When to Use `io.TeeReader` vs. `io.Copy` for Data Transfer and Processing?

Distinguishing io.TeeReader from io.Copy

io.TeeReader and io.Copy, two fundamental I/O constructs in Go, share a commonality in reading from a reader and writing to a writer. However, they exhibit subtle yet important differences.

io.Copy:

io.Copy is a straightforward function that mindlessly transfers data between an input reader and an output writer. Its sole purpose is to facilitate the data movement without providing access to the copied data.

io.TeeReader:

In contrast, io.TeeReader isn't a direct data copier. Instead, it creates and returns an intermediate io.Reader. Reading from this io.Reader simultaneously writes the data to an io.Writer provided during construction.

Practical Comparison:

The nuanced distinction between io.TeeReader and io.Copy becomes apparent in scenarios where access to the copied data is crucial. Consider a situation where you need to calculate the MD5 hash of data being transferred from a reader to a writer.

Using io.TeeReader:

<code class="go">r := io.TeeReader(strings.NewReader("Hello World"), os.Stdout)
// ... Perform calculations on data read from r</code>

This approach allows you to both write the data to a destination (os.Stdout in this case) and access it for calculations, such as computing the MD5 hash.

Using io.Copy and io.MultiWriter:

<code class="go">h := md5.New()
mw := io.MultiWriter(h, os.Stdout)
// ... Copy data to mw</code>

While this approach achieves the same goal as using io.TeeReader, it requires the explicit creation of an io.MultiWriter to write to both the destination and the MD5 hasher.

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