$golang$的$string$类型是不可修改的，对于拼接字符串来说，本质上还是创建一个新的对象将数据放进去。主要有以下几种拼接方式
拼接方式介绍
1.使用$string$自带的运算符$+$
ans = ans + s
2. 使用格式化输出$fmt.Sprintf$
ans = fmt.Sprintf("%s%s", ans, s)
3. 使用$strings$的$join$函数
一般适用于将字符串数组转化为特定间隔符的字符串的情况
ans=strings.join(strs,",")
4. 使用$strings.Builder$
builder := strings.Builder{
}
builder.WriteString(s)
return builder.String()
5. 使用$bytes.Buffer$
buffer := new(bytes.Buffer)
buffer.WriteString(s)
return buffer.String()
6. 使用$[]byte$，并且提前设置容量
ans := make([]byte, 0, len(s)*n)
ans = append(ans, s...)
性能对比
先写一个随机生成长度为$n$的字符串的函数
func getRandomString(n int) string {
var tmp = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
ans := make([]uint8, 0, n)
for i := 0; i < n; i++ {
ans = append(ans, tmp[rand.Intn(len(tmp))])
}
return string(ans)
}
接下来分别写出上述拼接方式的实现，假设每次都拼接n次字符串s后返回。
1.使用$string$自带的运算符$+$
循环$n$次，每次都令答案字符串$ans+$源字符串$s$
func plusOperatorJoin(n int, s string) string {
var ans string
for i := 0; i < n; i++ {
ans = ans + s
}
return ans
}
2. 使用格式化输出$fmt.Sprintf$
循环$n$次，使用$fmt.Sprintf$达到拼接的目的
func sprintfJoin(n int, s string) string {
var ans string
for i := 0; i < n; i++ {
ans = fmt.Sprintf("%s%s", ans, s)
}
return ans
}
3. 使用$strings$的$join$函数
拼接同一个字符串的话不适合用$join$函数，所以跳过这种方式
4. 使用$strings.Builder$
初始化$strings.Builder$，循环$n$次，每次调用$WriteString$方法
func stringBuilderJoin(n int, s string) string {
builder := strings.Builder{
}
for i := 0; i < n; i++ {
builder.WriteString(s)
}
return builder.String()
}
5. 使用$bytes.Buffer$
初始化$bytes.Buffer$，循环$n$次，每次调用$WriteString$方法
func bytesBufferJoin(n int, s string) string {
buffer := new(bytes.Buffer)
for i := 0; i < n; i++ {
buffer.WriteString(s)
}
return buffer.String()
}
6. 使用$[]byte$，并且提前设置容量
定义$ans$为$byte$数组，并提前设置容量为$len(s)*n$
func bytesJoin(n int, s string) string {
ans := make([]byte, 0, len(s)*n)
for i := 0; i < n; i++ {
ans = append(ans, s...)
}
return string(ans)
}
测试代码
先随机生成一个长度为10的字符串，然后拼接10000次。
package high_strings
import "testing"
func benchmark(b *testing.B, f func(int, string) string) {
var str = getRandomString(10)
for i := 0; i < b.N; i++ {
f(10000, str)
}
}
func BenchmarkPlusOperatorJoin(b *testing.B) {
benchmark(b, plusOperatorJoin)
}
func BenchmarkSprintfJoin(b *testing.B) {
benchmark(b, sprintfJoin)
}
func BenchmarkStringBuilderJoin(b *testing.B) {
benchmark(b, stringBuilderJoin)
}
func BenchmarkBytesBufferJoin(b *testing.B) {
benchmark(b, bytesBufferJoin)
}
func BenchmarkBytesJoin(b *testing.B) {
benchmark(b, bytesJoin)
}
测试结果：
使用$[]byte$ > $strings.Builder$ >= $bytes.Buffer$ > $fmt.Sprintf$ > $+$运算符
源码分析
1.使用$string$自带的运算符$+$
代码在runtime\string.go里
// concatstrings implements a Go string concatenation x+y+z+...
// The operands are passed in the slice a.
// If buf != nil, the compiler has determined that the result does not
// escape the calling function, so the string data can be stored in buf
// if small enough.
func concatstrings(buf *tmpBuf, a []string) string {
idx := 0
l := 0
count := 0
for i, x := range a {
n := len(x)
if n == 0 {
continue
}
if l+n < l {
throw("string concatenation too long")
}
l += n
count++
idx = i
}
if count == 0 {
return ""
}
// If there is just one string and either it is not on the stack
// or our result does not escape the calling frame (buf != nil),
// then we can return that string directly.
if count == 1 && (buf != nil || !stringDataOnStack(a[idx])) {
return a[idx]
}
s, b := rawstringtmp(buf, l)
for _, x := range a {
copy(b, x)
b = b[len(x):]
}
return s
}
- 首先计算拼接后的字符串长度
- 如果只有一个字符串并且不在栈上就直接返回
- 如果$buf$不为空并且$buf$可以放下这些字符串，就把拼接后的字符串放在$buf$里，否则在堆上重新申请一块内存
func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
if buf != nil && l <= len(buf) {
b = buf[:l]
s = slicebytetostringtmp(&b[0], len(b))
} else {
s, b = rawstring(l)
}
return
}
// rawstring allocates storage for a new string. The returned
// string and byte slice both refer to the same storage.
// The storage is not zeroed. Callers should use
// b to set the string contents and then drop b.
func rawstring(size int) (s string, b []byte) {
p := mallocgc(uintptr(size), nil, false)
return unsafe.String((*byte)(p), size), unsafe.Slice((*byte)(p), size)
}
- 然后遍历数组，将字符串$copy$过去
2. 使用$strings.Builder$
介绍：$strings.Builder$用于使用$Write$方法高效地生成字符串，它最大限度地减少了内存复制
拼接过程：$builder$里有一个$byte$类型的切片，每次调用$WriteString$的时候，是直接往该切片里追加字符串。因为切片底层的扩容机制是以倍数申请的，所以对比1而言，2的内存消耗要更少。
结果返回：在返回字符串的$String$方法里，是将$buf$数组转化为字符串直接返回的。
扩容机制： 想要缓冲区容量增加$n$个字节，扩容后容量变为$2len+n$
```go
// A Builder is used to efficiently build a string using Write methods.
// It minimizes memory copying. The zero value is ready to use.
// Do not copy a non-zero Builder.
type Builder struct {
addr Builder // of receiver, to detect copies by value
buf []byte
}
// String returns the accumulated string.
func (b *Builder) String() string {
return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
}
// grow copies the buffer to a new, larger buffer so that there are at least n
// bytes of capacity beyond len(b.buf).
func (b Builder) grow(n int) {
buf := make([]byte, len(b.buf), 2cap(b.buf)+n)
copy(buf, b.buf)
b.buf = buf
}
// WriteString appends the contents of s to b's buffer.
// It returns the length of s and a nil error.
func (b *Builder) WriteString(s string) (int, error) {
b.copyCheck()
b.buf = append(b.buf, s...)
return len(s), nil
}
## 3. 使用$bytes.Buffer$
**介绍**：$bytes.Buffer$跟$strings.Builder$的底层都是$byte$数组，区别在于扩容机制和返回字符串的$String$方法。
**结果返回：** 因为$bytes.Buffer$实际上是一个流式的字节缓冲区，可以向尾部写入数据，也可以读取头部的数据。所以在返回字符串的$String$方法里，只返回了缓冲区里**未读的部分**，所以需要重新申请内存来存放返回的结果。内存会比$strings.Builder$稍慢一些。
**扩容机制：** 想要缓冲区容量至少增加$n$个字节，$m$是未读的长度，$c$是当前的容量。
优化点在于如果$n <= c/2-m$，也就是当前容量的一半都大于等于现有的内容（未读的字节数）加上所需要增加的字节数，就复用当前的数组，把未读的内容拷贝到头部去。
> We can slide things down instead of allocating a new slice. We only need m+n <= c to slide, but we instead let capacity get twice as large so we don't spend all our time copying.
> 我们可以向下滑动，而不是分配一个新的切片。我们只需要m+n<=c来滑动，但我们让容量增加了一倍，这样我们就不会把所有的时间都花在复制上。
否则的话也是$2*len+n$的扩张
```go
// 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.
}
// 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:])
}
// 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
}
// 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.Reset()
}
// Try to grow by means of a reslice.
if i, ok := b.tryGrowByReslice(n); ok {
return i
}
if b.buf == nil && n <= smallBufferSize {
b.buf = make([]byte, n, smallBufferSize)
return 0
}
c := cap(b.buf)
if n <= c/2-m {
// We can slide things down instead of allocating a new
// slice. We only need m+n <= c 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:])
} else if c > maxInt-c-n {
panic(ErrTooLarge)
} else {
// Add b.off to account for b.buf[:b.off] being sliced off the front.
b.buf = growSlice(b.buf[b.off:], b.off+n)
}
// Restore b.off and len(b.buf).
b.off = 0
b.buf = b.buf[:m+n]
return m
}
