如果我的类型作为流没有意义，我应该实现io.Reader / io.Writer吗？

Question

Question regarding zip and io.Reader/io.Writer. As far as I understand, one of the purpose of io/Reader/io.Writer is streaming. But should I implement one of these if my type does not really make sense "as chunks"?

For more details:

Lets say I have this struct.

type MyZip struct {
    file1, file2 []byte
}

MyZip represents a particular structured zip. Let's say for example it represents a zip file containing exactly a file named file1 and a file named file2. MyZip has the responsibility of parsing a zip file to extract these two files into two []byte fields. It also should handle the other way around (turning these two []byte fields as two files named test1 and test2 archived into a zip file).

As far as I understand, the package archive/zip does not allow to decompress a zip file as a stream. We have to fully load the zip in memory or as a file and decompress afterwards.

So to refine my question, does it make sense for MyZip to implement io.Reader/io.Writer for reading/writing from/to the final zip file?

As said above, as I cannot extract the two files on the fly, I would have to add some sort of buffer to MyZip and just read/write from/to this buffer. So the zip would anyway be fully stored in memory before being streamed. Is it a counter indication for not using io.Reader/io.Writer?

Thanks a lot for shedding light!

Answer 1

As far as I understand, the package archive/zip does not allow to decompress a zip file as a stream. We have to fully load the zip in memory or as a file and decompress afterwards.

Wrong. Some metadata needs to be loaded into memory, yes, but you do not need to load everything into memory. You can extract individual files from a zip archive. See How to unzip a single file?

Yes, zip.Reader and zip.Writer doesn't implement io.Reader and io.Writer, because they are not a single source or target of bytes. But the files in them, they are. So the files in them implement io.Reader and io.Writer. More specifically a file in an archive is represented by a zip.File which may be used to obtain an io.Reader to get its (uncompressed) content using File.Open(). When you add a new entry to a zip archive using e.g. Writer.Create(), that returns you an io.Writer because that represents a target of bytes, you can write the file's content into it.

Back to your exmaple: MyZip also does not represent a single source or destination of bytes, so it doesn't make sense to itself implement io.Reader or io.Writer, so don't do it. Similarly to archive/zip, the individual files in it may do so.