io.Writer

Learn the Writer interface and the Write method.

io.Writer

io.Writer is the counterpart of Reader:

type Writer interface {
    Write(p []byte) (n int, err error)
}

os.Stdout, files, bytes.Buffer, http.ResponseWriter, hashes and compressors are Writers. fmt.Fprintf(w, ...) writes formatted output to any Writer, so the same function can produce output to the screen, a file or an HTTP response.

io.MultiWriter and short writes

io.MultiWriter(a, b) duplicates every write to several targets (e.g. both the screen and a log file). By contract, Write must return an error if it didn't write all bytes, so checking only err is usually enough.

f, _ := os.Create("log.txt")
w := io.MultiWriter(os.Stdout, f)
fmt.Fprintln(w, "başladı")  // written to both

Exercise Testable output

Write rapor(w io.Writer, adlar []string); it writes each name as "- " + name on its own line. Call it on a bytes.Buffer and print buf.String() with fmt.Print.

Show example solution

package main

import (
    "bytes"
    "fmt"
    "io"
)

func rapor(w io.Writer, adlar []string) {
    for _, ad := range adlar {
        fmt.Fprintln(w, "- "+ad)
    }
}

func main() {
    var buf bytes.Buffer
    rapor(&buf, []string{"Ada", "Can"})
    fmt.Print(buf.String())
}

Test yourself

Which signature best makes a report function testable?
- func rapor(dosya string)
- func rapor(w io.Writer, v Veri) error
- func rapor(v Veri) // writes to os.Stdout
- func rapor() string
Show answer

func rapor(w io.Writer, v Veri) error — A function taking a Writer can be called with a bytes.Buffer in tests and its output checked; in production os.Stdout or a file is passed.

Write to a Writer

func selam(w io.Writer, ad string) {
    fmt.____(w, "Merhaba %s\n", ad)
}

Show answer

func selam(w io.Writer, ad string) {
    fmt.Fprintf(w, "Merhaba %s\n", ad)
}

Writer target

os.Stdout

Screen
bytes.Buffer

Memory (tests)
http.ResponseWriter

HTTP response
io.MultiWriter

Multiple targets

Next lesson: os.Stdin and os.Stdout

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