**When Should You Use io.TeeReader vs. io.Copy in Go?**

Comparison of io.TeeReader and io.Copy

In Go, the io package offers multiple ways to handle data streams, including io.TeeReader and io.Copy. These functions share similar functionality: reading from a source and writing to a destination. However, there are key differences to consider.

io.Copy

io.Copy is straightforward in its operation. It efficiently transfers data from a supplied io.Reader to a io.Writer. This function does not return the copied data, making it suitable for scenarios where data modification or inspection is not necessary.

io.TeeReader

io.TeeReader, unlike io.Copy, does not perform an automatic copy. Instead, it returns a new io.Reader that, when read from, also sends the data to a specified io.Writer. This feature is particularly useful when both the original data and its copy are required for further processing.

For instance, consider a scenario where you want to write data to standard output while simultaneously computing its MD5 hash. io.TeeReader enables this by providing a way to access the data and redirect it to the MD5 calculation:

<code class="go">import (
    "bytes"
    "fmt"
    "hash/md5"
    "io"
    "os"
)

func main() {
    // Create a string to be written and copied
    data := "Hello World"

    // Create a tee reader that writes to standard output
    tee := io.TeeReader(bytes.NewReader([]byte(data)), os.Stdout)

    // Calculate the MD5 hash of the copied data
    h := md5.New()
    _, err := io.Copy(h, tee)
    if err != nil {
        panic(err)
    }

    // Print the hash
    fmt.Printf("\nHash: %x", h.Sum(nil))
}</code>

This code will display both the original data in standard output and its MD5 hash.

Summary

While io.Copy provides efficient data transfer, io.TeeReader offers more flexibility by allowing the retrieval and modification of copied data. The choice between the two functions depends on the specific requirements of the data handling task.

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