Most efficient way in Go to stream io.ReadCloser to a file?

Question

Given an io.ReadCloser, from the response of an HTTP request for example, what is the most efficient way both in memory overhead and code readability to stream the response to a File?

Answer 1

io.Copy is undoubtedly the most efficient in terms of code; you only need to

outFile, err := os.Create(filename)
// handle err
defer outFile.Close()
_, err = io.Copy(outFile, res.Body)
// handle err

it's also likely to be pretty efficient in terms of CPU and memory as well. You can peek at the implementation of io.Copy if you want; assuming that the body doesn't implement WriteTo and the file doesn't implement ReadFrom (a quick glance says that they don't), Copy will copy chunks of up to 32kB at a time. A bigger chunk would probably use a bit less CPU but more memory; the value they picked seems like a good tradeoff.