reflect.SliceHeader and reflect.StringHeader examples

example_test.go

package reflect_test

// Extracted from https://go-review.googlesource.com/c/go/+/401434

import (
  "fmt"
  "reflect"
  "unsafe"
)

func ExampleStringHeader() {
	src := "Hello World!"
	var dst string

	// The code in this example shows the correct way of using StringHeader by
	// obtaining a pointer to the string's header and then using the pointer to
	// read/write the header fields.
	//
	// On the contrary, all of the following are NOT correct and must NOT be used:
	//
	//   var s string
	//   *(*reflect.StringHeader)(unsafe.Pointer(&s)) = reflect.StringHeader{
	//	   Data: /* ... */, Len: /* ... */,
	//   }
	//
	//   s := *(*string)(unsafe.Pointer(&reflect.StringHeader{
	//     Data: /* ... */, Len: /* ... */,
	//   }))
	//
	//   var sh reflect.StringHeader
	//   sh.Data, sh.Len = /* ... */
	//   s := *(*string)(unsafe.Pointer(&sh))
	//
	// go vet attempts to identify and report suspicious uses of reflect.StringHeader.
	// See https://github.com/golang/go/issues/40701 for more details.
	srcHdr := (*reflect.StringHeader)(unsafe.Pointer(&src))
	dstHdr := (*reflect.StringHeader)(unsafe.Pointer(&dst))
	// Equivalent to dst = src[6:6+5], but without bounds checks!
	dstHdr.Data = srcHdr.Data + 6
	dstHdr.Len = 5
	runtime.KeepAlive(src) // Ensure src is not GCed prematurely.

	fmt.Println(dst)

	// Output:
	// World
}

func ExampleSliceHeader() {
	src := []byte("Hello World!")
	var dst []byte

	// The code in this example shows the correct way of using SliceHeader by
	// obtaining a pointer to the slice's header and then using the pointer to
	// read/write the header fields.
	//
	// On the contrary, all of the following are NOT correct and must NOT be used:
	//
	//   var s []byte
	//   *(*reflect.SliceHeader)(unsafe.Pointer(&s)) = reflect.SliceHeader{
	//	   Data: /* ... */, Len: /* ... */, Cap: /* ... */,
	//   }
	//
	//   s := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
	//     Data: /* ... */, Len: /* ... */, Cap: /* ... */,
	//   }))
	//
	//   var sh reflect.SliceHeader
	//   sh.Data, sh.Len, sh.Cap = /* ... */
	//   s := *(*[]byte)(unsafe.Pointer(&sh))
	//
	// go vet attempts to identify and report suspicious uses of reflect.SliceHeader.
	// See https://github.com/golang/go/issues/40701 for more details.
	srcHdr := (*reflect.SliceHeader)(unsafe.Pointer(&src))
	dstHdr := (*reflect.SliceHeader)(unsafe.Pointer(&dst))
	// Equivalent to dst = src[6:6+5:6+5], but without bounds checks!
	dstHdr.Data = srcHdr.Data + 6
	dstHdr.Len = 5
	dstHdr.Cap = 5
	runtime.KeepAlive(src) // Ensure src is not GCed prematurely.

	fmt.Println(string(dst))

	// Output:
	// World
}