Re: namespaces, class libraries, and VERY large projects

I have a question about the way people choose to partition large C++
projects (millions of LOC, lots of class libraries) using namespaces.

I have seen three ways of doing it which will be discussed below: As an
example, suppose we have libraries A, B, C and D. Suppose A is the
foundation library, B and C use it, and D uses B and C and also makes
direct calls to stuff in A.

1. Give each class library its own namespace, qualify all usage.
    Consider code in C. It will have to qualify stuff coming
    from A and B. Similarly, in D, the code will have to
    qualify stuff coming from A, B and C.

2. Give each class library its own namespace,
    employ "using namespace" for all used namespaces
    at the start of each ".cpp" file. For source in D this
    will often mean saying:
       using namespace A;
       using namespace B;
       using namespace C;
    at the start of each ".cpp".

3. Employ nested namespaces.
    A is the outermost, then B, then C (even though they
    are peers), then D. No qualifications are needed
    anywhere, neither are using directives.

Historically I have always used (1) but I worry about excessive
qualifications that could be distracting when applied to very large
projects.

I have always avoided (2) since it seems to me to open up ambiguities
that namespaces are supposed to avoid.

I have only recently come across (3) and have not yet made up my mind
about it.