// A Buffer is a variable-sized buffer of bytes with Read and Write methods.
// The zero value for Buffer is an empty buffer ready to use.
type Buffer struct {
 buf       []byte   // contents are the bytes buf[off : len(buf)]
 off       int      // read at buf[off], write at buf[len(buf)]--->指示读指针
 bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.
 lastRead  readOp   // last read operation, so that Unread* can work correctly.
}

// Write appends the contents of p to the buffer, growing the buffer as
// needed. The return value n is the length of p; err is always nil. If the
// buffer becomes too large, Write will panic with ErrTooLarge.
func (b *Buffer) Write(p []byte) (n int, err error) {
 b.lastRead = opInvalid
 m := b.grow(len(p))
 return copy(b.buf[m:], p), nil
}

// WriteString appends the contents of s to the buffer, growing the buffer as
// needed. The return value n is the length of s; err is always nil. If the
// buffer becomes too large, WriteString will panic with ErrTooLarge.
func (b *Buffer) WriteString(s string) (n int, err error) {
 b.lastRead = opInvalid
 //返回写入的index
 m := b.grow(len(s))
 return copy(b.buf[m:], s), nil
}

// Read reads the next len(p) bytes from the buffer or until the buffer
// is drained. The return value n is the number of bytes read. If the
// buffer has no data to return, err is io.EOF (unless len(p) is zero);
// otherwise it is nil.
func (b *Buffer) Read(p []byte) (n int, err error) {
 b.lastRead = opInvalid
 if b.off >= len(b.buf) {
  // Buffer is empty, reset to recover space.
  b.Truncate(0)
  if len(p) == 0 {
   return
  }
  return 0, io.EOF
 }
 n = copy(p, b.buf[b.off:])
 b.off += n
 if n > 0 {
  b.lastRead = opRead
 }
 return
}

// ReadString reads until the first occurrence of delim in the input,
// returning a string containing the data up to and including the delimiter.
// If ReadString encounters an error before finding a delimiter,
// it returns the data read before the error and the error itself (often io.EOF).
// ReadString returns err != nil if and only if the returned data does not end
// in delim.
func (b *Buffer) ReadString(delim byte) (line string, err error) {
 slice, err := b.readSlice(delim)
 return string(slice), err
}

// Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
// The slice is valid for use only until the next buffer modification (that is,
// only until the next call to a method like Read, Write, Reset, or Truncate).
// The slice aliases the buffer content at least until the next buffer modification,
// so immediate changes to the slice will affect the result of future reads.
func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }

// String returns the contents of the unread portion of the buffer
// as a string. If the Buffer is a nil pointer, it returns "nil>".
func (b *Buffer) String() string {
 if b == nil {
  // Special case, useful in debugging.
  return "nil>"
 }
 return string(b.buf[b.off:])
}

// Cap returns the capacity of the buffer's underlying byte slice, that is, the
// total space allocated for the buffer's data.
func (b *Buffer) Cap() int { return cap(b.buf) }

// Len returns the number of bytes of the unread portion of the buffer;
// b.Len() == len(b.Bytes()).
func (b *Buffer) Len() int { return len(b.buf) - b.off }

// grow grows the buffer to guarantee space for n more bytes.
// It returns the index where bytes should be written.
// If the buffer can't grow it will panic with ErrTooLarge.
func (b *Buffer) grow(n int) int {
 m := b.Len()
 // If buffer is empty, reset to recover space.
 if m == 0  b.off != 0 {
  b.Truncate(0)
 }
 //如果需要的容量大于现在的容量--->
 if len(b.buf)+n > cap(b.buf) {
  var buf []byte
  //现有的预备64byte可以满足
  if b.buf == nil  n = len(b.bootstrap) {
   buf = b.bootstrap[0:]
   //实际需要的小于本身切片容量
  } else if m+n = cap(b.buf)/2 {
   // We can slide things down instead of allocating a new
   // slice. We only need m+n = cap(b.buf) to slide, but
   // we instead let capacity get twice as large so we
   // don't spend all our time copying.
   copy(b.buf[:], b.buf[b.off:])
   buf = b.buf[:m]
  } else {
   // not enough space anywhere
   //不够，那就分配2倍加n的容量
   buf = makeSlice(2*cap(b.buf) + n)
   copy(buf, b.buf[b.off:])
  }
  b.buf = buf
  b.off = 0
 }
 b.buf = b.buf[0 : b.off+m+n]
 return b.off + m
}

type Buffer struct {
 buf      []byte // contents are the bytes buf[off : len(buf)]
 off      int    // read at buf[off], write at buf[len(buf)]
 lastRead readOp // last read operation, so that Unread* can work correctly.
}

var b bytes.Buffer           //直接定义一个Buffer变量，不用初始化，可以直接使用
b := new(bytes.Buffer)       //使用New返回Buffer变量
b := bytes.NewBuffer(s []byte)     //从一个[]byte切片，构造一个Buffer
b := bytes.NewBufferString(s string) //从一个string变量，构造一个Buffer

b.Write(d []byte) (n int, err error)      //将切片d写入Buffer尾部
b.WriteString(s string) (n int, err error)   //将字符串s写入Buffer尾部
b.WriteByte(c byte) error        //将字符c写入Buffer尾部
b.WriteRune(r rune) (n int, err error)      //将一个rune类型的数据放到缓冲区的尾部
b.ReadFrom(r io.Reader) (n int64, err error) //从实现了io.Reader接口的可读取对象写入Buffer尾部

//读取 n 个字节数据并返回，如果 buffer 不足 n 字节，则读取全部
b.Next(n int) []byte
//一次读取 len(p) 个 byte 到 p 中，每次读取新的内容将覆盖p中原来的内容。成功返回实际读取的字节数，off 向后偏移 n，buffer 没有数据返回错误 io.EOF
b.Read(p []byte) (n int, err error)
//读取第一个byte并返回，off 向后偏移 n
b.ReadByte() (byte, error)
//读取第一个 UTF8 编码的字符并返回该字符和该字符的字节数，b的第1个rune被拿掉。如果buffer为空，返回错误 io.EOF，如果不是UTF8编码的字符，则消费一个字节，返回 (U+FFFD,1,nil)
b.ReadRune() (r rune, size int, err error)
//读取缓冲区第一个分隔符前面的内容以及分隔符并返回，缓冲区会清空读取的内容。如果没有发现分隔符，则返回读取的内容并返回错误io.EOF
b.ReadBytes(delimiter byte) (line []byte, err error)
//读取缓冲区第一个分隔符前面的内容以及分隔符并作为字符串返回，缓冲区会清空读取的内容。如果没有发现分隔符，则返回读取的内容并返回错误 io.EOF
b.ReadString(delimiter byte) (line string, err error)
//将 Buffer 中的内容输出到实现了 io.Writer 接口的可写入对象中，成功返回写入的字节数，失败返回错误
b.WriteTo(w io.Writer) (n int64, err error)

b.Bytes() []byte  //返回字节切片
b.Cap() int    //返回 buffer 内部字节切片的容量
b.Grow(n int)   //为 buffer 内部字节切片的容量增加 n 字节
b.Len() int    //返回缓冲区数据长度，等于 len(b.Bytes())
b.Reset()     //清空数据
b.String() string  //字符串化
b.Truncate(n int)  //丢弃缓冲区中除前n个未读字节以外的所有字节。如果 n 为负数或大于缓冲区长度，则引发 panic
b.UnreadByte() error //将最后一次读取操作中被成功读取的字节设为未被读取的状态，即将已读取的偏移 off 减 1
b.UnreadRune() error //将最后一次 ReadRune() 读取操作返回的 UTF8 字符 rune设为未被读取的状态，即将已读取的偏移 off 减去 字符 rune 的字节数

My name is dablelv

package main
import (
 "os"
 "fmt"
 "bytes"
)
func main() {
    file, _ := os.Open("./test.txt")    
    buf := bytes.NewBufferString("Hello world ")    
    buf.ReadFrom(file)              //将text.txt内容追加到缓冲器的尾部    
    fmt.Println(buf.String())
}