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

From:
Howard Hinnant <howard.hinnant@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 02 Jul 2006 19:33:40 GMT
Message-ID:
<howard.hinnant-6989E2.15333902072006@syrcnyrdrs-02-ge0.nyroc.rr.com>
In article <JbTpg.28904$FR1.15119@dukeread05>,
 Chris Johnson <devcjohnson@gmail.com> wrote:

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)

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.

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


Your understanding is correct in on all counts. The standard is simply
deficient in this area. I had hoped that tuple I/O would save the day
here (pair simply being a special case of tuple length 2). However I/O
was stripped from the tuple library shortly before it was voted into
TR1. tuple is now in the C++0X working draft (still missing I/O). I am
hopeful that it can gain I/O in time for C++0X, but as yet there is no
such concrete proposal.

-Howard

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