The Go Programming Language

7.12 Querying Behaviors with Interface Type Assertions

The logic below is similar to the part of the net/http web server responsible for writing HTTP header fields such as "Content-type: text/html". The io.Writer w represents the HTTP response; the bytes written to it are ultimately sent to someone’s web browser.

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func writeHeader(w io.Writer, contentType string) error {
    if _, err := w.Write([]byte("Content-Type: ")); err != nil {
        return err
    }
    if _, err := w.Write([]byte(contentType)); err != nil {
        return err
    }
    // ...
}

Because the Write method requires a byte slice, and the value we wish to write is a string, a []byte(...) conversion is required. This conversion allocates memory and makes a copy, but the copy ...