Using with double-buffered transmission

[IndianBoy42]

So this is a feature request but a bit vague about what exactly the feature would be, I think we have to discuss it.

So basically I want to use this for printing to through the USB CDC interface of my microcontroller. For efficiency, we should use double buffering: so we set up one buffer to send and then continue filling the second buffer.

I have a std::array buffer, then create an emio::span_buffer, then create a emio::writer. when its full/close to full I can do the transmission from the underlying buffer.

The part that is missing is somehow in-place redirecting the writer and span_buffer to point to the other underlying buffer. Both span_buffer and writer have their copy assignment deleted. set_write_area doesn't reassign span_ member so it would be wrong to use it to change the buffer.

I think double buffering is a pretty common technique in embedded systems so it would be nice to support it.

Ultimately I would like the code to look like this:

// Global buffers exist `buffer[0]`
emio::span_buffer span{buffer};
emio::writer w{span};

format_to(w, ...);

flush(w); // Starts a transmission with the current buffer, writer now points to buffer[1]

format_to(w, ...);

The simplest solution might be that we get a writer.set_buffer() method