Value vs Reference Semantics

Go passes function arguments by value. Whether a mutation is visible elsewhere depends on the kind of value that was copied and what storage it refers to.

An int argument is copied, so changing the parameter leaves the caller’s integer alone. A slice value contains a view of a backing array; passing a slice copies that view, so changing an existing element is normally visible through other views of the same array.

Maps similarly refer to map storage; assigning a map variable or passing one does not clone all entries. Pointers copy an address while sharing its target. Do not describe Go as having a second pass-by-reference argument mode: it does not.

Read the labels and compare the identities, types and storage relationships shown here.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func edit(n int, values []int, labels map[string]int) {
	n = 99
	values[0] = 99
	labels["a"] = 99
}
func main() {
	n := 1
	values := []int{1, 2}
	labels := map[string]int{"a": 1}
	edit(n, values, labels)
	fmt.Println("Number:", n)
	fmt.Println("Slice:", values)
	fmt.Println("Map entry:", labels["a"])
}

Output

Number: 1
Slice: [99 2]
Map entry: 99

All three arguments are copied into parameters. The standalone integer remains 1, but the slice’s shared backing array element changes to 99, and the shared map entry changes to 99.

• Go does not have pass-by-reference parameters as a distinct calling convention.
• Appending to a copied slice header can reallocate storage and does not automatically update the caller’s slice length.

Modify the function to append to values, then explain why the caller must receive the returned slice if it needs the new length.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Value vs Reference Semantics in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.