Should I use copy on private slice fields before returning them from getter?

In object-oriented programming, the getter method is usually used to obtain the value of a private member variable. However, sometimes there is a question: Should I use copying of private slice fields before returning them from the getter method? The answer to this question is not absolute and depends on the specific circumstances. In some cases, copying a private slice field can prevent external code from modifying the field, thereby protecting data consistency and security. However, in some scenarios with high performance requirements, the copy operation may cause additional overhead. In this case, you can consider directly returning the reference to the slice field. Therefore, you need to weigh the pros and cons based on the specific situation and choose the appropriate way to handle the return of private slice fields.

Question content

If struct has a private slice field that requires a getter, should the slice be copied before returning it?

I want to confirm if it is safe to return private slices directly from the getter, since I don't want the caller to be able to modify them via the getter's return value.

From my testing so far, it seems that the slices returned are not linked to the private slice fields. However, I'm not sure if this works in all scenarios, and I don't want to use copy if it's not needed.

This is what I tried:

package main

import "fmt"

type basket struct {
    fruits []string
}

func (b *basket) fruits() []string {
    return b.fruits
}

func (b *basket) addfruit(fruit string) {
    b.fruits = append(b.fruits, fruit)
}

func main() {
    basket := &basket{}
    basket.addfruit("apple")
    basket.addfruit("banana")
    basket.addfruit("orange")

    fruits := basket.fruits()
    fmt.println(fruits) // [apple banana orange]

    fruits = append(fruits, "mango")
    fruits = append(fruits, "lemon")
    fruits = append(fruits, "pineapple")
    fmt.println(fruits)          // [apple banana orange mango lemon pineapple]
    fmt.println(basket.fruits()) // [apple banana orange]
}

The following is what the getter of copy looks like:

func (b *Basket) Fruits() []string {
    result := make([]string, len(b.fruits))
    copy(result, b.fruits)
    return result
}

Workaround

As @kostix mentioned in the comments, it depends on the scenario.

For the one in the OP, we want to use copy because we want to separate the getter's output from the private field so the caller can't modify it.

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