Is "using std::begin;" a good practice?

Question

As I have read, begin(some_vector) is more standard than some_vector.begin() because of array support... and as I know also, the use of using keyword is not really desirable behavior. However, I also see lot of code that contains just these two usings:

using std::begin;
using std::end;

Is that considered good or bad practice? Especially when many begin and end are needed?

Answer 1

It can be helpful for ADL help. Something like:

template<typename T, typename F>
void apply(T& v, const F& f)
{
   using std::begin;
   using std::end;
   std::for_each(begin(v), end(v), f);
}

So, than we just can call apply with types, that have begin/end functions and classic C arrays.

In other case it's actually okay to use using directive in source file, in little scope and so on, but it's bad to use in header.

Answer 2

As I have read, begin(some_vector) is more standard than some_vector.begin() because of array support

It's not "more standard", both are 100% standard. It is more generic, because it works for arrays, but it's not actually very common to have an object and not know whether it's an array or a container type. If you have something that you know is an array then use std::begin(a) but if you have something that you know is not an array then there is no advantage to using the form that also works with arrays. If you're in a generic context where you might have either, then std::begin works for both cases.

the use of using keyword is not really desirable behavior.

That's debatable. There are very good reasons to avoid using-directives in most contexts (i.e. using namespace foo) but the same arguments don't apply to using-declarations that introduce a single name (i.e. using foo::bar). For example the recommended way to use std::swap is via using std::swap so it's certainly not true that the using keyword is undesirable in general.

Is that considered good or bad practice? Especially when many begin and end are needed?

I would say that in general it's a bad idea. As the other answers explain, it allows begin and end to be found by ADL, but that's not necessarily a good thing. Enabling ADL for begin and end can cause problems, that's why we changed range-based for at the last minute to not use using std::begin; using std::end; to avoid ambiguities, see N3257 for the details.