Avoiding namespace ambiguities - c++

Matti Rintala wrote:

> I'm in a situation where I'd like to create a new namespace which is
> identical to an old one except that one type is different:
>
> namespace Original
> {
> class A;
> class B;
> // ...
> }
>
> namespace AlmostOriginal
> {
> using namespace Original;
> class B;
> }
>
> The above works fine as long as the contents of AlmostOriginal are
> used with qualification (AlmostOriginal::A, AlmostOriginal::B).
> However, if someone writes "using namespace AlmostOriginal" and uses
> unqualified B, an ambiguity results (since in that case both
> AlmostOriginal::B and Original::B are visible).
>
> Is there a solution? Any tricks which would allow unqualified B to
> unambiguously refer to AlmostOriginal::B when a using namespace
> AlmostOriginal is used?

There is a solution, but it is not going to be nice.

namespace AlmostOriginal
{
using Original::A;
// list every name from Original in a using declaration,
// *except* B.

class B;
}

As I said, it is not nice, but it will work.
Unfortunately, there is no easy syntax to get access to everything from
a namespace except the parts you list. It is either everything or only
the explicitly mentioned things.

Bart v Ingen Schenau
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On Thu, 23 Aug 2007 10:30:46 CST, Matti Rintala
<matti.rintala@tut.fi> wrote:

>I'm in a situation where I'd like to create a new namespace which is
>identical to an old one except that one type is different:
>
>namespace Original
>{
> class A;
> class B;
> // ...
>}
>
>namespace AlmostOriginal
>{
> using namespace Original;
> class B;
>}
>
>The above works fine as long as the contents of AlmostOriginal are used
with
>qualification (AlmostOriginal::A, AlmostOriginal::B). However, if
someone
>writes "using namespace AlmostOriginal" and uses unqualified B, an
ambiguity
>results (since in that case both AlmostOriginal::B and Original::B are
>visible).
>
>Is there a solution? Any tricks which would allow unqualified B to
>unambiguously refer to AlmostOriginal::B when a using namespace
>AlmostOriginal is used?

The "solution", if you want to call it that, is to make sure that
none of the header files in your project pollutes the global
namespace (by putting extraneous "using" directives in header files).

But I know that it is very difficult to prevent, especially with
legacy code. Even one particular compiler, very popular on Windows --
not Microsoft -- was guilty of this (maybe still is...can you guess
which one? <g>)

--
Bob Hairgrove
NoSpamPlease@Home.com


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