Re: cout << vector<string>

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 11 Nov 2008 07:59:31 -0800 (PST)

Message-ID:

<b0176e24-2d9e-49ac-9ff6-de4cda06d72d@35g2000pry.googlegroups.com>

On Nov 11, 3:36 pm, Hendrik Schober <spamt...@gmx.de> wrote:

Maxim Yegorushkin wrote:

On Nov 11, 2:40 pm, Hendrik Schober <spamt...@gmx.de> wrote:

Maxim Yegorushkin wrote:

[...]

    #include <map>
    #include <iostream>
    #include <iterator>
    // should be in namespace std::
    template<class T, class U>
    std::ostream& operator<<(std::ostream& s, std::pair<T, U> con=

st& p)

    {
        return s << p.first << ' ' << p.second;
    }
    int main()
    {
        typedef std::map<int, int> Map;
        Map m;
        std::copy(
              m.begin()
            , m.end()
            , std::ostream_iterator<Map::value_type>(std:=

:cout)

);
}
It won't compile unless operator<<(std::ostream& s, std::pair<T, U>
const& p) is in namespace std.

  I would have asked the same question for this code. :)
  I don't understand why it doesn't compile. It comes down
  to this
    ostr << val;
  with 'ostr' being an 'std::basic_ostream<>' and 'val'
  being an 'std::pair<>'. Why doesn't this find the global
  operator?

Because expression "ostr << val" is template argument dependent and
thus is bound at the second phase of the two-phase name lookup. At the
second phase it uses ADL only to search for functions within
namespaces associated with ostr and val. ostr is std::basic_ostream
and val is std::pair<int, int>, thus one associated namespace is std.
int has no associated namespaces. So, the only namespace considered
for expression "ostr << val" is std, which lacks a suitable
operator<<().

  But lookup isn't ADL only. The enclosing namespaces are considered,
  too, aren't they? And the global namespaces is always enclosing.
  (I'm not saying you're wrong. I just don't understand this.)

At the second stage of the two-phase name lookup (at the point of
template instantiation) it is ADL only.

Schobi

--
Max