When have we any practical use for hierarchical namespaces in c++?

Question

I can understand the use for one level of namespaces. But 3 levels of namespaces. Looks insane. Is there any practical use for that? Or is it just a misconception?

Answer 1

Big codebases will need it. Look at boost for an example. I don't think anyone would call the boost code 'insane'.

If you consider the fact that at any one level of a hierarchy, people can only comprehend somewhere very roughly on the order of 10 items, then two levels only gives you 100 maximum. A sufficiently big project is going to need more, so can easily end up 3 levels deep.

Answer 2

Obviously it's a matter of opinion. But it really boils down to organization. For example, I have a project which has a plugin api that has functions/objects which look something like this:

plugins::v1::function

When 2.0 is rolled out they will be put into the v2 sub-namespace. I plan to only deprecate but never remove v1 members which should nicely support backwards compatibility in the future. This is just one example of "sane" usage. I imagine some people will differ, but like I said, it's a matter of opinion.

Answer 3

I work on XXX application in my company yyy, and I am writing a GUI subsystem. So I use yyy::xxx::gui as my namespace.

Answer 4

Hierarchical namespaces do have a use in that they allow progressively more refined definitions. Certainly a single provider may produce two classes with the same name. Often the first level is occupied by the company name, the second specifies the product, the third (and possibly more) my provide the domain.

There are also other uses of namespace segregation. One popular situation is placing the base classes for a factory pattern in its own namespace and then derived factories in their own namespaces by provider. E.g. System.Data, System.Data.SqlClient and System.Data.OleDbClient.

Answer 5

It depends on your needs and programming style. But one of the benefits of namespace is to help partition name space (hence the name). With a single namespace, as your project is increases in size and complexity, so does the likelihood of name-collision.

If you're writing code that's meant to be shared or reused, this becomes even more important.

Answer 6

You can easily find yourself in a situation when you need more than one level. For example, your company has a giant namespace for all of its code to separate it from third party code, and you are writing a library which you want to put in its own namespace. Generally, whenever you have a very large and complex system, which is broken down hierarchically, it is reasonable to use several namespace levels.

Answer 7

I agree for applications. Most people that use multiple levels of namespaces (in my experience) come from a Java or .NET background where the noise is significantly less. I find that good class prefixes can take the place of multiple levels of namespaces.

But I have seen good use of multiple namespace levels in boost (and other libraries). Everything is in the boost namespace, but libraries are allowed (encouraged?) to be in their own namespace. For example - boost::this_thread namespace. It allows things like...

boost::this_thread::get_id()
boost::this_thread::interruption_requested()

"this_thread" is just a namespace for a collection of free functions. You could do the same thing with a class and static functions (i.e. the Java way of defining a free function), but why do something unnatural when the language has a natural way of doing it?

Answer 8

Just look at the .Net base class library to see a namespace hierarchy put to good use. It goes four or five levels deep in a few places, but mostly it's just two or three, and the organization is very nice for finding things.

Answer 9

The bigger the codebase the bigger the need for hierarchical namespaces. As your project gets bigger and bigger you find you need to break it out in ways to make it easier to find stuff.

For instance we currently use a 2 level hierarchy. However some of the bigger portions we are now talking about breaking them out into 3 levels.