class FooFilter
{
    public virtual bool operator()(const Foo& obj) const = 0;
};

namespace fooFilter
{
    class FooFilter ...
    class HappyFooFilter ....
    class SadFooFilter ...
    etc ....
}

namespace fooFilter
{
    class Base ...
    class Happy ....
    class Sad ...
    etc ....
}