C++ How to declare a namespace in a class using an alias?

Using g++ 7.3.0.

I have a global data structure (spaceA::bars) that is used throughout my program. I am trying to pass SharedBarMap into class Foo which will operate on the data in bars.

The problem is, the namespace doesn't appear to be seen within class Foo.

error: 'spaceA' has not been declared
spaceA::SharedBarMap sbm;
~~~~~~

globals.h

namespace spaceA
{
    using MapGroup = std::vector<std::shared_ptr<Bars>>;
    using BarMap = std::map<int, MapGroup>;
    using SharedBarMap = std::shared_ptr<BarMap>;

    extern SharedBarMap bars;
}

globals.cpp

namespace spaceA
{
    SharedBarMap bars;
}

foo.h

#include "globals.h"

class Foo
{
    private:
        spaceA::SharedBarMap sbm; // <--- error here

    public:
        Foo(spaceA::SharedBarMap bars) : sbm(bars) { }
};