Using Namespaces Properly

by Dejan Jelovic

Namespaces are a very powerful C++ language feature. This article does not teach you the syntax of namespaces. Rather, it shows you how to use them properly.

Namespaces simply wrap all enclosed names with another name. For example:

namespace net {
    class Socket {
        ...
    };
}

...

net::Socket socket;

By doing that, they make sure that if two libraries both implement the Socket class, if they name their namespaces differently your program can use both without a conflict.

But this brings up another question: If two independent companies both decide to write network libraries, what are the chances that they are going to implement a class named Socket? My guess is somewhere around 100 percent.

We also like it when namespace names are easy to type, which means that they should be 2-4 characters long. With that in mind, what are the chances that both companies are going to name their namespace net? 5 percent? 10 percent?

Whatever it is, it shows that namespaces do not solve the problem, they only make it less severe.

An Industrial Strength Solution

The solution to this problem is to use long, unique namespace names, and then bring the namespaces into a program by using short aliases.

So a company writing the network library should write something like:

namespace net_33843894 {
    class Socket {
        ...
    };
}

where the number after net_ is going to be generated using a random number generator. Say this code is placed in a header file called <netlib>

Then the library is sold to a client, who decides to use it on a project. The client then writes his own project-local header file named <mynetlib>, with the following content:

#include <netlib>

namespace net = net_33843894;

He has just created a project-local alias for the namespace of his library vendor. If the namespace name net had already been taken by another library, the user can choose another name: net2, sock, or something else. There will be no name conflicts.

Lowering Barriers

A smart thing to do with your library is to make it easy for people to start using it. In an ideal world, they should be able to double-click on an installation file and the library would be immediately available inside their development environment. Next thing they are typing is #include <yourlib> and they are using it to do something useful.

However, if the user has to make his own header for every header in your library, then he has to suffer a little bit in order to use it. Not every user will be willing to do so.

The solution to this problem is to provide reasonable defaults, but to let the users cop out of them if they are not suitable. The way to do this is with preprocessor directives in your header file:

namespace net_33843894 {
    class Socket {
        ...
    };
}

#ifndef NO_NET_33843894_ALIAS
    namespace net = net_33843894;
#endif

This way, we provide a reasonable default for the namespace name. If that name is already taken, then the user can define the macro NO_NET_33843894_ALIAS and no alias will be defined.

Current Compilers

Error messages are already a nightmare with templates. With long namespace names, we make them even worse.

Unfortunately, none of the compilers I use are smart enough to display an error with the shortest available namespace alias at the point of error. So even though you may be using the alias net, if you make an error using the Socket class the error will mention net_33843894::Socket. Not very readable.

So I use a little trick. It works only for headers that contain only inline functions (as it affects the actual names used by the linker), but I have plenty of those. If the macro NO_NET_33843894_ALIAS is not defined, I use the short name as the namespace name, and the long name as the alias:

#ifdef NO_NET_33843894_ALIAS
namespace net_33843894 {
#else
namespace net {
#endif
    class Socket {
        ...
    };
}

#ifndef NO_NET_33843894_ALIAS
    namespace net_33843894 = net;
#endif

And the error messages become bearable again.

 

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