How do you clear a slice in Go

Question

What is the appropriate way to clear a slice in Go   Here s what I ve found in the go forums      test go package main  import        fmt     func main         letters      string  a    b    c    d       fmt Println cap letters       fmt Println len letters          clear the slice     letters   letters  0      fmt Println cap letters       fmt Println len letters       Is this correct   To clarify  the buffer is cleared so it can be reused   An example is Buffer Truncate function in the bytes package   Notice that Reset just calls Truncate 0   So it appears that in this case line 70 would evaluate      b buf   b buf 0   0   http   golang org src pkg bytes buffer go     Truncate discards all but the first n unread bytes from the buffer  60     It panics if n is negative or greater than the length of the buffer  61  func  b  Buffer  Truncate n int    62      b lastRead   opInvalid 63      switch   64      case n  lt  0    n  gt  b Len    65          panic  bytes Buffer  truncation out of range   66      case n    0  67             Reuse buffer space  68          b off   0 69        70      b buf   b buf 0   b off n  71    72   73     Reset resets the buffer so it has no content  74     b Reset   is the same as b Truncate 0   75  func  b  Buffer  Reset     b Truncate 0

User · Answer

I was looking into this issue a bit for my own purposes; I had a slice of structs (including some pointers) and I wanted to make sure I got it right; ended up on this thread, and wanted to share my results.

To practice, I did a little go playground: https://play.golang.org/p/9i4gPx3lnY

which evals to this:

package main

import "fmt"

type Blah struct {
    babyKitten int
    kittenSays *string
}

func main() {
    meow := "meow"
    Blahs := []Blah{}
    fmt.Printf("Blahs: %v\n", Blahs)
    Blahs = append(Blahs, Blah{1, &meow})
    fmt.Printf("Blahs: %v\n", Blahs)
    Blahs = append(Blahs, Blah{2, &meow})
    fmt.Printf("Blahs: %v\n", Blahs)
    //fmt.Printf("kittenSays: %v\n", *Blahs[0].kittenSays)
    Blahs = nil
    meow2 := "nyan"
    fmt.Printf("Blahs: %v\n", Blahs)
    Blahs = append(Blahs, Blah{1, &meow2})
    fmt.Printf("Blahs: %v\n", Blahs)
    fmt.Printf("kittenSays: %v\n", *Blahs[0].kittenSays)
}

Running that code as-is will show the same memory address for both "meow" and "meow2" variables as being the same:

Blahs: []
Blahs: [{1 0x1030e0c0}]
Blahs: [{1 0x1030e0c0} {2 0x1030e0c0}]
Blahs: []
Blahs: [{1 0x1030e0f0}]
kittenSays: nyan

which I think confirms that the struct is garbage collected. Oddly enough, uncommenting the commented print line, will yield different memory addresses for the meows:

Blahs: []
Blahs: [{1 0x1030e0c0}]
Blahs: [{1 0x1030e0c0} {2 0x1030e0c0}]
kittenSays: meow
Blahs: []
Blahs: [{1 0x1030e0f8}]
kittenSays: nyan

I think this may be due to the print being deferred in some way (?), but interesting illustration of some memory mgmt behavior, and one more vote for:

[]MyStruct = nil

User · Answer

It all depends on what is your definition of  clear   One of the valid ones certainly is   slice   slice  0    But there s a catch  If slice elements are of type T   var slice   T   then enforcing len slice  to be zero  by the above  trick    doesn t make any element of  slice  cap slice     eligible for garbage collection  This might be the optimal approach in some scenarios  But it might also be a cause of  memory leaks  - memory not used  but potentially reachable  after re-slicing of  slice   and thus not garbage  collectable

User · Answer

Setting the slice to nil is the best way to clear a slice   nil slices in go are perfectly well behaved and setting the slice to nil will release the underlying memory to the garbage collector   See playground  package main  import        fmt     func dump letters   string        fmt Println  letters      letters      fmt Println cap letters       fmt Println len letters       for i    range letters           fmt Println i  letters i            func main         letters      string  a    b    c    d       dump letters         clear the slice     letters   nil     dump letters         add stuff back to it     letters   append letters   e       dump letters      Prints  letters     a b c d  4 4 0 a 1 b 2 c 3 d letters       0 0 letters     e  1 1 0 e   Note that slices can easily be aliased so that two slices point to the same underlying memory   The setting to nil will remove that aliasing   This method changes the capacity to zero though

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