C++ namespaces advice

Question

Just teaching myself c++ namespaces (coming from a c# background) and i'm really starting to think that all the things that c++ does better then most other languages, nested namespaces isn't one of them!

Am i right in thinking that in order to declare nested namespaces i HAVE to do the following:

namespace tier1
{
    namespace tier2
    {
        namespace tier3
        {
            /* then start your normal code nesting */
        }
    }
}

as apposed to:

namespace tier1::tier2::tier3
{
}

ala c#.

This becomes even more demented when i need to forward declare:

namespace tier1
{
    namespace tier2
    {
        namespace forward_declared_namespace
        {
            myType myVar; //forward declare
        }
        namespace tier3
        {
            /* then start your normal code nesting */
            class myClass
            {
                forward_declared_namespace::myType myMember;
            }
        }
    }
}

Bearing in mind that a typical system that i develop consists of:

MyCompany::MySolution::MyProject::System::[PossibleSections]::Type

Is this why you don't tend to see much use of namespaces in c++ examples? or usually on single (un-nested) namespaces?

Answer 1

C++ namespaces were a vast improvement over the previous offering (i.e. No namespaces at all).

C# namespaces have extended the concept and ran with it.

I would advise you to keep your namespaces in a simple flat structure.

EDIT

Simply "Yes"

C++ namespaces weren't designed to help you partition your logic & libraries the way they do in C#.

The pruposes of C++ namespaces are to stop the real world problem encountered by C developers, where they're using two third party libs that export the same function name(s). C developers had various workarounds for it, but it could be a serious pain.

The idea was that the STL etc has the std:: namespace, libs provided by "XYZ Corp" would have an xyz:: namespace, you working for "ABC corp" would put ALL your stuff in a single abc:: namespace.

Answer 2

First of all you can avoid namespace indentation, because there is no reason for that.

Using namespaces in the examples won't show namespaces power. And their power as for me is dividing Domain areas one from another. Divide utility classes from the business ones.

Just don't mix different namespace hierarchies in the one .h file. Namespaces are kind of extra comment for your interface of functions declaration. Looking on namespaces and the class names should explain a lot of stuff.

namespace product
{
namespace DAO
{

class Entity
{
};

Answer 3

C++ namespaces were not intended to be a design mechanism - they are there simply to prevent name clashes. You really don't want or need to use nested namespaces in 99.99% of situations.

A good example of the correct use of namespaces in C++ is the C++ Standard Library. Everything in this quite large library is placed in a single namespace called std - there is no attempt or need to break the library up into (for example) an I/O sub-namespace, a math sub-namespace, a container sub-namespace etc.

The basic tool for modelling in C++ is the class (and to some extent the template), not the namespace. If you feel the need for nesting, you should consider using nested classes, which have the following advantages over namespaces:

• they have methods
• they can control access
• they cannot be re-opened

Having considered these, if you still wish to use nested namespaces by all means do so - there is nothing technically wrong with using them in this way.

Answer 4

You're over-using them (and you'll get nothing in return).

Answer 5

Atleast as a small help, in some cases you can do this:

namespace foo = A::B::C::D;

And then reference A::B::C::D as foo. But only in some cases.

Answer 6

I agree with Adam. C++ is syntactically most powerful imperative language, but in aspect of modularity this language is a terrible mistake.

And I'm really anxious when I hear C++ fanatics defending even poorest C++ concepts. Adam accurately pointed out that C++ namespaces are unconvenient (and this is not their only sin), so fanatics replied that they are not intended to. The question araises why not? The fanatic won't tell you - they are just flawly designed.

Anyone tried integrate C++ code with Java? Anyone used CORBA? Guess what is generated from IDL, a hierarchical namespaces structure. For WSDL/SOAP large namespace structures are also welocme. And in many other applications.

Go for the battle and tell Java developers that they are all wrong. Tell them that java.lang should be named package std and for example org.eclipse.swt.graphics should be package ibm. Tell them that they shall introduce 5 new keywords for simply including a module and its namespace into program (#ifndef, #define, #include, namespace, using) and veeeeery usefull features like anonymous namespace or aliases and additional grammar. And of course let this commands work against each other. Ehh.

Answer 7

You can skip indentation. I often write

namespace myLib { namespace details {

/* source code */

}; }; /* myLib::details */

C++ source code is eventually compiled into binary, unlike C#/Java which stays in the binary. Therefore, namespace just provides a fine solution for variable naming conflict. It is not intended for class hierarchy.

I often keep one or two namespace levels in the code.