Using directive

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The keyword ‘using’ has three flavors. Combined with keyword ‘namespace’ you write a ‘using directive’:

If you don’t want to write Foo:: in front of every stuff in the namespace Foo, you can use using namespace Foo; to import every single thing out of Foo.

namespace Foo
    void bar() {}
    void baz() {}

//Have to use Foo::bar()

//Import Foo
using namespace Foo;
bar(); //OK
baz(); //OK

It is also possible to import selected entities in a namespace rather than the whole namespace:

using Foo::bar;
bar(); //OK, was specifically imported
baz(); // Not OK, was not imported

A word of caution: using namespaces in header files is seen as bad style in most cases. If this is done, the namespace is imported in every file that includes the header. Since there is no way of “un-using” a namespace, this can lead to namespace pollution (more or unexpected symbols in the global namespace) or, worse, conflicts. See this example for an illustration of the problem:

/***** foo.h *****/
namespace Foo
    class C;

/***** bar.h *****/
namespace Bar
    class C;

/***** baz.h *****/
#include "foo.h"
using namespace Foo;

/***** main.cpp *****/
#include "bar.h"
#include "baz.h"

using namespace Bar;
C c; // error: Ambiguity between Bar::C and Foo::C

A using-directive cannot occur at class scope.

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