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?
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.