Summary - io package

Combine Reader and Writer to write a file copier.

io package summary

I/O in Go is built on two small interfaces: io.Reader and io.Writer. Functions that accept them work the same with files, networks, memory and HTTP, and are easy to test. Your toolbox: io.ReadAll, io.Copy, io.LimitReader, io.TeeReader, io.MultiWriter, bufio.Scanner, bufio.Writer.

Rule: for large data, don't read it all — stream it.

Your own Reader

Implementing the interface is easy. A Reader wrapper that upper-cases every letter:

type buyukReader struct{ r io.Reader }

func (b buyukReader) Read(p []byte) (int, error) {
    n, err := b.r.Read(p)
    copy(p, bytes.ToUpper(p[:n]))
    return n, err
}

This wrapper works directly with io.Copy, bufio.Scanner and every other tool.

Exercise Wrapper Reader

Write the buyukReader above and run it with io.Copy(os.Stdout, buyukReader{strings.NewReader("gopher\n")}).

Show example solution

package main

import (
    "bytes"
    "io"
    "os"
    "strings"
)

type buyukReader struct{ r io.Reader }

func (b buyukReader) Read(p []byte) (int, error) {
    n, err := b.r.Read(p)
    copy(p, bytes.ToUpper(p[:n]))
    return n, err
}

func main() {
    io.Copy(os.Stdout, buyukReader{strings.NewReader("gopher\n")})
}

Test yourself

What happens if you use <code>bufio.NewWriter</code> and don't call <code>Flush</code>?
- It has no effect
- The last data in the buffer is never written
- The program crashes
- The data is written twice
Show answer

The last data in the buffer is never written — bufio.Writer keeps data in memory until 4 KB fills; Flush sends the remaining bytes to the underlying Writer.

Flush the buffer

w := bufio.NewWriter(os.Stdout)
fmt.Fprintln(w, "son satır")
w.____()

Show answer

w := bufio.NewWriter(os.Stdout)
fmt.Fprintln(w, "son satır")
w.Flush()

Group summary

io.Reader

Read(p []byte)
io.Writer

Write(p []byte)
bufio.Scanner

Line-by-line reading
io.Copy

Streaming transfer

Next lesson: Test function

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