Re: Why koenig lookup?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Mon, 11 Aug 2008 03:37:01 +0200

Message-ID:

<54udnevLw_bfCgLVnZ2dnUVZ_szinZ2d@posted.comnet>

* kwikius:

kwikius wrote:

Jerry Coffin wrote:

In article <2a0f00c3-10cd-4b64-8203-57c5e681b565
@k13g2000hse.googlegroups.com>, james.kanze@gmail.com says...

[ ... ]

But doesn't use sin in a dependent context in a dependent
context in the template. I was thinking of the classical case,
where the operator<< for std::vector<int> defined in global
namespace doesn't get found when it is used in
std::ostream_iterator. Curiously, I can't reproduce the
symptomes in a simple example, but I've definitely had the
problem myself, and people post it here from time to time.

Here's a fairly simple example:

#include <map>
#include <string>
#include <iostream>
#include <algorithm>

typedef std::map<std::string, int> s_int;

//namespace std {
std::ostream &operator<<(std::ostream &os, s_int::value_type const &v)
{ return os << v.first << ": " << v.second;
}

//}

int main() { s_int m;

    m["a"] = 123;
    m["b"] = 345;

    std::copy(m.begin(), m.end(),
std::ostream_iterator<s_int::value_type>(std::cout, "\n"));

    return 0;
}

As it stands right now, it won't compile -- but remove the two
comment delimiters so operator<< is in namespace std, and it compiles
and works just fine.

Lemme guess. operator<<(std::ostream&, int ) is ***NOT*** in global
namespace Right? .. Which is the whole point!

I'm with Alf Steinbach on this one....

Oh I see..

hmm C++ is too complicated. Bring back BASIC I say :-(

Well, the rule's simple: if you place an overload in the original namespace, it
will most likely be considered by original code there, otherwise it may not.

It's like this:

     #include <math.h>

     namespace james
     {
         struct BigInt {};
         double sin( BigInt ) { return 0; }
         void foo() { sin( 3.14 ); } // *\/o
     }

     int main()
     {
         james::foo();
     }

Here the sinful overload of sin placed in namespace james causes compilation
failure of foo. So I guess you're still with me on this... Yes? <g>

Cheers,

- Alf

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