3

This is confusing me a great deal. All code examples being namespaceA and namespaceB and the method names being foo() and bar() are not helping, either. The way everyone explains it makes it seem as though namespaces are a relic of pre-OOP times where you could not say 'class car give fuel level' but had to go to this from another approach. But when I now want to do a C++ level, what is the point of using namespaces? Not that headers are confusing enough already, namespaces make absolutely no sense to me, in either how they work or why to use them.

Let's say I have a project built around Traffic, for example. You'd have classes for the Cars and its components, the Drivers and Passengers and the Road. Now, the Road has Cars and each Car has Persons. What would this look like?

Would you have the namespaces Road, Car, Person? Would the main() use the namespace Road to access the stuff in the header? Would the namespace Road include the namespace Car, and the Car include the namespace Person, and would through this the main() have access to the methods in Person? This is the way most guides explain this, but I don't really see the advantage of this over just importing the header file, wouldn't that have the same effect?

Or would you put multiple headers in the same namespace, such as namespace Traffic with all those classes? Can you nest namespaces?

I know C# and never knew it had namespaces until I looked it up just now, and never needed it, and in Java, Python, and Dart, those also never came up. Since I am trying to teach myself C++, I am kind of stranded here now, asking this question here. So far, I also never used them in C++, but I want to learn it properly.

Remy Lebeau
  • 555,201
  • 31
  • 458
  • 770
Maritn Ge
  • 997
  • 8
  • 35
  • 1
    *in Java, Python and Dart those also never came up* Python modules create implicit namespaces. I don't know about Java and Dart. – R Sahu Aug 04 '20 at 15:36
  • 1
    Java has packages, those partition names as well – StoryTeller - Unslander Monica Aug 04 '20 at 15:37
  • related/maybe dupe: https://stackoverflow.com/questions/4211827/why-and-how-should-i-use-namespaces-in-c – NathanOliver Aug 04 '20 at 15:37
  • 1
    Also: https://stackoverflow.com/questions/41590/how-do-you-properly-use-namespaces-in-c – NathanOliver Aug 04 '20 at 15:38
  • 1
    In your Road/Car/Person example I would expect those classes to perhaps be in a namespace called Models or something of that nature. Can't imagine why you'd want to create a namespace for each class, since the purpose is to logically group classes together. – David Aug 04 '20 at 15:38
  • 1
    Before C++ added namespace to the language, some people would use a struct as a makeshift namespace, to help collect and organize components. But that has some undesirable behaviors that a namespace doesn't have, so during the evolution of C++ the `namespace` was formally added to the language, and the standard C++ library was put into the `::std` namespace. – Eljay Aug 04 '20 at 15:44

4 Answers4

7

For small, self-contained projects, there's not much need for namespaces, and you'd never create a namespace for each object or concept in your code.

Larger projects using libraries benefit from being isolated from names introduced by those libraries, as well as some internal organisation to make readability easier.

Similarly, when creating a library, it's a good idea to put its contents into a namespace so as not to cause headaches and conflicts for your users (as you don't know how large their projects will be, and what names they may want to use themselves).

To use an analogy: if you have three books, you don't bother organising them alphabetically. But, once you have a hundred, you might decide to categorise them on your bookshelf for easier reference and mental health.

And, if you now borrow another twenty books from a friend, you'd probably keep those in a separate pile so they're easier to find when you need to give them back.

So, to some degree, this is a case of… you'll know why you need it, when you need it.

Asteroids With Wings
  • 17,071
  • 2
  • 21
  • 35
2

If I make a library with a function calculateStuff() in it and you also make a library with a calculateStuff() function, then some other person who wants to use both our libraries at the same time is going to have a bad day. But, if we both used namespaces there's no problem since he/she can then distinguish the functions as myNamespace::calculateStuff() and yourNamespace::calculateStuff() and there's no ambiguity.

For example: std::shared_ptr vs boost::shared_ptr. Without namespaces you wouldn't be able to use both in the same program as the name shared_ptr would be ambiguous.

Jesper Juhl
  • 30,449
  • 3
  • 47
  • 70
  • 1
    Another common example that comes up often is `std::bind()` in the standard C++ library vs `::bind()` in the Winsock library. Without namespaces, which `bind()` should be called? The compiler has trouble figuring that out. Qualifying the call to `bind()` with the appropriate namespace solves that. – Remy Lebeau Aug 04 '20 at 16:52
1

"[Named] Namespaces" are, as the name perhaps suggests, a way to subdivide the identifier space. In addition to solving literal conflicts ("you have more than one foo ..."), it also makes it considerably easier to find foo in a big, mature program that might well consist of hundreds or even thousands of modules.

The "name" of a variable or routine might(?) suggest what it is, but might not give any clue as to where it is, nor the context (not a technical term) of what it relates to: "it's just one name among many thousands." But, if you now group these into intelligently-chosen namespaces, you're adding a level of helpful organization to them. In a typical "great big program," especially one that is (as is also typical ...) "not entirely familiar to you," this extra level of bread-crumbs is a big bonus.

Mike Robinson
  • 8,490
  • 5
  • 28
  • 41
1

Simply put, namespaces allow to use the same names for different contexts.

Let's say you want to create two functions that take the same parameters and output text in two different ways, for the sake of simplicity you'd want to call them both print().

Since they both take the same parameters, there is no chance for function overloading here, but if you put each function in a separate namespace and then you call print(), you can simply change what the function will be doing by calling a different namespace each time.

Remy Lebeau
  • 555,201
  • 31
  • 458
  • 770
The Riser
  • 309
  • 2
  • 4
  • 12
  • In that example, it would be much preferable to use distinct, self-documenting function names. – Asteroids With Wings Aug 04 '20 at 15:44
  • True, but if you're working on a bigger scale, and you want to easily switch between too large sets of functions, this method may come in handy, it's very specific example but it demonstrates one of the ways you can use namespaces to your advantage. – The Riser Aug 04 '20 at 15:46