如何通过函数/方法将数据保存到Go结构中？ [重复]

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I'm just starting out with Go and I'm having a hard time saving data in a struct. Coming from other languages I learned there is no such thing as a class in Go. For similar purposes, the struct can be used, and functions can be "added" to the struct. So I wrote the following simple program:

package main

import "fmt"

type MyStruct struct {
    the_number int
}
func (self MyStruct) add(another_number int) int {
    self.the_number += another_number  // I save the result to the struct the_number
    return self.the_number
}

func main() {
    my_struct := MyStruct{1}
    result := my_struct.add(2)
    fmt.Println(result)               // prints out 3 as expected
    fmt.Println(my_struct.the_number) // prints out 1. Why not also 3?
}

As you can see from the comments I'm puzzled by the fact that the result is not saved in self.the_number in the instantiated my_struct.

So I found out I can get around this by doing

my_struct.the_number = my_struct.add(2)

But I methods/functions can sometimes become complex in which I want to save a lot of data to the my_struct from within the function.

Could any smarter soul than me give me a tip on what I'm missing here?

How can I save data to an instantiated struct from within a function?

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Answer 1

You should use pointer in struct method func (self *MyStruct) add(another_number int) int as without the * variable (self) is passed by value, not by references. E.g. you are updating a copy of an original object and this changes are discarded.

It's a basic stuff and well covered in Tour of Go - everyone should take it before starting coding in Go.

Another option would be return the self from the method - it would follow "immutable" style so you can write code like: my_struct = my_struct.add(1).add(2)

package main

import "fmt"

type MyStruct struct {
    the_number int
}
func (self *MyStruct) add(another_number int) int {
    self.the_number += another_number
    return self.the_number
}

func main() {
    my_struct := MyStruct{1}
    result := my_struct.add(2)
    fmt.Println(result)               // prints out 3 as expected
    fmt.Println(my_struct.the_number) // prints out 1. Why not also 3?
}