在golang中，如何为现有struct对象分配方法？

Question

I was wondering if doing something like this in golang is even possible --

type MyStruct struct {
    id int
}

func (ms *MyStruct) PrintHello() {
    fmt.Printf("Hello from original method %v", ms.id)
}

func main() {
    fmt.Println("Hello, playground")
    m := MyStruct{}
    m.PrintHello()

    m.PrintHello = func() {fmt.Printf("Hello from newer method 2")}
}

Error: cannot assign to m.PrintHello

https://play.golang.org/p/2oJQFFH4O5

Sorry if this doesn't make sense for Go programmers, I am new to Go and wondering if some of the things that can be done in dynamically typed languages can be done in Go. Thanks! :-)

Answer 1

Given that Go is a statically typed language, you cannot do this specific piece of code. However, functions are variables, so you CAN do something like this. Just remember that this is technically NOT the same as assigning a new method, as JimB states in the comments.

https://play.golang.org/p/rfuCzXD8fP

package main

import (
    "fmt"
)

type MyStruct struct {
    id         int
    PrintHello func(ms * MyStruct)
}

func (ms *MyStruct) init() {
    ms.PrintHello = func(ms *MyStruct) { fmt.Printf("Hello from original method %v
", ms.id) }
}

func main() {
    m := &MyStruct{id: 42}
    m.init()
    m.PrintHello(m)

    m.PrintHello = func(ms *MyStruct) { fmt.Printf("Hello from newer method 2 %d
", ms.id) }
    m.PrintHello(m)
}

Because this function is not really a method, you can only access the internal struct values by passing it as an argument. This means that if this ever has to cross package boundaries, unexported values will not be available.

It is also important to note that the function type needs to be the same. So if you defined the function as PrintHello func(ms * MyStruct) error in the struct, you would need to assign a function that returns an error.