Go's Value vs. Pointer Semantics: When Do Functions Modify Original Variables?

Value and Pointer Semantics in Go

Value semantics refers to the passing of a copy of a variable's value to a function or method, allowing the function to only modify this copy without affecting the original value.

Pointer semantics, on the other hand, allows a function to modify the original value by passing a pointer to it. Although Go passes everything by value, some types exhibit pointer semantics.

Example: Value Semantics

Consider the following example:

func main() {
    i := 1
    fmt.Println("double:", double(i))
    fmt.Println("original i:", i)
}

func double(i int) int {
    i *= 2
    return i
}

Here, passing i to double creates a copy, and any modification in double doesn't affect the original i variable.

Example: Pointer Semantics

To demonstrate pointer semantics, let's use a slice:

func main() {
    is := []int{1, 2}
    fmt.Println("double:", doubles(is))
    fmt.Println("original is:", is)
}

func doubles(is []int) []int {
    for i := range is {
        is[i] *= 2
    }
    return is
}

In this case, passing is to doubles creates a copy of the slice header, but the pointer to the underlying array remains the same. Any modification to the elements in doubles is therefore reflected in the original is slice.

Reasoning

In Go, everything is passed by value, but slices are composed of a structure containing a pointer to an array. When a slice is passed, the slice header copy includes the same pointer, allowing functions to modify the original array elements. This behavior is known as pointer semantics.

Conclusion

Understanding value and pointer semantics is crucial in Go. It determines whether functions can modify original values or only copies.

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