Reference to another field with in a func in the same struct in golang

Question

I have a struct looks something like this:

type Job struct {
    Action  func()
    Cron    string
}

So for an instance of this struct, is it possible for the code inside the implementation of Action to get a reference to Cron? Thanks.

Answer 1

Use a closure to create the Action function:

func MakeAction(job *Job) func() {
    return func() {
        // function that uses the variable job
    }
}

You can use the MakeAction function like this:

j := &Job{ nil, "foo" }
j.Action = MakeAction(j)

Notice that this will behave in possibly unexpected ways when somebody makes a copy of the Job structure—the reference inside the closure created by MakeAction() will still point to the original instance (instead of the copy).

Answer 2

You seem to have to pass a Job reference to the Action function in order for said function to access Cron.

See for instance:

package main

import "fmt"

type Job struct {
    Action func(*Job)
    Cron   string
}

func main() {
    j := &Job{f, "test"}
    fmt.Println("Hello, playground", j)
    j.Action(j)
}

func f(j *Job) {
    fmt.Println(j.Cron)
}

Output:

Hello, playground &{0x201a0 test}
test

None of the closures examples allows you to reference your own struct.

---

The OP adds:

The external framework expects a function like this: func().
I'm wrapping this function into my homebrewed struct Job, and I put something else into the Job struct.
Now I hope the Action could access other fields in Job, but it doesn't seem to be possible.
Later I will pass the Action directly into the external framework.

I confirm that, just by wrapping func() (from external framework) in a struct of yours doesn't means f has access to the struct fields through closure.
You need a separate initialization, as FUZxxl's answer shows.

Otherwise, how that function (from an external framework) would even know about Job?
That could only work if that function expect an interface with a Cron() function, not a struct which it knows nothing about, with a Cron field.
As in this example.