C++ namespaces advice

user_17540 0
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I'm just teaching myself C++ namespaces (coming from a C# background) and I'm really starting to think that even with all the things that C++ does better than most other languages, nested namespaces isn't one of them!

Am I right in thinking that in order to declare some nested namespaces I have to do the following: 

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

As opposed to: 

namespace tier1::tier2::tier3
{
}

à la 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 only single (not nested) namespaces?

UPDATE

For anyone interested, this is how I ended up tackling this issue.

namespaces c++
9 Answers

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.

Admin1

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 Do you advise that due to the short comings i've outlined here?

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

The purpose of C++ namespaces is to stop the real world problem encountered by C developers, where they experience name collisions when 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.

user_187970

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.

user_474530

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
{
};
user_546840

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

user_738690
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