Re: Nested namespace syntactic sugar
Greg Herlihy wrote:
All other issues aside, I would question the alleged benefit itself:
that is, whether streamlining nested namespace declarations is a
desirable goal. I argue that it is not. I think that declaring deeply
nested namespaces in C++ should be something of a chore - because
writing client code that uses deeply nested names is not in the least
convenient. In other words, there is cost in creating nested namespaces
that should not minimized for anyone inclined to make such
declarations. So if some implementor is determined to declare a set of
nested namespaces five levels deep - I would rather have them have to
open each namespace explicitly and do so five times - because there is
a greater chance that at some point in this process they may ask
themselves: "hmmm...do I really this many levels of namespaces?"
Whereas if the task can be accomplished with a one-line declaration -
then the chance that that question would ever be raised is remote. And
in that case it would be clients of this interface who would be the
ones to bear the cost of this convenience.
The actual problem is uniqueness. Uniqueness is an important benefit
for the concurrent/distributed development, because anybody can be sure
that his declarations will never conflict with others' work.
If you want your namespace to have unique name, you should follow some
hierarchical convention, like
namespace net {
namespace pacbell {
namespace cool {
class A {
};
}
}
}
using net::pacbell::cool::A;
With this convention, the namespace net::pacbell is reserved for the
owner of http://pacbell.net domain and the declarations in this
namespace are both mnemonic and world-unique.
There should be some solution for uniqueness problem: it must be
convenient to declare unique namespaces, and
1. Simplifying declaration of hierarchical namespace is the most
obvious way to go.
Other possible solutions are
2. Use undersore-separated namespace names:
namespace net_pacbell_cool {
class A {
};
}
using net_pacbell_cool::A;
3. Use some ugly macros:
#define NET_PACBELL_COOL_BEGIN namespace net { ... { ... {
#define NET_PACBELL_COOL_END }}}
#define NET_PACBELL_COOL_NS ::net::pacbell::cool
NET_PACBELL_COOL_BEGIN
class A {
};
NET_PACBELL_COOL_END
using NET_PACBELL_COOL_NS::A;
4. Allow unique string instead of namespace name:
namespace "http://pacbell.net/cool/" {
class A {
};
}
using "http://pacbell.net/cool/"::A;
Which one do you prefer?
--
Andrei Polushin
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