Go Programming - bypassing access privileges using pointers

Question

Let's say I have the following hierarchy for my project:

fragment/fragment.go
main.go

And in the fragment.go I have the following code, with one getter and no setter:

package fragment

type Fragment struct {
    number int64   // private variable - lower case
}

func (f *Fragment) GetNumber() *int64 {
    return &f.number
}

And in the main.go I create a Fragment and try to change Fragment.number without a setter:

package main

import (
    "fmt"
    "myproject/fragment"
)

func main() {
    f := new(fragment.Fragment)

    fmt.Println(*f.GetNumber()) // prints 0

    //f.number = 8 // error - number is private

    p := f.GetNumber()
    *p = 4                      // works. Now f.number is 4
    fmt.Println(*f.GetNumber()) // prints 4
}

So by using the pointer, I changed the private variable outside of the fragment package. I understand that in for example C, pointers help to avoid copying large struct/arrays and they are supposed to enable you to change whatever they're pointing to. But I don't quite understand how they are supposed to work with private variables.

So my questions are:

1. Shouldn't the private variables stay private, no matter how they are accessed?
2. How is this compared to other languages such as C++/Java? Is it the case there too, that private variables can be changed using pointers outside of the class?

My Background: I know a bit C/C++, rather fluent in Python and new to Go. I learn programming as a hobby so don't know much about technical things happening behind the scenes.

Answer 1

You're not bypassing any access privilegies. If you acquire a *T from any imported package then you can always mutate *T, ie. the pointee at whole, as in an assignment. The imported package designer controls what you can get from the package, so the access control is not yours.

The restriction to what's said above is for structured types (structs), where the previous still holds, but the finer granularity of access control to a particular field is controlled by the field's name case even when referred to by a pointer to the whole structure. The field name must be uppercase to be visible outside its package.

Wrt C++: I believe you can achieve the same with one of the dozens C++ pointer types. Not sure which one, though.

Wrt Java: No, Java has no pointers. Not really comparable to pointers in Go (C, C++, ...).