Go: cannot call pointer method getFirstName on "Struct" [duplicate]

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
– mkopriva, Commented Jun 24, 2022 at 7:27

Shubham Dixit · Accepted Answer · 2022-06-24 07:14:19Z

3

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())



}

answered Jun 24, 2022 at 7:14

Shubham Dixit

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2 Comments

user63898 Over a year ago

what is the best practice if i dont want to copy values , the first example ? what is uselly used in go code?

Shubham Dixit Over a year ago

receiver pointer is a better approach ,but if you dont have large structs then value reeciver is also fine

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