Go: new vs make

Tags: #go #performance

1. README.md

Summary

make: allocates memory to hold a built-in type (slice, map or channel) with len and cap (capacity) set accordingly to the type’s zero value (immediately usable).
new: allocates memory to hold both built-in and custom user types with fields zeroed, not immediately usable for some types.

NOTE: This means t := new(T) and var t *T are equivalent.

Example:

m := new(int)
fmt.Printf("%+v | %p | %T\n", *m, m, m) // 0 | 0xc000110008 | *int`

main.go

package main

import (
	"fmt"
)

func main() {
	// The new built-in function allocates memory.
	// The first argument is a type, not a value.
	// The value returned is a pointer to a newly allocated zero value of that type.
	p := new([]int)
	// len, cap == 0

	fmt.Println("underlying array should be nil, i.e. comparison should return true:", *p == nil)

	// we can't append to p (as p is a pointer)
	// so we have to dereference p to append to it
	// also *p == nil until we append to the underlying array
	// once we've appended some data the underlying array is no longer nil
	*p = append(*p, 1)

	fmt.Println("underlying array should no longer be nil, i.e. comparison should return false:", *p == nil)

	// The make built-in function allocates and initializes an object of type slice, map, or chan (only).
	// Like new, the first argument is a type, not a value.
	// Unlike new, make's return type is the same as the type of its argument, not a pointer to it.
	v := make([]int, 5, 10)
	// len = 5, cap = 10

	fmt.Println("v should not be nil, i.e. comparison should return false:", v == nil)

	fmt.Printf("\np:\n%+v\n%T\n\n", p, p) // &[]
	fmt.Printf("v:\n%+v\n%T\n\n", v, v)   // [0 0 0 0 0]

}