Is it safe to swap two integers by `a, b = b, a` in golang?

Question

package main

import "fmt"

func main() {
    a := 1
    b := 2

    fmt.Printf("Before Swap: %v %v\n", a, b)
    a, b = b, a
    fmt.Printf(" After Swap: %v %v\n", a, b)
}

The output is:

Before Swap: 1 2  
 After Swap: 2 1

It looks good. I just wonder:

Is it safe to swap two integers by a, b = b, a in golang?

Answer 1

Yes, it's safe to swap two variables using:

a, b = b, a

The assignment happens in two steps:

• The right hand side of the assignment is evaluated first.
• Then the evaluated values will be assigned to the left hand side in left-to-right order.

In other words, a, b = b, a will be rewritten roughly to:

tmpB := b
tmpA := a
a = tmpB
b = tmpA

That's why the assignment is safe.

---

In fact, the compiler is smart enough to avoid a second temporary variable, so a, b = b, a will be rewritten as:

tmpA := a
a = b
b = tmpA

$ cat x.go 
package p

func f(a, b int) {
    a, b = b, a
}
$ go tool compile -W x.go

before walk f
.   AS2 tc(1) # x.go:4:7
.   AS2-Lhs
.   .   NAME-p.a esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:8
.   .   NAME-p.b esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:11
.   AS2-Rhs
.   .   NAME-p.b esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:11
.   .   NAME-p.a esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:8
after walk f
.   BLOCK # x.go:4:7
.   BLOCK-List
.   .   AS tc(1) # x.go:4:7
.   .   .   NAME-p..autotmp_2 esc(N) Class:PAUTO Offset:0 AutoTemp OnStack Used int tc(1) # x.go:4:7
.   .   .   NAME-p.a esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:8
.   .   AS tc(1) # x.go:4:7
.   .   .   NAME-p.a esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:8
.   .   .   NAME-p.b esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:11
.   .   AS tc(1) # x.go:4:7
.   .   .   NAME-p.b esc(no) Class:PPARAM Offset:0 OnStack Used int tc(1) # x.go:3:11
.   .   .   NAME-p..autotmp_2 esc(N) Class:PAUTO Offset:0 AutoTemp OnStack Used int tc(1) # x.go:4:7