Why would I make() or new()?

make function allocates and initializes an object of type slice, map, or chan only. Like new, the first argument is a type. But, it can also take a second argument, the size. Unlike new, make’s return type is the same as the type of its argument, not a pointer to it. And the allocated value is initialized (not set to zero value like in new). The reason is that slice, map and chan are data structures. They need to be initialized, otherwise they won't be usable. This is the reason new() and make() need to be different.

The following examples from Effective Go make it very clear:

p *[]int = new([]int) // *p = nil, which makes p useless
v []int = make([]int, 100) // creates v structure that has pointer to an array, length field, and capacity field. So, v is immediately usable

Go has multiple ways of memory allocation and value initialization:

&T{...}, &someLocalVar, new, make

Allocation can also happen when creating composite literals.

new can be used to allocate values such as integers, &int is illegal:

new(Point)
&Point{}      // OK
&Point{2, 3}  // Combines allocation and initialization

new(int)
&int          // Illegal

// Works, but it is less convenient to write than new(int)
var i int
&i

The difference between new and make can be seen by looking at the following example:

p := new(chan int)   // p has type: *chan int
c := make(chan int)  // c has type: chan int

Suppose Go does not have new and make, but it has the built-in function NEW. Then the example code would look like this:

p := NEW(*chan int)  // * is mandatory
c := NEW(chan int)

The * would be mandatory, so:

new(int)        -->  NEW(*int)
new(Point)      -->  NEW(*Point)
new(chan int)   -->  NEW(*chan int)
make([]int, 10) -->  NEW([]int, 10)

make(Point)  // Illegal
make(int)    // Illegal

Yes, merging new and make into a single built-in function is possible. However, it is probable that a single built-in function would lead to more confusion among new Go programmers than having two built-in functions.

Considering all of the above points, it appears more appropriate for new and make to remain separate.


Things you can do with make that you can't do any other way:

  • Create a channel
  • Create a map with space preallocated
  • Create a slice with space preallocated or with len != cap

It's a little harder to justify new. The main thing it makes easier is creating pointers to non-composite types. The two functions below are equivalent. One's just a little more concise:

func newInt1() *int { return new(int) }

func newInt2() *int {
    var i int
    return &i
}

Tags:

Go

Recent Posts