c++ - Using namespace std vs other alternatives

using namespace std;

So far in my computer science courses, this is all we have been told to do. Not only that, but it's all that we have been allowed to do, otherwise we get penalized on our code. I understand, through looking at code posted online, that you can use ::std or std:: to accomplish the same thing.

My question is generall why? Obviously for the sake of learners and simplicity using the global declaration is simpler, but what are the draw backs? Is it more realistic to expect ::std in a real world application? And I guess to add on to this, what is the logic/concept behind the using declaration? None of this has been explained during my courses, and I'd like to get a better grasp on it.

As a general question: if I haven't been taught this content, vectors, templates, classes, or error handling does it seem like I'm missing a lot of essential C++ functionality?

Thanks in advance!

Answer

This is really one of those things that you can discuss over beer for hours and hours, and still not have an answer everyone is happy with.

If you are nearly always using std:: functionality, then adding using namespace std; at the beginning of the file is not that bad an idea. On the other hand, if you are using things from more than one namespace (e.g. writing a compiler using llvm:: and also using std::, it may get confusing as to which parts are part of llvm and which parts are std:: - so in my compilter project, I don't have a single file with using namespace ...; - instead I write out llvm:: and std:: as needed. There are several functions (unwisely, perhaps) called Type(), and some places use something->Type()->Type() to get the type-thing that I need... Yes, it confuses even me a little at times...

I also have many things that look like Constants::ConstDecl and Token::RightParen, so that I can quickly see "what is what". All of these COULD be made shorter and "simpler", but I prefer to see where things belong most of the time.

Being more verbose helps making it easier to see where things belong - but it makes for more typing and more reading, so it is a balance.