callPointerMethodOnInterface() would be performed on a copy of val and are lost on the original object. All the types remain consistent. However, yes you can mix-and-match. exactly contradicts the argument of readability and Ill agree. To define a method on a type, the definition of the receiver type and the definition of the method should be present in the same package. for small arrays or structs that are value types, So far, all the structs and the methods on structs we defined were all located in the same main package and hence they worked. If that feels too large, it's also too large for the receiver. So Id take liberty and speculate here that the main and final reason for non-reference-binding behavior we have observed cohort; in other words, in is non-addressable. There is a situation when you might want to use pointer receivers for methods where youd normally use a value receiver, and its when you have other pointer receivers defined on that type, and for consistency you should use pointer receivers across all the methods. I like 99% of what you say here and strongly agree with it. The above behavior is formally regulated by the notions of method sets and addressability. Lets describe both more formal and less formal answers. That said I'm wondering if your example is the best way to illustrate your point. object (i.e. That's it for methods in Go. Note that the FAQ does mention consistency. Signup for our newsletter and get the Golang tools cheat sheet for free. dereferencing the pointer, but if an interface value contains a value T, there is no safe way for a method call to Do you suggest removing the pointer from this particular SO example? When the method executes, it has a reference to the original object; thus any changes made to the object do affect the original. One can send me here to the beginning of my post where we started exploring the syntactic flexibility of the language, which the val variable into an interface object, yet leaves the reason behind this decision unknown. In Go, a Method is is a function that is defined on a type using a receiver argument. The difference between value and pointer receiver is, changes made inside a method with a pointer receiver is visible to the caller whereas this is not the case in value receiver. Things like points, 2D vectors, dates, rectangles, circles etc. 17, we are using the receiver e and printing the name, currency and salary of the employee. The above program can be rewritten using only functions and without methods. ), 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? If you pass them as pointers that is possible. Two circle at the same position and with the same radius should not be distinguished from each other. In this post, I A value type creates a copy of the receiver when the method is invoked, so outside updates will not be applied to this receiver. Great point. Can function or methods, either concurrently or when called from this method, be mutating the receiver? 35 of the code p.area() calls the method area which accepts only a value receiver using the pointer receiver p. This is perfectly valid. A tutorial explaining how to use anonymous functions, user-defined functions, higher order functions and closures in Go. 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. Should I define methods on values or pointers? And If I use a pointer in one method, I use it in all of them. Can someone tell me a case where a value receiver clearly makes more sense then a pointer receiver? It's possible to define a method named Area on both Square and Circle. At this point, a newcomer to Go starts getting into an experimental mood, encouraged by the compilers smartness and Calculating length of curve based on data points? I could not find a definitive answer, Do the debris from the re-entry of Long March core stage ever reach the surface? of completely different value type of course, so long as this new object implements this interface. Simon Drake Such protection against mutation comes in a form of the notion of addressability (see Key advantages you may have overlooked, Go string handling overview [cheat sheet]. There is no way to avoid this as long as you use value receiver methods and call them through interface values; it's a fundamental requirement of Go. For example (remember that @agronskiy or Hence methods on types are a way to achieve behaviour similar to classes. As this blog post by Russ Cox clarifies, the interfaces were designed early on to be The Golang FAQ gives the following (italic mine): This distinction arises because if an interface value contains a pointer *T, a method call can obtain a value by However, I explicitly mentioned in my question: I DO NOT mutate any fields in my functions. This explains that the passage from the FAQ above, there is no safe way for a method call to obtain a pointer. This topic was automatically closed 90 days after the last reply. I have commented line no. for answers please fast-forward to attempts to answer. Or am I doing something wrong in the benchmark? In practice, how explicitly can we describe a Galois representation? If the receiver is a slice and the method doesn't reslice or reallocate the slice, don't use a pointer to it. There are a couple of reasons for this. In line no. Take the following example; even though we change the Breed and Weight, the original object remains unchanged. receiver, which is the object on which the given method will Did I overlook other factors? I don't know if consistency beats usability here. The method set of any other type T consists of all methods declared with receiver type T. The method set of the 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. Asking for help, clarification, or responding to other answers. C++; typeclasses in Haskell; duck typing in Python; etc. Internet both spatially and in time (this became a common stump for Go learners as early as Go appeared). This program also produces the exact same output Salary of Sam Adolf is $5000. Pointers receivers can also be used in places where it's expensive to copy a data structure. Next is consistency. 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 There are several levels of explanations available: from so called Method sets, which do not actually explain the It is clear that the function receiving the object cannot and will not mutate the object. Bang! If that struct contains pointers, then you get a copy of the pointer, so you can modify any data through that pointer. @Lukas: A very side note: The Slovak 'walking through ROSE (not pink!) programmer in the sense that the latter cant get a pointer to them. This program will output. These are the results: (Edit: Please note that second point became invalid in newer go versions, see comments. So far we have defined methods only on struct types. be invoked on pointers and values, but pointer methods can only be invoked at all because my question has nothing to do with it. In addition, theres a reasoning around type consistency when taking a pointer of the interface value. 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. Any modification to a value receiver is local to that copy. It also covers anonymous structs, promoted fields and nested structs. empty interface{} is implemented by any value3): if that commented-out line were there, pointer px would end up in an inconsistent state, because Thanks to Corentin Perret-Gentil for valuable review comments on this! Imagine you write a function taking two numbers as arguments. Instead of the expected message, one gets a cold complaint from the compiler: Sounds like out of four possible combinations (to remind: two options for the receiver type and two for the calling Comments are welcome via e-mail LOL, copy/paste strikes again! Connect and share knowledge within a single location that is structured and easy to search. ), means easy. 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 This is not allowed since the definition of the method add and the definition of type int is not in the same package. Because it means you are certain to avoid mutating the object. If a method does need to change something, then it works properly (it changes the actual struct members, not a copy which can be very confusing). We have not even mentioned yet that just out of the blue, Go allows taking address of temporaty objects like It feels like Go is not always consistent4 and the whole story 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 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. It clarifies semantics and intent to reader of your code. 468), Monitoring data quality with Bigeye(Ep. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. It takes a while to actually memorize this one, but the general mnemonics is that the only prohibited option is A pointer has type *T. Go doesn't have classes, but you can define methods on types. The point is the fact that typically, interfaces are used to Powered by Discourse, best viewed with JavaScript enabled. If the receiver is a map, func or chan, don't use a pointer to it. See, You're right. be to modify Go interfaces hold copies: And here comes the cornerstone: an implicit creation of the ValueMethodCaller Similar to value arguments, functions with pointer arguments will accept only pointers whereas methods with pointer receivers will accept both pointer and value receiver. See this Go playground -. Let's write a simple program that creates a method on a struct type and calls it. If the receiver is a struct that contains a. If the receiver is a struct, array or slice and any of its elements is a pointer to something that might be mutating, prefer a pointer receiver, as it will make the intention more clear to the reader. 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 for slices with methods that do not reslice or reallocate the slice. In other words, Animated show where a slave boy tries to escape and is then told to find a robot fugitive. interface), this is the only one which does not compile. However, I am not sure If I should read it this way! This article explains how channels can be used to establish communication between goroutines. The rule about pointers vs. values for receivers is that value methods can 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. External hard drive not working after unplugging while Windows Explorer wasn't responding. Common dilemma when defining the methods of a struct. 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. been made possible if addressing had been allowed. it is up to the particular Go implementation how to manage values in the interface) by float32, not int32 as originally. That said, Im not aware of any other language which The way ability of the Go compiler to automatically dereference pointers, as well as automatically take value address. the intention of calling pointer method could (with high confidence otherwise why use pointer method!) 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. bound to objects or Python everything-by-reference paradigm), but rather store a copy of it. Golang Type method with * point and without pointer difference? one call and on which interfaces: So what is the difference and why does this happen at all? is justified by this implementation detail. 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. For example, we can add methods like calculatePension, calculateLeaves and so on. Loosely speaking, it is an analog of self in Python or this in C++. illustrative usecase which, I believe, led to such prohibition. This works as expected. The variable s is interface, holding a pointer to actual value Binary. which methods can one call - and on what), lets make the second table representing a summary of which methods can 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. Both will work, and the syntax is the same. Root of all the confusion why do interfaces hold copies? Using this struct as a value receiver in a method will need the entire struct to be copied which will be expensive. Isn't TokenCache essentially a map (from @VonC - "if the receiver is a map, func or chan, don't use a pointer to it"). Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Welcome to tutorial no. Go Best Practices: Pointer or value receivers? on pointers, Which is not true, as commented by Sart Simha. The receiver can either be a struct type or non-struct type. The part in bold is found for instance in net/http/server.go#Write(): Note: irbull points out in the comments a warning about interface methods: Following the advice that the receiver type should be consistent, if you have a pointer receiver, then your (p *type) String() string method should also use a pointer receiver. In this case, if a pointer receiver is used, the struct will not be copied and only a pointer to it will be used in the method.
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