Question: Let's say that I am building an application to a DEB package, which has as recommended dependency a package, let's call it package1.

So, package1, if installed, can provide extra functionality to my program, but not directly related to my program's basic functionality (that's why it is a recommended package)

So, the user doesn't choose initially to install the recommended package (in Ubuntu Software Center you can choose by ticking/unticking to install or not to install the recommended packages), but in the future, while using my application, it asks for the corresponding functionality.

What is the best way to tell him that an extra package has to be installed in order for this functionality to work? What program can I use so as to install this package directly from my program with a dialog indicating how the installation process is going?

I want to do this via the default way, and, preferably a way that is Linux Desktop Environment independent.

In short, I want a way to install a package from my program neither by opening Ubuntu Software Center (via an apt:// URL) nor by doing it at the background.

Ideally, it would be a dialog like the one that is used by the Update Manager (or, for the ones that remember, from the Synaptic Package Manager).

PS: I would prefer something that could be implemented using C++ !

Answer: 
You might take example on the totem media player when it prompts the user to install missing codecs, or when you open share options of a directory in nautilus via right click, it prompts to install samba.

I think they use the `sessioninstaller` package to do that. See `man session-installer`, and <https://launchpad.net/sessioninstaller>

You could also use the command `xdg-open apt://package`: it will open the default package manager with the possibility to install your package (ubuntu software center in recent releases).
This is an old question but I've found the answer, which is `apturl`.

It works as follows:

`apturl apt:packagename` and it is exactly what I need and what nautilus uses!

Importantly, it is available pre-installed on many linux distros :)