Summary - io package
- Go course
- Intermediate
- Lesson 40/80
- 4 min
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.
Practice Wrapper Reader
Write the buyukReader above and run it with io.Copy(os.Stdout, buyukReader{strings.NewReader("gopher\n")}).
Starter code
package main
import (
"bytes"
"io"
"os"
"strings"
)
func main() {
// buraya yaz
}Sample 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")})
}Expected output
GOPHER
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- The program crashes
- The data is written twice
Explanation: bufio.Writer keeps data in memory until 4 KB fills; Flush sends the remaining bytes to the underlying Writer.
2 Flush the buffer
w := bufio.NewWriter(os.Stdout)
fmt.Fprintln(w, "son satır")
w.____()w := bufio.NewWriter(os.Stdout)
fmt.Fprintln(w, "son satır")
w.Flush()Explanation: Flush writes the bytes waiting in the buffer to the underlying Writer.
3 Group summary
| Term | Meaning |
|---|---|
io.Reader |
Read(p []byte) |
io.Writer |
Write(p []byte) |
bufio.Scanner |
Line-by-line reading |
io.Copy |
Streaming transfer |