为什么 Golang string.Builder String() 可以在没有 reflect.StringHeader 的情况下将 []byte 转换为 slice？

Question

Go's strings package defines a Builder type which has a String() method

func (b *Builder) String() string {
    return *(*string)(unsafe.Pointer(&b.buf))
}

as reflect pacakge indicates, a byte slice is defined as a SliceHeader and an associated data block pointed by the header's Data field:

type SliceHeader struct {
  Data uintptr
  Len  int
  Cap  int
}

and string is defined as a StringHeader and an associated data block pointed by the header's Data field

type StringHeader struct {
  Data uintptr
  Len  int
}

the SliceHeader has 3 fields(Data, Len and Cap) and StringHeader has only 2(Data and Len), so how can one convert byte slice to string directly like this?

*(*string)(unsafe.Pointer(&buf))

in my understanding, we should code following:

func byte2str(b []byte) string {
  hdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
  return *(*string)(unsafe.Pointer(&reflect.StringHeader{Data: hdr.Data, Len: hdr.Len}))
}

Answer 1

Let's break it down.

The expression &buf is a pointer to the slice header. A pointer to a slice is a pointer to the slice header.

The expression (unsafe.Pointer(&buf) is a conversion from the slice header pointer to an unsafe.Pointer . The unsafe.Pointer type has magical a magical property -- it can be converted to and from any other pointer type.

The expression is (*string)(unsafe.Pointer(&buf)) is a conversion from a slice header pointer to a string header pointer (a pointer to a string is a pointer to the header).

The expression *(*string)(unsafe.Pointer(&buf)) dereferences the string header pointer to get the string.

This all works because the the memory layout of a string header is a prefix of the memory layout for a slice header. The final dereference of the string header pointer copies the two string header fields only.