Go: cannot call pointer method getFirstName on "Struct"

Question

I have a simple struct and receiver. I try to set the map with the struct and then call the receiver.

Like that:

package main

import (
    "fmt"
)

type myStruct struct {
    FirstName string 
    LastName string
}

func (m *myStruct) GetFirstName() string {
    return m.FirstName
}

func (m *myStruct) GetLastName() string {
    return m.LastName
}


func main() {
    


    testMyStruct := myStruct {
        FirstName: "x1",
        LastName: "x2",
    }

    myMapStruct["test2"] = testMyStruct 

     

    fmt.Println(myMapStruct["test2"].GetFirstName())


}

I'm getting this error:

 cannot call pointer method getFirstName on myStruct

Why can I call the receiver method from the map?

The expression `x.M()`, where `x` is of type `T` and `M` is a method of `*T`, is shorthand for `(&x).M()`. However, for `(&x).M()` to be valid `x` MUST be addressable. A map index expression, e.g. `m[k]`, is NOT addressable. Therefore `m[k].M()` is illegal if `m[k]` is of type `T` and `M` is a method of `*T`. One of the reasons why `m[k]` is not addressable is that *"growing a map might cause rehashing of existing elements into new storage location, thus potentially invalidating the address"* (from The Go Programming Language book). — mkopriva, Jun 24 '22 at 07:27

score 2 · Accepted Answer · answered Jun 24 '22 at 07:14

As you are passing a receiver pointer ,you have to use pointer to the struct to call it ,Please look at the below code

package main

import (
    "fmt"
)

type myStruct struct {
    FirstName string 
    LastName string
}

func (m *myStruct) GetFirstName() string {
    return m.FirstName
}

func (m *myStruct) GetLastName() string {
    return m.LastName
}


func main() {
    


    testMyStruct := &myStruct {
        FirstName: "x1",
        LastName: "x2",
    }

  myMapStruct:=make(map[string]*myStruct)

    myMapStruct["test2"] = testMyStruct 

     

    fmt.Println(myMapStruct["test2"].GetFirstName())



}

See working here

or You can use like below if passing receiver as value

package main

import (
    "fmt"
)

type myStruct struct {
    FirstName string 
    LastName string
}

func (m myStruct) GetFirstName() string {
    return m.FirstName
}

func (m myStruct) GetLastName() string {
    return m.LastName
}


func main() {
    


    testMyStruct := myStruct {
        FirstName: "x1",
        LastName: "x2",
    }

  myMapStruct:=make(map[string]myStruct)

    myMapStruct["test2"] = testMyStruct 

     

    fmt.Println(myMapStruct["test2"].GetFirstName())



}

what is the best practice if i dont want to copy values , the first example ? what is uselly used in go code? — user63898, Jun 24 '22 at 09:18
receiver pointer is a better approach ,but if you dont have large structs then value reeciver is also fine — Shubham Dixit, Jun 24 '22 at 09:32

Go: cannot call pointer method getFirstName on "Struct"

1 Answers1