Wait, do you want to write an online game, or do you want to use a C++ compiler through your browser? They're different things, and writing a game using a compiler inside a browser is insane, when you could just do it using a proper compiler.
Wouldn't You also need to find a online compiler with a graphics library?
You could just use a compiler on a computer and save the projects to a program like dropbox which can be accessed from anywhere with the application.
You're going to have to download files to your computer in any manner, a file's data must downloaded in order for you to view it (lol).
There is an enormous difference between downloading a file from the internet and saving it as a file on your computer, and having information sent from the internet that is only kept in RAM on your computer. My point is, the thing you get from a server is usually not even an existing file, just a generated document, and it is your choice whether you save what you receive as a file on your hard drive or if you just keep it in memory.
It still has to be transferred from a server to your computer which is "downloading". In his case, I would think the files would be cached for offline referencing in best case scenario. But who knows...
Well, your definition is wrong. To download is to receive data from a remote source. To upload is to send data to a remote target. It doesn't matter what piece of hardware the data comes from or goes to.