一，Benchmark 比较

下表是在VMware VM（centos7）上做的测试，拼接一个字符串包含1000个字符x。

BenchmarkSimple-2                7265664               164 ns/op             530 B/op          0 allocs/op
BenchmarkSprintf-2               4183986               298 ns/op             546 B/op          1 allocs/op
BenchmarkBytesBuffer-2          90349341              11.9 ns/op               2 B/op          0 allocs/op
BenchmarkStringBuilder-2        370823845             3.23 ns/op               2 B/op          0 allocs/op

我们可以得出以下结果：

从数量级上看，bytes.Buffer (BenchmarkBytesBuffer-2) 和 strings.Builder (BenchmarkStringBuilder-2) 要优于 “+” operator (BenchmarkSimple-2) 和 fmt.Sprintf (BenchmarkSprintf-2)
细微上看，strings.Builder (BenchmarkStringBuilder-2) 优于 bytes.Buffer (BenchmarkBytesBuffer-2)。“+” operator (BenchmarkSimple-2) 优于 fmt.Sprintf (BenchmarkSprintf-2)。

对待一些简单得场景，“+” operator足够好用。而一些复杂字符串的拼接，strings.Builder 完全可以替代 fmt.Sprintf，提高性能。

二，bytes.Buffer

有时候我们不仅仅是拼接字符串，而是字节“串”，例如文件IO，网络IO等。bytes.Buffer 绝对是一个最佳选择，提供丰富易用的函数集。
主要的数据结构：

// 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)]
  lastRead readOp // last read operation, so that Unread* can work correctly.
}

建立新 Buffer - 没有初始值

new(bytes.Buffer)

建立新 Buffer - 有初始值

// NewBuffer creates and initializes a new Buffer using buf as its
// initial contents. The new Buffer takes ownership of buf, and the
// caller should not use buf after this call. NewBuffer is intended to
// prepare a Buffer to read existing data. It can also be used to set
// the initial size of the internal buffer for writing. To do that,
// buf should have the desired capacity but a length of zero.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
// NewBufferString creates and initializes a new Buffer using string s as its
// initial contents. It is intended to prepare a buffer to read an existing
// string.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
func NewBufferString(s string) *Buffer {
  return &Buffer{buf: []byte(s)}
}

写 Buffer

// 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, ok := b.tryGrowByReslice(len(p))
  if !ok {
    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
  m, ok := b.tryGrowByReslice(len(s))
  if !ok {
    m = b.grow(len(s))
  }
  return copy(b.buf[m:], s), nil
}
// WriteByte appends the byte c to the buffer, growing the buffer as needed.
// The returned error is always nil, but is included to match bufio.Writer's
// WriteByte. If the buffer becomes too large, WriteByte will panic with
// ErrTooLarge.
func (b *Buffer) WriteByte(c byte) error {
  b.lastRead = opInvalid
  m, ok := b.tryGrowByReslice(1)
  if !ok {
    m = b.grow(1)
  }
  b.buf[m] = c
  return nil
}
// WriteRune appends the UTF-8 encoding of Unicode code point r to the
// buffer, returning its length and an error, which is always nil but is
// included to match bufio.Writer's WriteRune. The buffer is grown as needed;
// if it becomes too large, WriteRune will panic with ErrTooLarge.
func (b *Buffer) WriteRune(r rune) (n int, err error) {
  if r < utf8.RuneSelf {
    b.WriteByte(byte(r))
    return 1, nil
  }
  b.lastRead = opInvalid
  m, ok := b.tryGrowByReslice(utf8.UTFMax)
  if !ok {
    m = b.grow(utf8.UTFMax)
  }
  n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
  b.buf = b.buf[:m+n]
  return n, nil
}
// WriteTo writes data to w until the buffer is drained or an error occurs.
// The return value n is the number of bytes written; it always fits into an
// int, but it is int64 to match the io.WriterTo interface. Any error
// encountered during the write is also returned.
func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
  b.lastRead = opInvalid
  if nBytes := b.Len(); nBytes > 0 {
    m, e := w.Write(b.buf[b.off:])
    if m > nBytes {
      panic("bytes.Buffer.WriteTo: invalid Write count")
    }
    b.off += m
    n = int64(m)
    if e != nil {
      return n, e
    }
    // all bytes should have been written, by definition of
    // Write method in io.Writer
    if m != nBytes {
      return n, io.ErrShortWrite
    }
  }
  // Buffer is now empty; reset.
  b.Reset()
  return n, nil
}

读 Buffer

// 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.empty() {
    // Buffer is empty, reset to recover space.
    b.Reset()
    if len(p) == 0 {
      return 0, nil
    }
    return 0, io.EOF
  }
  n = copy(p, b.buf[b.off:])
  b.off += n
  if n > 0 {
    b.lastRead = opRead
  }
  return n, nil
}
// Next returns a slice containing the next n bytes from the buffer,
// advancing the buffer as if the bytes had been returned by Read.
// If there are fewer than n bytes in the buffer, Next returns the entire buffer.
// The slice is only valid until the next call to a read or write method.
func (b *Buffer) Next(n int) []byte {
  b.lastRead = opInvalid
  m := b.Len()
  if n > m {
    n = m
  }
  data := b.buf[b.off : b.off+n]
  b.off += n
  if n > 0 {
    b.lastRead = opRead
  }
  return data
}
// ReadByte reads and returns the next byte from the buffer.
// If no byte is available, it returns error io.EOF.
func (b *Buffer) ReadByte() (byte, error) {
  if b.empty() {
    // Buffer is empty, reset to recover space.
    b.Reset()
    return 0, io.EOF
  }
  c := b.buf[b.off]
  b.off++
  b.lastRead = opRead
  return c, nil
}
// ReadRune reads and returns the next UTF-8-encoded
// Unicode code point from the buffer.
// If no bytes are available, the error returned is io.EOF.
// If the bytes are an erroneous UTF-8 encoding, it
// consumes one byte and returns U+FFFD, 1.
func (b *Buffer) ReadRune() (r rune, size int, err error) {
  if b.empty() {
    // Buffer is empty, reset to recover space.
    b.Reset()
    return 0, 0, io.EOF
  }
  c := b.buf[b.off]
  if c < utf8.RuneSelf {
    b.off++
    b.lastRead = opReadRune1
    return rune(c), 1, nil
  }
  r, n := utf8.DecodeRune(b.buf[b.off:])
  b.off += n
  b.lastRead = readOp(n)
  return r, n, nil
}
// UnreadRune unreads the last rune returned by ReadRune.
// If the most recent read or write operation on the buffer was
// not a successful ReadRune, UnreadRune returns an error.  (In this regard
// it is stricter than UnreadByte, which will unread the last byte
// from any read operation.)
func (b *Buffer) UnreadRune() error {
  if b.lastRead <= opInvalid {
    return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
  }
  if b.off >= int(b.lastRead) {
    b.off -= int(b.lastRead)
  }
  b.lastRead = opInvalid
  return nil
}
var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
// UnreadByte unreads the last byte returned by the most recent successful
// read operation that read at least one byte. If a write has happened since
// the last read, if the last read returned an error, or if the read read zero
// bytes, UnreadByte returns an error.
func (b *Buffer) UnreadByte() error {
  if b.lastRead == opInvalid {
    return errUnreadByte
  }
  b.lastRead = opInvalid
  if b.off > 0 {
    b.off--
  }
  return nil
}
// ReadBytes reads until the first occurrence of delim in the input,
// returning a slice containing the data up to and including the delimiter.
// If ReadBytes encounters an error before finding a delimiter,
// it returns the data read before the error and the error itself (often io.EOF).
// ReadBytes returns err != nil if and only if the returned data does not end in
// delim.
func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
  slice, err := b.readSlice(delim)
  // return a copy of slice. The buffer's backing array may
  // be overwritten by later calls.
  line = append(line, slice...)
  return line, err
}
// 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
}

置零 Buffer

// Reset resets the buffer to be empty,
// but it retains the underlying storage for use by future writes.
// Reset is the same as Truncate(0).
func (b *Buffer) Reset() {
  b.buf = b.buf[:0]
  b.off = 0
  b.lastRead = opInvalid
}

导出 Buffer

// 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>".
//
// To build strings more efficiently, see the strings.Builder type.
func (b *Buffer) String() string {
  if b == nil {
    // Special case, useful in debugging.
    return "<nil>"
  }
  return string(b.buf[b.off:])
}
// 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 }
// 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) }

一个例子：来自以太坊 Discovery 中的 Packet Encode 代码。

// Encode encodes a discovery packet.
func Encode(priv *ecdsa.PrivateKey, req Packet) (packet, hash []byte, err error) {
  b := new(bytes.Buffer)
  b.Write(headSpace)
  b.WriteByte(req.Kind())
  if err := rlp.Encode(b, req); err != nil {
    return nil, nil, err
  }
  packet = b.Bytes()
  sig, err := crypto.Sign(crypto.Keccak256(packet[headSize:]), priv)
  if err != nil {
    return nil, nil, err
  }
  copy(packet[macSize:], sig)
  // Add the hash to the front. Note: this doesn't protect the packet in any way.
  hash = crypto.Keccak256(packet[macSize:])
  copy(packet, hash)
  return packet, hash, nil
}

参考链接：

Golang 点滴

Golang 之字符串拼接

一，Benchmark 比较

二，bytes.Buffer