Well, even if there is a built-in way of declaring integers larger than __int64, if you're dealing with bignums, you'll eventually run into a problem where you'll need something larger than 128bits, or say, your friend's 4096-bit integer class. That's where the power of C++ kicks in, once you dominate all the class-bulding techniques you'll have no limits to the size of your ints.
I suggest you get to learn about classes and operator overloading now, because, eventually you'll have to learn it anyway. Of course, you can always google for a pre-built large-integer class, if you don't want to build one yourself or dig into the deep C++ details. Anyway, in my opinion the best place to get to learn gory C++ details is in Bjarne Stroustrup - The C++ Proggraming Language 3rd edition. Bjarne Stroustrup is the author of C++.