[go] From io.Reader to string in Go

I have an io.ReadCloser object (from an http.Response object).

What's the most efficient way to convert the entire stream to a string object?

data, _ := ioutil.ReadAll(response.Body)
fmt.Println(string(data))

func copyToString(r io.Reader) (res string, err error) {
    var sb strings.Builder
    if _, err = io.Copy(&sb, r); err == nil {
        res = sb.String()
    }
    return
}

Answers so far haven't addressed the "entire stream" part of the question. I think the good way to do this is ioutil.ReadAll. With your io.ReaderCloser named rc, I would write,

Go >= v1.16

if b, err := io.ReadAll(rc); err == nil {
    return string(b)
} ...

Go <= v1.15

if b, err := ioutil.ReadAll(rc); err == nil {
    return string(b)
} ...

The most efficient way would be to always use []byte instead of string.

In case you need to print data received from the io.ReadCloser, the fmt package can handle []byte, but it isn't efficient because the fmt implementation will internally convert []byte to string. In order to avoid this conversion, you can implement the fmt.Formatter interface for a type like type ByteSlice []byte.

I like the bytes.Buffer struct. I see it has ReadFrom and String methods. I've used it with a []byte but not an io.Reader.

var b bytes.Buffer
b.ReadFrom(r)

// b.String()