~/Go Reader and Writer Interfaces Explained with Code

May 14, 2021


Gos Reader and Writer interfaces standardize streaming data operations. The Reader interface requires a Read(p []byte) (n int, err error) method. The Writer interface requires a Write(p []byte) (n int, err error) method.

Code Example 1: Implement the Reader interface

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type MyReader struct{}
func (m MyReader) Read(p []byte) (int, error) {
    copy(p, "abc")
    return 3, io.EOF
}
var r io.Reader = MyReader{}
buf := make([]byte, 10)
n, err := r.Read(buf)

Code Example 2: Implement the Writer interface

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type MyWriter struct{}
func (m MyWriter) Write(p []byte) (int, error) {
    fmt.Print(string(p))
    return len(p), nil
}
var w io.Writer = MyWriter{}
w.Write([]byte("hello"))

Code Example 3: Using the built-in strings.Reader

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r := strings.NewReader("golang")
buf := make([]byte, 4)
n, _ := r.Read(buf)
// buf now contains "gola"

Code Example 4: Use bytes.Buffer as both Reader and Writer

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var buf bytes.Buffer
buf.Write([]byte("abc"))
output := make([]byte, 2)
n, _ := buf.Read(output)
// output now contains "ab"

Code Example 5: Copy from Reader to Writer

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src := strings.NewReader("1234")
dst := &bytes.Buffer{}
io.Copy(dst, src)
// dst.String() is "1234"

These interfaces are fundamental for I/O in Go. For more details, see the Go io package.

Tags: [golang]