Question about "using" keyword

Question

Heya,

I'm well aware of using namespaces however, every now and then I'm stumbling upon a using, which uses a specific class. For instance :

#include <string>
using namespace std;
(...)

However - every now and then, I'm seeing :

using std::string;

How should I interpret the "using" in this case ?

Cheers

Answer 1

using std::string simply imports std::string into the current scope (aka, you can just use 'string' rather than 'std::string') without importing everything from ::std into the current scope.

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edit: clarification after comment.

Answer 2

In this case it allows you to bind to a specific type within a namespace without qualification. As opposed to the first case which allows you to bind to any type.

Answer 3

You will be able to use string class without putting std:: before it. However if you want to use something else for example a vector then you need to use std::vector

Answer 4

using namespace foo allows you to access all names in the namespace foo without qualification. using foo::bar allows you to use bar without qualification, but not any other names in foo.

Answer 5

too make things more complicated it's possible to do that:

class Base {
protected:
    void f();
};
class Fun: public Base {
public:
    using Base::f;
};

and now you have nice public method.