Where do the square brackets come from?

Question

package main

import (
    "fmt"
    "log"
)

func main() {
    a := []string{"abc", "edf"}
    log.Println(fmt.Sprint(a))
}

The above Go program will print the following output, with slice value inside the square brackets "[]".

2009/11/10 23:00:00 [abc edf]

And I want to know where in the source code that [] is added into the formatted string.

I checked the source code src/fmt/print.go file, but couldn't find the exact line of code that does this.

Could anyone provide a hint?

Answer 1

You are printing the value of a slice. It is formatted / printed in print.go, unexported function printReflectValue(), currently line #980:

855 func (p *pp) printReflectValue(value reflect.Value, verb rune, depth int)
                                           (wasString bool) {

        // ...

947     case reflect.Array, reflect.Slice:
            // ...

979         } else {
980             p.buf.WriteByte('[')
981         }

and line #995:

994         } else {
995             p.buf.WriteByte(']')
996         }

Note that this is for "general" slices (like your []string), byte slices are handled differently:

948         // Byte slices are special:
949         // - Handle []byte (== []uint8) with fmtBytes.
950         // - Handle []T, where T is a named byte type, with fmtBytes only

[]byte is printed in the unexported function fmtBytes():

533 func (p *pp) fmtBytes(v []byte, verb rune, typ reflect.Type, depth int) {

            // ...

551         } else {
552             p.buf.WriteByte('[')
553         }

            // ...

566         } else {
567             p.buf.WriteByte(']')
568         }