Using std Namespace

Question

There seem to be different views on using \'using\' with respect to the std namespace.

Some say use \' using namespace std\', other say don\'t but rathe

Answer 1

Most C++ users are quite happy reading std::string, std::vector, etc. In fact, seeing a raw vector makes me wonder if this is the std::vector or a different user-defined vector.

I am always against using using namespace std;. It imports all sorts of names into the global namespace and can cause all sorts of non-obvious ambiguities.

Here are some common identifiers that are in the std namespace: count, sort, find, equal, reverse. Having a local variable called count means that using namespace std won't enable you to use count instead of std::count.

The classic example of an unwanted name conflict is something like the following. Imagine that you are a beginner and don't know about std::count. Imagine that you are either using something else in or it's been pulled in by a seemingly unrelated header.

#include 
using namespace std;

int count = 0;

int increment()
{
    return ++count; // error, identifier count is ambiguous
}

The error is typically long and unfriendly because std::count is a template with some long nested types.

This is OK though, because std::count goes into the global namespace and the function count hides it.

#include 
using namespace std;

int increment()
{
    static int count = 0;
    return ++count;
}

Perhaps slightly surprisingly, this is OK. Identifiers imported into a declarative scope appear in the common namespace that encloses both where they are defined and where they are imported into. In other words, std::count is visible as count in the global namespace, but only inside increment.

#include 

int increment()
{
    using namespace std;
    static int count = 0;
    return ++count;
}

And for similar reasons, count is ambiguous here. using namespace std doesn't cause std::count, hide the outer count as it might be expected. The using namespace rule means that std::count looks (in the increment function) as though it was declared at the global scope, i.e. at the same scope as int count = 0; and hence causing the ambiguity.

#include 

int count = 0;

int increment()
{
    using namespace std;
    return ++count; // error ambiguous
}