How do I open a URL from C++?

Your question may mean two different things:

1.) Open a web page with a browser.

#include <windows.h>
#include <shellapi.h>
...
ShellExecute(0, 0, L"http://www.google.com", 0, 0 , SW_SHOW );

This should work, it opens the file with the associated program. Should open the browser, which is usually the default web browser.

2.) Get the code of a webpage and you will render it yourself or do some other thing. For this I recommend to read this or/and this.

I hope it’s at least a little helpful.

EDIT: Did not notice, what you are asking for UNIX, this only work on Windows.