Structs, methods, and small interfaces

The idea

Enter with structs and function calls understood. A struct groups named fields into one value. A method is a function associated with a receiver type. A value receiver works with a copy of the receiver; a pointer receiver can modify the original record through its pointer. Choose the receiver deliberately rather than expecting every method call to mutate its caller’s variable.

An interface describes required methods. A type satisfies it by having the appropriate method set, without an explicit implements declaration. Keep interfaces focused on what a consumer needs, such as a single Size method, instead of collecting unrelated behavior. Pointer and value method sets differ, so passing a value and passing its address are not interchangeable in every interface assignment. This chapter uses a value method for read-only behavior.

package main
import "fmt"
type Box struct { Count int }
func (b *Box) Add(n int){b.Count+=n}
func main(){b:=Box{Count:2}; b.Add(3); fmt.Println(b.Count)}

5

The pointer receiver refers to b’s original storage, so Add changes its Count field from two to five.