I've done a bunch of Java coding recently and have got used to very specific package naming systems, with deep nesting e.g. com.company.project.db. This works fine in Java, AS3/Flex and C#. I've seen the same paradigm applied in C++ too, but I've also heard that it's bad to view C++ namespaces as direct counterparts to Java packages.
Is that true, and why? How are namespaces/packages alike and different? What problems are likely to be seen if you do use deep nested namespaces?
Java packages are not nested, they're flat. Any apparent nesting is nothing more than a naming convention.
For example, the package com.company.project.db has no relation whatsoever to com.company.project or com.company.project.db.x. Code in com.company.project.db has no more access to code in com.company.project.db.x than would code in a.b.c.
In C++ namespaces are just about partitioning the available names. Java packages are about modules. The naming hierarchy is just one aspect of it.
There's nothing wrong, per-se, with deeply nested namespaces in C++, except that they're not normally necessary as there's no module system behind them, and the extra layers just add noise. It's usually sufficient to have one or two levels of namespace, with the odd extra level for internal details (often just called Details).
There are also extra rules to C++ namespaces that may catch you out if overused - such as argument-dependent-lookup, and the rules around resolving to parent levels. WRT the latter, take:
namespace a{ namespace b{ int x; } }
namespace b{ string x; }
namespace a
{
b::x = 42;
}
Is this legal? Is it obvious what's happening? You need to know the precendence of the namespace resolution to answer those questions.
You can have nested namespaces in C++.
They don't work the same as in java, though, obviously. Java packages are really much better defined and there is no real static init weirdness. With C++ namespaces are helpful but still have a fair bit of danger involved.
In C++, the basic unit of design and implementation is the class, not the namespace. Namespaces were intended as a means of preventing name clashes in large libraries, not for expressing concepts.
Classes have several advantages over namespaces:
However, I would look twice at any deeply nested relationship. That is really not a good way of designing software, and leads to unreadable code, whether you juse classes or namespaces.
I'll throw in a couple of things I've heard but don't know the truthfulness of, please help confirm/dispel them.
1)C++ performance is (somehow) affected by long fully-specified method names e.g namespace1::namespace2::namespace3::classX::method123()
2)You might hit a limit on allowed symbol length in the compiler/linker