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From: Kaz Kylheku <kaz@kylheku.com>
Newsgroups: comp.programming
Subject: Re: Here is why C++ and C are bad...
Date: Tue, 29 Dec 2015 21:20:17 +0000 (UTC)
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On 2015-12-29, Richard Heathfield <rjh@cpax.org.uk> wrote:
> On 26/12/15 00:54, Ramine wrote:
>>
>> Hello,
>>
>> I was working on a project in C++, but i have encountered
>> a problem with the language that we call C++, this problem
>> is inherent to C++ and C, here is a small example that
>> shows the problem:
>>
>> ===
>>
>> #include <iostream>
>> using namespace std;
>
> This is a very bad idea. The *whole point* of namespaces is to avoid 
> name clashes. What you should do (and indeed what you *do* do) is to use 
> the std:: prefix for symbols that you pull in from the std namespace. So 
> you don't need using namespace std at all! Just remove it.

Indeed, "using namespace std" means "bring into my namespace here
all symbols in std, current and future, vendor-specific and standard".

It's a transition mechanism for programmers who want namespaces to
go away and deal with clashes, or who want to do a quick and dirty
port of pre-namespace C++ code to namespaces.

Of course, the "using namespace" mechanism is perfectly useful, if namespaces
are sanely designed by someone who pins down the exact contents. For instance:

  namespace foo_api {
    // everything

    int a, b, c, d, e;
    void f(), g(), h();

    // Only a and f existed in the 1.0 API
    namespace v1_0 {
      using foo_api::a;
      using foo_api::f;
    }

    // 2.0 has all of 1.0 plus c, and g.
    namespace v2_0 {
      using namespace foo_api::v1_0;
      using foo_api::c;
      using foo_api::g;
    }

    // 3.0 has everything.
    // (In the future it won't!)
    namespace v3_0 {
      using namespace foo_api;
    }
  }

Now you can do "using namespace foo_api::v1_0" with the confidence
that you're getting an exact, documented set of identifier which shall
never change.