Go error collection | talk about the difference between the value type and pointer type of the method receiver

Hello everyone , I am a 『Go School 』 Fisherman's son . Today, I want to talk about the type of method receiver . Link to the original text ：https://mp.weixin.qq.com/s/Av3DrzDXa2cjjBbtkj6wuw We know , stay Go After the structure is defined in , You can define methods for this structure . as follows ：```gotype customer struct { balance string}func (c customer) SetBalance(v float64) { s.balance = v}func (c *customer) UpdateBalance(v float64) { c.balance = v}``` there c It's the receiver of the method . The type of the receiver can be the value of an object , It can also be a pointer to an object . So what is the difference between the value type and pointer type of the receiver ？ When we define methods , Should the receiver choose to use value type or pointer type ？​**01 The method receiver is a value type ** stay Go in , Everything you have heard of is a copy . therefore , When the receiver of a method is a value type , It is actually a copy of the original object , Then let the copy of the object call the corresponding method . Any change to the recipient in the method , Will not affect the original object .​ The following is illustrated by a specific example .```gotype customer struct { balance float64}func (c customer) add(v float64) { c.balance += v}func main() { c := customer{balance: 100.} c.add(50.) fmt.Printf("balance: %.2f\n", c.balance)}``` Because in add In the method , The receiver is a value type , In execution c.add(50.) Function time , Actually, it's true c Made copies , Then change the new copy of the object balance. therefore , Final c.balance There's no change in the outcome of , Is still 100. As shown in the figure ：​**02 The method receiver is a pointer type ** If the type of receiver is pointer , that , What we pass to the method is the address of the original object , It is still a value copy , The value here is the address value , Instead of a copy of the original object . At this time , Any change to the recipient in the method , Will act on the original object .​ Still the example above , We change the receiver type to pointer .​```gotype customer struct { balance float64}func (c *customer) add(v float64) { c.balance += v}func main() { c := customer{balance: 100.} c.add(50.) fmt.Printf("balance: %.2f\n", c.balance)}``` Because the receiver is a pointer type , therefore , Yes balance The change of is actually a change to the original object , The final result will be output 150. As shown in the figure ：​**03 How to choose the type of receiver ** When defining the structure method , Is the receiver type a value type or a pointer type ？ Below we list some common selection criteria to help us choose which type to use .​ The receiver must be a scenario of pointer type ：- If the method needs to change the receiver , Must be a pointer type . This rule also applies to slice types . If the receiver type is a slice , At the same time, when we want to add elements to the slice in the method , as follows ：```gotype slice []intfunc (s *slice) add(element int) { *s = append(*s, element)}```- If the receiver contains fields that cannot be copied , Must be a pointer type . for example sync The type field in the package cannot be copied . The receiver suggests using the scenario of pointer type ：- If the receiver is a large object , It is recommended to use pointer type first . Using pointer type enables fast copying , It can improve the efficiency of calling methods . that , How big is the big object , There is no standard , The general recommendation is to decide through benchmarking in actual projects . The receiver must be a scenario of value type ：- When the invariance of the receiver must be maintained , That is, when the original object cannot be changed in the function .- When the recipient is map、function or channel Type . otherwise , Causes a compilation error . The receiver suggests using the scenario of value type ：- When the receiver is an unchanging slice type .- When the type of receiver is a basic type .Go The basic types of include Numbers、strings、boolean.- When the receiver is a small object and does not meet the conditions for using pointers .**04 An example ** Let's take a slightly more complicated example . In this example ,customer The structure contains a pointer type field . Examples are as follows ：```gotype customer struct { card *card}type card struct { balance float64}func (c customer) add(operation float64) { c.card.balance += operation}func main() { c := customer{card: &card{ balance: 100, }} c.add(50.) fmt.Printf("balance: %.2f\n", c.card.balance)}``` In this example ,balance yes card Fields in structure , and customer Through the pointer card. meanwhile , The receiver type of the method we still use is the value type , But the final result will still change the original object balance Value .​