Immutable Struct in Golang

Question

Is it possible to define an immutable struct in Golang? Once initialized then only read operation on struct\'s field, no modification of field values. If so, how to do that.

Answer 1

There is no way to mark fields/variables as read only in a generic way. The only thing you could do is marking fields/variable as unexported (first letter small) and provide public getters to prevent other packages editing variables.

Answer 2

It is possible to make a struct read-only outside of its package by making its members non-exported and providing readers. For example:

package mypackage

type myReadOnly struct {
  value int
}

func (s myReadOnly) Value() int {
  return s.value
}

func NewMyReadonly(value int) myReadOnly{
  return myReadOnly{value: value}
}

And usage:

myReadonly := mypackage.NewMyReadonly(3)
fmt.Println(myReadonly.Value())  // Prints 3

Answer 3

There is no way to define immutable structures in Go: struct fields are mutable and the const keyword doesn't apply to them. Go makes it easy however to copy an entire struct with a simple assignment, so we may think that passing arguments by value is all that is needed to have immutability at the cost of copying.

However, and unsurprisingly, this does not copy values referenced by pointers. And the since built-in collections (map, slice and array) are references and are mutable, copying a struct that contains one of these just copies the pointer to the same underlying memory.

Example :

type S struct {
    A string
    B []string
}

func main() {
    x := S{"x-A", []string{"x-B"}}
    y := x // copy the struct
    y.A = "y-A"
    y.B[0] = "y-B"

    fmt.Println(x, y)
    // Outputs "{x-A [y-B]} {y-A [y-B]}" -- x was modified!
}

Note : So you have to be extremely careful about this, and not assume immutability if you pass a parameter by value.

There are some deepcopy libraries that attempt to solve this using (slow) reflection, but they fall short since private fields can't be accessed with reflection. So defensive copying to avoid race conditions will be difficult, requiring lots of boilerplate code. Go doesn't even have a Clone interface that would standardize this.

Credit : https://bluxte.net/