Answer

In Go, by default, all function arguments are passed by value: the value of the variable is copied. However, some types (for example, slices, maps, channels) are "wrappers" around internal structures (pointers). Passing a slice by value only copies the slice descriptor, not the data; both variables refer to the same array. In the case of structs — the entire structure is copied.

If you want to avoid copying and work with the original structure, you use pointer passing (*Struct).

Example:

type User struct {
    Name string
    Age  int
}

func updateUser(u User) {
    u.Age = 30 // will only change the copy
}

func updateUserPtr(u *User) {
    u.Age = 30 // will change the original
}

func main() {
    u := User{"Ivan", 25}
    updateUser(u)
    fmt.Println(u.Age) // 25
    updateUserPtr(&u)
    fmt.Println(u.Age) // 30
}

Trick Question

Are changes to a slice passed into a function always visible outside the function?

No!

• If the contents of the slice are modified (slice[i] = ...), it is visible outside.
• If the slice itself is modified (for example, slice = append(slice, ...)), and the result is not returned from the function — the new elements will be in a local copy, and you will lose them.

Example:

func addElem(s []int) {
    s = append(s, 100)
}

func main() {
    arr := []int{1,2,3}
    addElem(arr)
    fmt.Println(arr) // [1 2 3] — 100 was not added
}

Examples of Real Errors Due to Ignorance of Nuances

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Story

In one project, data structures with large struct fields (200+ bytes) were passed by value through channels between goroutines, causing huge overheads in copying and performance loss. After switching to pointer passing, latency decreased significantly.

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Story

In an audit logging service, a developer passed a map between functions without explicit cloning. Changes made by one function unexpectedly altered data in another part of the program, causing confusion in the log.

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Story

In a function for dynamically increasing a slice, the new slice was forgotten to be returned. As a result, changes were not reflected in the calling code, leading to the loss of some transactions. It was decided to return the new slice from the function.