我们在对文件进行io操作的时候，经常看到需要我们传递一个 io.reader 或者 io.writer 对象作为读写的入参， 那么我们该如何或者这些个rw对象呢？ 其实很简单，你只需要查找一下哪些对象实现了 read或者 writer方法，那么你只需要创建一个实现了这2个方法之一的对象 ， 那他就可以是一个 io.reader 或者 io.writer 。
当然最常见的应该就是我们的 os.file对象了， 另外还有 bufio.reader， bytes.buffer 等对象都可以作为io的rw入参。
当然你也可以自己定义一个对象，实现 io.reader 或者 io.writer 接口中定义的方法，那么你的对象也可以作为一个rw入参来使用了。 这个也就是go语言中面向接口编程的完美体现。
go中reader writer接口定义
type reader interface {
read(p []byte) (n int, err error)
}
type writer interface {
write(p []byte) (n int, err error)
}
os.file对象中的rw实现代码
// read reads up to len(b) bytes from the file and stores them in b.
// it returns the number of bytes read and any error encountered.
// at end of file, read returns 0, io.eof.
func (f *file) read(b []byte) (n int, err error) {
if err := f.checkvalid("read"); err != nil {
return 0, err
}
n, e := f.read(b)
return n, f.wraperr("read", e)
}
// write writes len(b) bytes from b to the file.
// it returns the number of bytes written and an error, if any.
// write returns a non-nil error when n != len(b).
func (f *file) write(b []byte) (n int, err error) {
if err := f.checkvalid("write"); err != nil {
return 0, err
}
n, e := f.write(b)
if n < 0 {
n = 0
}
if n != len(b) {
err = io.errshortwrite
}
epipecheck(f, e)
if e != nil {
err = f.wraperr("write", e)
}
return n, err
}
bufio.reader中的rw实现代码
// read reads data into p.
// it returns the number of bytes read into p.
// the bytes are taken from at most one read on the underlying reader,
// hence n may be less than len(p).
// to read exactly len(p) bytes, use io.readfull(b, p).
// if the underlying reader can return a non-zero count with io.eof,
// then this read method can do so as well; see the [io.reader] docs.
func (b *reader) read(p []byte) (n int, err error) {
n = len(p)
if n == 0 {
if b.buffered() > 0 {
return 0, nil
}
return 0, b.readerr()
}
if b.r == b.w {
if b.err != nil {
return 0, b.readerr()
}
if len(p) >= len(b.buf) {
// large read, empty buffer.
// read directly into p to avoid copy.
n, b.err = b.rd.read(p)
if n < 0 {
panic(errnegativeread)
}
if n > 0 {
b.lastbyte = int(p[n-1])
b.lastrunesize = -1
}
return n, b.readerr()
}
// one read.
// do not use b.fill, which will loop.
b.r = 0
b.w = 0
n, b.err = b.rd.read(b.buf)
if n < 0 {
panic(errnegativeread)
}
if n == 0 {
return 0, b.readerr()
}
b.w = n
}
// copy as much as we can
// note: if the slice panics here, it is probably because
// the underlying reader returned a bad count. see issue 49795.
n = copy(p, b.buf[b.r:b.w])
b.r = n
b.lastbyte = int(b.buf[b.r-1])
b.lastrunesize = -1
return n, nil
}
// writebuf writes the reader's buffer to the writer.
func (b *reader) writebuf(w io.writer) (int64, error) {
n, err := w.write(b.buf[b.r:b.w])
if n < 0 {
panic(errnegativewrite)
}
b.r = n
return int64(n), err
}
bytes.buffer中的rw实现代码
// 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
}
// 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
}
注意这些方法一般是绑定在指针类型的对象上， 所以你在创建你需要的rw对象的时候需要使用&指针符号或者使用 new函数来创建对象， 如：w := &bytes.buffer{} 等效于 w := new(bytes.buffer)
