How to Alias a namespace and extend the original namespace based on the alias

Question

I have the current problem in C++:

I have a namespace for a given external library. I didn't like the name of this external namespace, so I did this following:

namespace mynamespace = othernamespace;

and this works fine if I want to say do the following:

mynamespace::foo(...);

but I found that I needed to have a special function to convert values found in othernamespace to values found in an external library. I defined a function the following way:

Y xToY(mynamespace::X x);

I realized I only wanted to use this function in the context of mynamespace, and I still didn't like using the namespace name for othernamespace so I thought simple, I would do the following:

namespace mynamespace = othernamespace;

namespace mynamespace{
    Y xToY(mynamespace::X x);
}

however I get the following compiler error telling me that this can't be done:

myheader.h:13:11: error: namespace alias 'mynamespace' not allowed here, assuming 'othernamespace'

and thus it doesn't compile. Note I'm currently using c++14. I would like to know if it is possible for me to extend this namespace using my alias name for the namespace othernamespace.

Answer 1

Nope. It's unfortunately not possible. The rule, from [namespace.def], explicitly excludes aliases:

In a named-namespace-definition, the identifier is the name of the namespace. If the identifier, when looked up (3.4.1), refers to a namespace-name (but not a namespace-alias) that was introduced in the namespace in which the named-namespace-definition appears or that was introduced in a member of the inline namespace set of that namespace, the namespace-definition extends the previously-declared namespace. Otherwise, the identifier is introduced as a namespace-name into the declarative region in which the named-namespace-definition appears.

You cannot extend a namespace by alias, you can only extend a namespace by the original namespace name.

Answer 2

Do it by building a new namespace, rather than an alias:

// external namespace

namespace othernamespace {

    struct X {};
    void foo(X& x) {};
}

// my namespace

namespace mynamespace
{
    using namespace othernamespace; // equivalent to import * from othernamespace
}


int main()
{
    mynamespace::X x;

    foo(x);
}