C++ using namespaces into same namespace nested

Question

Let's say I have a namespace:

namespace UI
{
}

And I have another namespace:

namespace Domain
{
}

Now let's say in the Domain namespace I also have a UI namespace that is specific for that domain.

namespace Domain
{
    namespace UI
    {
    }
}

Is it possible to import the global UI namespace into the domain UI namespace without importing it in the Domain namespace? I know using namespace is often not a good idea but conceptually the domain UI namespace should be both in the global UI namespace and in the domain namespace.

Enoc Martinez · Accepted Answer · 2019-03-12 19:11:08Z

2

The only possible ways are namespace alias or use the entire namespace UI into Domain::UI. In both cases theres a way for access it from Domain namespace.

Examples:

namespace UI {
    class Test {

    };
}

namespace Domain {

}

namespace Domain {
    namespace UI {
        namespace _UI = ::UI;

        struct Test2 {
            _UI::Test param;
        };
    }
}

namespace Domain2 {

    namespace UI {
        using namespace ::UI;

        struct Test2 {
            Test param;
        };
    }
}

answered Mar 12, 2019 at 19:11

Enoc Martinez

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4 Comments

hmojtaba Over a year ago

yes, exactly, use ::UI when referencing to the global namespace.

Andreas Loanjoe Over a year ago

Wow you can have namespace aliases didn't even know that. One more question if I do the 2nd solution the header, does this it import the global UI for all uses of the domain UI namespace?

Enoc Martinez Over a year ago

No, only works in files including "using namespace ::UI" or files including a header with the using. Namespace in C++ not work like for example packages in Go. If you don't have a header defining something, theres no visibiltiy for that.

Nick Matteo Jul 16 at 16:53

Using _UI as an identifier is undefined behavior. All identifiers beginning with an underscore and a capital letter are reserved for the implementation.

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