Re: Namespace inclusion

James Kanze <>
Thu, 28 Feb 2008 01:15:23 -0800 (PST)
On Feb 27, 5:15 pm, Jeff Schwab <> wrote:

D. Susman wrote:

What is the difference between using "using std::X" instead
of writing "std::XInstance" every time I use the X instance?
Is the latter one more efficient in the context of compiler

First, of course, the choice is between "using std::X ; X" and
simple "std::X". If you declare an instance of std::X
(regardless of how you specified X), the name of that instance
doesn't reside in std, but in the namespace and scope it was
declared in.

The former tells the compiler: "When you're looking for
something called X, please consider the thing called std::X."

The latter tells the compilre: "I specifically want to use
std::X here."

The former is generally preferable for functions, because it
allows argument-dependent lookup to be considered.

I don't see any relationship with ADL here. The whole point of
ADL is that you don't need either:

    std::complex z ;
    std::cout << sin( z ) << std::endl ;

The whole point of ADL is that the compiler will find std::sin
for the function call in the second line, even if there is no
using declaration for it.

For example, suppose there
is a type "thing" in a namespace "things":

     namespace things {
         struct thing { ... };

         void swap (thing&, thing&);

Suppose you later want to swap two things. If you say:

     using std::swap;
     swap(thing1, thing2);

The compiler has a chance to find things::swap.

Even without the std::, this is true.

I presume, here, that your talking about template code, of
course, because in normal code, it doesn't necessarily matter.
If you're dealing with a non-class type, you automatically write
std::swap, and if you're dealing with a class type, you either
specify the namespace you want, or just let ADL take care of it.

In a template, of course, you don't know whether you're dealing
with a non-class type or not, and the using makes sense: it
ensures that the right swap is found for the non-class types,
and ADL assures that it is found for the class types.

Of course, this is just one convention. The convention I've
usually seen is for the template code to write std::swap, with
user classes which want a specialization to explicitly
specialize std::swap.

I has a long discussion about this with someone else here, a
while ago. I forget what we finally concluded with regards to
which is better, but IIRC, we both agreed that a good programmer
will code defensively: if you're writing a class for which you
want swap specialized, you explicitly specialize std::swap,
which works with either philosophy in the template, and if
you're writing a template which uses swap, you do as above,
which works regardless of how the author of the instantiation
class implemented his specialization.

Abstractly, I find that if what you want is a specialization of
std::swap, that's what you should do. A swap that is not a
specialization of std::swap would only be justified if it had
different semantics or different use cases. But that's just
abstractly; there may be pragmatic issues related to how the
language works which argue otherwise. (I think that there were
formally issues if thing were a class template, instead of a
class. You can't "explicitly specialize" for a template; you
have to define an overload. Which is, formally, a new function,
and you're not allowed to add new functions to the std
namespace, although in practice, it works.)

That's usually a good thing, since it may use a special
thing-specific swapping technique, e.g. just swapping the
things' p_impls.

If instead you say:

     std::swap(thing1, thing2);

You will specifically get std::swap, and things::swap will not
even be considered. This is not necessarily wrong, but it is
probably sub-optimal.

If the author of thing did the right thing, and explicitly
specialized std::swap, it should be OK:-). But you're probably
right for the special case of templates. Outside of a template,
it's less obvious; I tend to spell the name out completely. Or
use typedef's for class templates, e.g.:
    typedef std::map< std::string, std::complex< double > >
                        Map ;
I wouldn't rule out an occasional using, however, particularly
in a function. (I also don't hesitate using it in unit tests.
If the unit test is designed to test
Namespace1::NestedNamespace2::Class3, I'll usually use a using
for the Class3, right up front.)

James Kanze (GABI Software)
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