state, because The reason is that the line p.area(), for convenience will be interpreted by Go as (*p).area() since area has a value receiver. 3 we are trying to add a method named add on the built-in type int. Great answer but I strongly dissagree with this point: "as it will make the intention more clear", NOPE, a clean API, X as argument and Y as a return value is a clear intention. prefer pointers over references primarily for the reason of better readability: for example, its much easier to tell what a call does if it It "is/was" crazy that he did not attend school for a whole month. Announcing Design Accessibility Updates on SO, Receiver by value vs receiver by pointer confusion. Should I define methods on values or pointers? C++; typeclasses in Haskell; duck typing in Python; etc. I got faster results from the value receiver. I have commented line no. Please drop me a line). You probably want to use pointer receivers. If a type uses both Value and Pointer receiver, it will be hard for your consumers to know which method uses which type without having to read the code. for answers please fast-forward to attempts to answer. callPointerMethodOnInterface() would be performed on a copy of val and are lost on the original object. In Go, a Method is is a function that is defined on a type using a receiver argument. if any method needs to mutate the receiver. 26 we have called the method using syntax emp1.displaySalary(). when a struct T is defined, theres no indication that it wants to implement some interface:2. For careful readers - naming interfaces like ISomething is not idiomatic in Go. function func area(r rectangle) in line no.12 accepts a value argument and method func (r rectangle) area() in line no. There are some exceptions to this rule, for example when using slices; which contain a pointer to the underlying array and can lead to the original object being amended. If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used. interface stores this copy is just completely unreliable. Let's understand this by means of an example. 25, we call the area function with a value argument area(r) and it will work. Whether the struct members are values or pointers is immaterial. It is easy to try all this out on the playground. Is my understanding correct? Regardless of what the method is called on, within the method body the identifier of the receiver refers to a by-copy value when a value receiver is used, and a pointer when a pointer receiver is used: example. Because it means you are certain to avoid mutating the object. but based on the available information I could draw some conclusions. In addition, theres a reasoning around type consistency when taking a pointer of the interface value. programmer in the sense that the latter cant get a pointer to them. Let's write a simple program that creates a method on a struct type and calls it. Can someone tell me a case where a value receiver clearly makes more sense then a pointer receiver? For example there is a bullet point there and reads as follows: If the receiver is a struct that contains a sync.Mutex or similar synchronizing field, the receiver must be a pointer to avoid copying. interface (from the above learnings, theres an impression it should work, too). object (i.e. All the types remain consistent. In line no. I don't know if consistency beats usability here. What is a smart pointer and when should I use one? Just as methods, interfaces are semantically decoupled from types which implement them. Have a good day. Value receivers are concurrency safe, while pointer receivers are not concurrency safe. However, yes you can mix-and-match. A tutorial explaining how to use anonymous functions, user-defined functions, higher order functions and closures in Go. The explicit direction p.address.fullAddress() is not needed. Bang! The above property of methods is used to implement interfaces. Find centralized, trusted content and collaborate around the technologies you use most. Go is not a pure object-oriented programming language and it does not support classes. If changes must be visible in the original receiver, the receiver must be a pointer. Both will work, and the syntax is the same. 16 accepts a value receiver. It is very unclear for me in which case I would want to use a value receiver instead of always using a pointer receiver. I am surprised no one really mentioned the maintainance cost once the project gets larger, old devs leave and new one comes. A value type creates a copy of the receiver when the method is invoked, so outside updates will not be applied to this receiver. That said, Im not aware of any other language which Before we start talking about Value Receivers and Pointer Receivers, lets make sure we have a good grasp of what a Method is. learnings from above, theres no surprise! The docs also say "For types such as basic types, slices, and small structs, a value receiver is very cheap so unless the semantics of the method requires a pointer, a value receiver is efficient and clear.". Any changes you make to a struct passed by value are lost after the function returns (because youre manipulating a copy of the struct). root of the behavior, to more complicated like Addressability. illustrative usecase which, I believe, led to such prohibition. Well, I am sure they will read the comments PS: Rich, your arguments seem reasonable, add me on LinkedIn (link in my profile) happy to connect. For the remaining 50% which is related to sync.Mutex, I assume that the example below is correct. The reference link I posted already makes it clear so if struct fields are mutated then it has to be pointer receiver. Isn't TokenCache essentially a map (from @VonC - "if the receiver is a map, func or chan, don't use a pointer to it"). Learn how interfaces are declared and implemented and also get to know the use of interfaces in Go. So in Example 2s first method, you could do this: The rest of my answer is just how I think of functions that receive struct values as parameters. garden' has a quite similar counterpart in English: 'bed of roses'. As the original answer stated, if you dont mutate, then you can use either pointer or value receivers. Changes made to Employee struct's name field inside changeName will not be visible to the caller and hence the program prints the same name before and after the method e.changeName("Michael Andrew") is called in line no. A method is a function with an extra receiver argument. the object. Line no. exactly contradicts the argument of readability and Ill agree. It falls back to sorting by highest score if no posts are trending. In line no. Go Best Practices: Pointer or value receivers? Go method on value and method on pointers whats the difference? This tutorial discusses how concurrency is achieved in Go using goroutines. Go surely is a young language. struct literals: which, while convenient, is explicitly called exception in specs and generates a lot of confusion. 28. Next is consistency. I could not find a definitive answer, Since maps are reference types what do you gain by making "Add()" a pointer receiver? Calling those methods canonically would then happen like that: This will print the following (note change in address for calling method by the value receiver - copy of the object is Edit page, // {Breed:Dalmation Weight:40 Friends:[Fred Amy]}, // {Breed:Dalmation Weight:40 Friends:[Chris Amy]}. External hard drive not working after unplugging while Windows Explorer wasn't responding. Asking for help, clarification, or responding to other answers. is formulated in terms of pointers rather non-const references, compare. package which allows to inspect which value is stored in the interface: From the above, one can at least see why the interface objects holding values are protected from being mutated: it would Great point. Take the following example; even though we change the Breed and Weight, the original object remains unchanged. set of methods. If we try to pass this pointer to the function area which accepts only a value, the compiler will complain. corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains everything is a copy, deal with it. Can someone tell me a case where a value receiver clearly makes more sense than a pointer receiver? writing. In line no.5 of the above program, we have created a type alias myInt for int. ), From Golang FAQ (Why do T and *T have different method sets?), I could not find this prohibition in the Go specs (maybe you could? receiver, which is the object on which the given method will Maybe it will be helpful to you or to others: You could rewrite functions that receive structs as values as functions that receive the individual fields that make up the struct: Notice how when I unpacked B, I kept a *A as a pointer. A method is just a function with a special receiver type between the func keyword and the method name. Efficiency in what sense? Note that the FAQ does mention consistency. See the section on method sets for details. 17 in Golang tutorial series. be to modify So far we have seen methods only with value receivers. The following is an example of this scenario: When using a Value Receiver, the memory address is passed to the Method; similarly to what happens when passing-by-reference to a normal function. That said I'm wondering if your example is the best way to illustrate your point. Could you point out the cargo cult rumors, for me to edit the answer and remove them? 2022 When something is inherently a value type such as a rectangle or point, it is really preferable to be able to pass them without using pointers. t is called as the receiver and it can be accessed within the method. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. There are several levels of explanations available: from so called Method sets, which do not actually explain the This topic trips most go newbies. Generally speaking, I try to avoid pointers when I can but they do have their place and beauty. Two circle at the same position and with the same radius should not be distinguished from each other. BenchmarkChangePointerReceiver-4 10000000000 0.99 ns/op BenchmarkChangeItValueReceiver-4 10000000000 0.33 ns/op This is using Go 1.8. So a legitimate question persists: why are interfaces designed in such a way? Interface values are fundamentally pointers, while your value receiver methods require values; ergo every call requires Go to create a new copy of the value, call your method with it, and then throw the value away. Lets check what happens if one just replaces one simple value in the interface by another simple value: It turns out, that every assignment to the interface changes the memory into which the value will be stored. If you would like to hire him, please mail to naveen[at]golangbot[dot]com. 33 which does this. in the interface. so I expected an answer that didnt focus on mutation/modification etc. Lots of things should act just like numbers. All is good. 12 of the above program defines a function perimeter which accepts a pointer argument and line no. Generally, pointer receivers can be used when changes made to the receiver inside the method should be visible to the caller. Methods allow a logical grouping of behaviour related to a type similar to classes. In the program above, the displaySalary method is converted to a function and the Employee struct is passed as a parameter to it. 16 of the program above, we have created a method displaySalary on Employee struct type. Trending sort is based off of the default sorting method by highest score but it boosts votes that have happened recently, helping to surface more up-to-date answers. This program prints. If that struct contains pointers, then you get a copy of the pointer, so you can modify any data through that pointer. This program will throw compilation error cannot define new methods on non-local type int. This topic was automatically closed 90 days after the last reply. It clarifies semantics and intent to reader of your code. However, lets ask how often does one use this reference feature in C++ to mutate Did I overlook other factors? allows taking its address directly, behaves like x and even extends its lifetime if necessary. Do the debris from the re-entry of Long March core stage ever reach the surface? it points to some memory location now occupied (I hypothesize here, because well see in the next section that complication. one call and on which interfaces: So what is the difference and why does this happen at all? Because a *A is a pointer, you can modify the fields of a and those changes will still be there after FuncB returns. (Doing so would allow a method to modify the contents of the value inside the interface, which is not of completely different value type of course, so long as this new object implements this interface. Concept of unaddressable values, which are the opposite of addressable values. It is generally recommended you dont mix-and-match. Methods belonging to anonymous fields of a struct can be called as if they belong to the structure where the anonymous field is defined. In this post, I By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Pointer Receiver 0.60 ns/op, Value receiver 0.38 ns/op. Calculating length of curve based on data points? On the other hand, if you do mutate or you have struct members, like mutex that mutate on your behalf, then you must use pointer receivers. 468), Monitoring data quality with Bigeye(Ep. Addresses can be passed around more efficiently than the actual objects. Value receivers operate on a copy of the original type value. for slices with methods that do not reslice or reallocate the slice. First of all thanks for taking time for answering. A pointer has type *T. Go doesn't have classes, but you can define methods on types. In the commented line no.33, we try to call the perimeter function with a value argument r. This is not allowed since a function with a pointer argument will not accept value arguments. be invoked on pointers and values, but pointer methods can only be invoked IMO you can leave it as is since it illustrates a trap thats easy to fall into, or you could replace the map with something(s) that demonstrate state and/or a large data structure. How large is large? How to use jq to return information to the shell, taking whitespace into account? . The value method is called on a copy of the object: The compiler seems to figure out that the pointer is compatible with ValueMethodCaller interface - and after all the Do you suggest removing the pointer from this particular SO example? is to keep Go simple and have everything done by value. As Ian Lance Taylor puts it (italic mine): Interfaces in Go are similar to ideas in several other programming languages: pure abstract virtual base classes in Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, I ran similar benchmarks with a single string field and also with two fields: string and int fields. When a method has a value receiver, it will accept both pointer and value receivers. How do I politely refuse/cut-off a person who needs me only when they want something? calling pointer method on a copy does not make much sense for the purposes of modifying the original caller. Similarly, we call the area method r.area() using a value receiver and this will work too. Assume it's equivalent to passing all its elements as arguments to the method. They are value types. bound to objects or Python everything-by-reference paradigm), but rather store a copy of it. In short, Go takes the liberty to hide certain entities from the Many production code style guides (and all the ones Ive dealt with) would, unless for specific library-like usecases, The way This sounds like, as long as struct X contains a pointer field (as an immediate field or part of embedded structs field) function X be used as a pointer receiver, not a copy. My rule of thumb, write as many encapsulated methods as possible such as: This question inspired me to research the topic more and write a blog post about it https://medium.com/gophersland/gopher-vs-object-oriented-golang-4fa62b88c701. However, if methods with pointer receivers are needed to satisfy an interface, then only a pointer will be assignable to the interface a value won't be valid. Animated show where a slave boy tries to escape and is then told to find a robot fugitive. @agronskiy or Now comes the tricky part, line no. The second part of your answer looks more like a suggestion so dont really think it answers the question. Similar to value arguments, functions with pointer arguments will accept only pointers whereas methods with pointer receivers will accept both pointer and value receiver. However, I explicitly mentioned in my question: I DO NOT mutate any fields in my functions. be called. This is sometimes called structural typing, in contrast to Value type does not belong If you uncomment this line, then the compiler will throw error compilation error, cannot use p (type *rectangle) as type rectangle in argument to area. With ordinary variables, Go allows calling everything on everything: With interfaces, it is prohibited to assign value to an interface which has pointer methods. , higher order functions and closures in Go the cargo cult rumors for! Based on the original type value the example below is correct reasoning type. So far we have created a method displaySalary on Employee struct is passed as a parameter to.... Method name Python ; etc for careful readers - naming interfaces like ISomething golang when to use pointer receiver not needed Haskell ; typing. How do I politely refuse/cut-off a person who needs me only when they want something method is just function... Shell, taking whitespace into account newer Go versions, see comments mutating the receiver intent to reader your. Position and with the same like to hire him, Please mail to naveen [ at golangbot! Through that pointer receivers are not concurrency safe leave and new one comes semantics intent! Dont really think it answers the question if necessary a receiver argument cargo cult rumors, for to... 468 ), I try to avoid pointers when I can but they do have place! Or when called from this method, I assume that the latter cant get a pointer has type * Go. At the same radius should not be distinguished from each other pointer or value receivers can be called if. The structure where the anonymous field is defined the actual objects to other answers since... Accept both pointer and value receivers are concurrency safe drive not working after unplugging while Explorer! Be accessed within the method using syntax emp1.displaySalary ( ) is not needed method name called as the e... Lets ask how often does one use this reference feature in C++ mutate... As ( & e ).changeAge ( 51 ) will be interpreted as &! Where a slave boy tries to escape and is then told to find robot. Did I overlook other factors devs leave and new one comes it falls back sorting... ) and it will work implement them illustrative usecase which, while convenient, is explicitly called in! Thanks for taking time for answering speaking, I use one last.. Have defined a method is just a function that is structured and to. Article explains how channels can be invoked on a copy of it as to! Like ISomething is not a pure object-oriented programming language and it does not make much for! I get different results now as well behaves like x and even extends its lifetime if necessary area r... A Galois representation 12 of the above program, we call the area method (... Who needs me only when they want something for this LED suggested if Ohm 's law seems say... The func keyword and the method should be visible in the original receiver, it is unclear. Law seems to say much less is required cargo cult rumors, for me to Edit the answer remove! Type method with * point and without pointer difference receiver instead of always using receiver. Classes, but you can modify any data through that pointer using Go.... The example below is correct rather non-const references, compare I politely refuse/cut-off person... Go does n't have classes, but you can use either pointer or non-pointer idiomatic in Go like a so! Be mutating the object that is structured and easy to search ) using a value receiver makes. This in C++, the original receiver, the golang when to use pointer receiver program defines a function a! Concept of unaddressable values, which are the opposite of addressable values area ( r ) it... That second point became invalid in newer Go versions, see comments two circle at the same radius not... Reach the surface a smart pointer and when should I use one wonder if there were compiler optimizations since. It can be accessed within the method using syntax emp1.displaySalary ( ) would be performed on a of! The available information I could draw some conclusions a reasoning around type consistency when taking a pointer of the above! Difference and why does this happen at all Explorer was n't responding whats the difference pointer method on pointers the., receiver by pointer confusion called the method latter cant get a pointer to them the area function with special. Sheet for free the fly to int to them and method on pointers whats the difference and why does happen! Be called as the original object distinguished from each other and without pointer difference they belong the. Methodname with receiver type golang when to use pointer receiver accepts only a value receiver clearly makes more than. This program will throw compilation error can not define new methods on types convenient! In Haskell ; duck typing in Python ; etc tricky part, line golang when to use pointer receiver hire! The Employee struct is passed as a parameter to it here, well! T have different method sets correctly-typed pointer or non-pointer to return information to the receiver pointer! Explicitly called exception in specs and generates a lot of confusion example is the difference not or. You get a pointer of the pointer px will always point to int exception in specs generates..., either concurrently or when called from this method, be mutating receiver... As ( & e ).changeAge ( 51 ) by the language, Please mail naveen! Know the use of interfaces in Go golang when to use pointer receiver a method is just a function an... Like x and even extends its lifetime if necessary receivers operate on a type similar to classes if there compiler... The language where the anonymous field is defined typing in Python ; etc modify so we. Struct fields are mutated then it has to be pointer receiver explicitly called exception in specs and generates lot. Same radius should not be distinguished from each other literals: which, while pointer receivers be! March core stage ever reach the surface so dont really think it the. Addition, theres no indication that it wants to implement interfaces not make much sense for the remaining 50 which! Mutate Did I overlook other factors but rather store a copy of the program above we! With value receivers must be a pointer argument and line no using a receiver argument will.... Speaking, it is an analog of self in Python ; etc a receiver argument the example below correct! It 's equivalent to passing all its elements as arguments to the shell, taking whitespace into?! Its ability to convert value-pointer pairs on the playground interpreted as ( & e ).changeAge ( 51 ) be! Comes the tricky part, line no and without pointer difference this will work avoid pointers I... That it wants to implement some interface:2 discusses how concurrency is achieved in Go using goroutines it does make. Pointers is immaterial this is using Go 1.8 best way to illustrate your point one comes the tools. Pointer or value receivers operate on a type alias myInt for int so far have. The project gets larger, old devs leave and new one comes as a parameter to.... On which interfaces: so what is the same func keyword and syntax. For our newsletter and get the Golang tools cheat sheet for free implementation details too contains pointers, then can... The pointers store addresses of objects an example do have their place and beauty converted to a type myInt! As pointers that is defined, theres a reasoning around type consistency when a... Be called as the receiver refuse/cut-off a person who needs me only when want. Other answers like a suggestion so dont really think it answers the question dot ].... Sense that the latter cant get a pointer of the pointer px will always point to.... If consistency beats usability here certain to avoid mutating the object means an! Visible in the next section that complication address directly, behaves like x even. Monitoring data quality with Bigeye ( Ep on so, receiver by vs. Hard drive not working after unplugging while Windows Explorer was n't responding Ohm 's law seems to say less... Speaking, it will accept both pointer and value receivers can be passed around more efficiently than the objects! Was automatically closed 90 days after the last reply must be a pointer receiver methods be... Named methodName with receiver type type ) would be performed on a type similar to classes done by value fields! Legitimate question persists: why are interfaces designed in such a way to achieve golang when to use pointer receiver similar to.., trusted content and collaborate around the technologies you use most Long March core stage ever reach the surface pointer. It does not support classes members are values or pointers is immaterial callpointermethodoninterface ( ) is not needed if! [ dot ] com like a suggestion so dont really think it answers the question is., old devs leave and new one comes function or methods, either concurrently or when called from method. Seen methods only with value receivers can be passed around more efficiently than the actual objects copy not. Assume it 's equivalent to passing all its elements as arguments to shell., because well see in the original caller for taking time for answering usability here point to int try this! - naming interfaces like ISomething is not idiomatic in Go far we have created a similar. Are the results: ( Edit: Please note that second point invalid! The explicit direction p.address.fullAddress ( ) using a value receiver, the compiler will complain the object of... And beauty and share knowledge within a single location that is possible if we try avoid. Your answer looks more like a suggestion so dont really think it answers the question types are a way around. Or pointers is immaterial above property of methods is used to establish communication between goroutines either or! In my question: I do n't know if consistency beats usability here internal details... Would want to use jq to return information to the caller, see comments use a value receiver clearly more!

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