Pointers and controlled mutation

The idea

A pointer holds the address of a value. The address operator & obtains a pointer, and * accesses the pointed-to value. Passing an ordinary struct to a function copies that struct value; passing its pointer lets the function modify the shared original. A nil pointer must not be dereferenced. Make mutation visible in the function contract so callers understand which state can change.

A copied struct can still contain slices, maps, or pointers that refer to shared data. Copying the outer struct therefore does not promise a completely independent object graph. A pointer receiver is useful when a method intentionally updates the receiver. A value receiver operates on a copy of the receiver, although nested references can still expose shared mutable state.

package main
import "fmt"
type Counter struct{ N int }
func bump(c *Counter){ c.N++ }
func main(){ a := Counter{2}; b := a; bump(&b); fmt.Println(a.N,b.N) }

2 3

The initial assignment copies the integer field. Passing b’s address permits bump to update b alone.