C++ modules and namespaces are said to be orthogonal. You can have a module X and a namespace X. They do not relate to each other.

The declarations in a namespace block can be exported from a module interface by writing export in front of the namespace keyword:

export module A;

export namespace red
{
    int func1()
    {
        return 41;
    }
}

which is equivalent to exporting all the declarations inside the namespace block:

export module A;

namespace red
{
    export int func1()
    {
        return 41;
    }
}

An interesting question is, what

export using namespace red;

is supposed to do.

Consider the following program:

// translation unit #1
export module A;

namespace red
{
    export int func1()
    {
        return 41;
    }
}

export namespace blue
{
    int func2()
    {
        return 42;
    }
}

namespace red
{
    int func3()
    {
        return 43;
    }
}

namespace green1
{
    export using namespace red;
    export using namespace blue;
}

export namespace green2
{
    using namespace red;
    using namespace blue;
}

// translation unit #2 (file "main.cpp")
import A;

int main()
{
    green1::func1();
    green1::func2();
    green1::func3();  // error

    green2::func1();
    green2::func2();
    green2::func3();  // error
}

In module A, we have the namespaces red and blue.

The module exports the functions red::func1 and blue::func2. Function red::func3 is not exported.

The module additionally has the namespaces green1 and green2. These provide alternative methods to access the contents of the namespaces red and blue, by using the keyword sequence using namespace.

Those are exported too.

In function main(), the namespaces green1 and green2 can then be used to call func1 and func2.

The namespace green1 in effect combines the names from the namespaces red and blue.

This only works, if both the function declarations and the “using namespace” are exported.

Function red::func3 is not exported, so it cannot be called by importing module A.

The example shown above was tested with MSVC version 19.52.36629 for x64 (PREVIEW) and gcc version 16.2.0.

(last edited 2026-08-17)