C++：为命名空间内的类型定义的无符号整数向量提供流插入运算符

Question

首先：我不是 C++ 专家，但我正在为低级 C 库编写 C++ 绑定，并且需要一些帮助才能采取正确的方法。

以下是将我面临的问题抽象为一个简单的示例（请参阅评论）：

#include <iostream>
#include <vector>
#include <iterator>
#include <map>

// We have a namespace N.
namespace N {

// And a nested namespace NN.
namespace NN {

// A scoped enum E.
enum class E {
    FIRST = 1,
    SECOND,
};

// And a typedef for a vector holding E values.
using EV = ::std::vector<E>;

// We want to make E printable so let's provide a mapping of values
// to human-readable names ...
const ::std::map<E, ::std::string> E_names = {
    { E::FIRST, "FIRST" },
    { E::SECOND, "SECOND" }
};

// ... and stream insertion operator for the enum ...
::std::ostream& operator<<(::std::ostream& out, E e)
{
    out << E_names.at(e);

    return out;
}

// ... and for the vector.
::std::ostream& operator<<(::std::ostream& out, const EV& ev)
{
    out << "(";
    ::std::copy(ev.begin(), ::std::prev(ev.end()),
            ::std::ostream_iterator<E>(out, ", "));
    out << ev.back();
    out << ")";

    return out;
}

} /* namespace NN */

// Now back in namespace N, let's define the following class:
class A
{
public:
    explicit A(void) {}

    void m(const NN::EV& ev) {}

    NN::EV get_ev(void) const { return NN::EV({ NN::E::FIRST, NN::E::SECOND}); }
};

// And its own stream insertion operator:
::std::ostream& operator<<(::std::ostream& out, const A& a)
{
    out << "A(" << a.get_ev() << ")";

    return out;
}

} /* namespace N */

// And do the following in main():
int main(int argc, char **argv)
{
    ::std::cout << N::NN::E::FIRST << ::std::endl;

    ::std::cout << N::NN::EV({ N::NN::E::FIRST, N::NN::E::SECOND }) << ::std::endl;

    ::std::cout << N::A().get_ev() << ::std::endl;

    ::std::cout << N::A() << ::std::endl;

    return 0;
}

上面的工作正常。现在让我们按如下方式扩展示例：

#include <iostream>
#include <vector>
#include <iterator>
#include <map>

namespace N {

namespace NN {

enum class E {
    FIRST = 1,
    SECOND,
};

using EV = ::std::vector<E>;
// Let's add another typedef for a vector holding unsigned integers.
using UV = ::std::vector<unsigned int>;

const ::std::map<E, ::std::string> E_names = {
    { E::FIRST, "FIRST" },
    { E::SECOND, "SECOND" }
};

::std::ostream& operator<<(::std::ostream& out, E e)
{
    out << E_names.at(e);

    return out;
}

::std::ostream& operator<<(::std::ostream& out, const EV& ev)
{
    out << "(";
    ::std::copy(ev.begin(), ::std::prev(ev.end()),
            ::std::ostream_iterator<E>(out, ", "));
    out << ev.back();
    out << ")";

    return out;
}

// And define the following stream insertion operator.
::std::ostream& operator<<(::std::ostream& out, const UV& uv)
{
    out << "(";
    ::std::copy(uv.begin(), ::std::prev(uv.end()),
            ::std::ostream_iterator<unsigned int>(out, ", "));
    out << uv.back();
    out << ")";

    return out;
}

} /* namespace NN */

class A
{
public:
    explicit A(void) {}

    void m(const NN::EV& ev) {}

    NN::EV get_ev(void) const { return NN::EV({ NN::E::FIRST, NN::E::SECOND}); }

    NN::UV get_uv(void) const { return NN::UV({ 10, 20 }); }
};

::std::ostream& operator<<(::std::ostream& out, const A& a)
{
    out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";

    return out;
}

} /* namespace N */

int main(int argc, char **argv)
{
    ::std::cout << N::NN::E::FIRST << ::std::endl;

    ::std::cout << N::NN::EV({ N::NN::E::FIRST, N::NN::E::SECOND }) << ::std::endl;

    ::std::cout << N::A().get_ev() << ::std::endl;

    ::std::cout << N::A().get_uv() << ::std::endl;

    ::std::cout << N::A() << ::std::endl;

    return 0;
}

现在以下 splat 是由 gcc 生成的：

ostest.cpp: In function ‘std::ostream& N::operator<<(std::ostream&, const N::A&)’:
ostest.cpp:68:36: error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’})
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~ ~~~~~~~~~~
      |                            |                  |
      |                            |                  N::NN::UV {aka std::vector<unsigned int>}
      |                            std::basic_ostream<char>
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:108:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  108 |       operator<<(__ostream_type& (*__pf)(__ostream_type&))
      |       ^~~~~~~~
/usr/include/c++/10/ostream:108:36: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&)’ {aka ‘std::basic_ostream<char>& (*)(std::basic_ostream<char>&)’}
  108 |       operator<<(__ostream_type& (*__pf)(__ostream_type&))
      |                  ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/10/ostream:117:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]’
  117 |       operator<<(__ios_type& (*__pf)(__ios_type&))
      |       ^~~~~~~~
/usr/include/c++/10/ostream:117:32: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&)’ {aka ‘std::basic_ios<char>& (*)(std::basic_ios<char>&)’}
  117 |       operator<<(__ios_type& (*__pf)(__ios_type&))
      |                  ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/usr/include/c++/10/ostream:127:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  127 |       operator<<(ios_base& (*__pf) (ios_base&))
      |       ^~~~~~~~
/usr/include/c++/10/ostream:127:30: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘std::ios_base& (*)(std::ios_base&)’
  127 |       operator<<(ios_base& (*__pf) (ios_base&))
      |                  ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/usr/include/c++/10/ostream:166:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  166 |       operator<<(long __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:166:23: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘long int’
  166 |       operator<<(long __n)
      |                  ~~~~~^~~
/usr/include/c++/10/ostream:170:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  170 |       operator<<(unsigned long __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:170:32: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘long unsigned int’
  170 |       operator<<(unsigned long __n)
      |                  ~~~~~~~~~~~~~~^~~
/usr/include/c++/10/ostream:174:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  174 |       operator<<(bool __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:174:23: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘bool’
  174 |       operator<<(bool __n)
      |                  ~~~~~^~~
In file included from /usr/include/c++/10/ostream:784,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/bits/ostream.tcc:91:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]’
   91 |     basic_ostream<_CharT, _Traits>::
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/10/bits/ostream.tcc:92:22: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘short int’
   92 |     operator<<(short __n)
      |                ~~~~~~^~~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:181:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  181 |       operator<<(unsigned short __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:181:33: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘short unsigned int’
  181 |       operator<<(unsigned short __n)
      |                  ~~~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/10/ostream:784,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/bits/ostream.tcc:105:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]’
  105 |     basic_ostream<_CharT, _Traits>::
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/10/bits/ostream.tcc:106:20: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘int’
  106 |     operator<<(int __n)
      |                ~~~~^~~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:192:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  192 |       operator<<(unsigned int __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:192:31: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘unsigned int’
  192 |       operator<<(unsigned int __n)
      |                  ~~~~~~~~~~~~~^~~
/usr/include/c++/10/ostream:201:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  201 |       operator<<(long long __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:201:28: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘long long int’
  201 |       operator<<(long long __n)
      |                  ~~~~~~~~~~^~~
/usr/include/c++/10/ostream:205:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  205 |       operator<<(unsigned long long __n)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:205:37: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘long long unsigned int’
  205 |       operator<<(unsigned long long __n)
      |                  ~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10/ostream:220:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  220 |       operator<<(double __f)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:220:25: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘double’
  220 |       operator<<(double __f)
      |                  ~~~~~~~^~~
/usr/include/c++/10/ostream:224:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  224 |       operator<<(float __f)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:224:24: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘float’
  224 |       operator<<(float __f)
      |                  ~~~~~~^~~
/usr/include/c++/10/ostream:232:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  232 |       operator<<(long double __f)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:232:30: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘long double’
  232 |       operator<<(long double __f)
      |                  ~~~~~~~~~~~~^~~
/usr/include/c++/10/ostream:245:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
  245 |       operator<<(const void* __p)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:245:30: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘const void*’
  245 |       operator<<(const void* __p)
      |                  ~~~~~~~~~~~~^~~
/usr/include/c++/10/ostream:250:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::nullptr_t) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::nullptr_t = std::nullptr_t]’
  250 |       operator<<(nullptr_t)
      |       ^~~~~~~~
/usr/include/c++/10/ostream:250:18: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘std::nullptr_t’
  250 |       operator<<(nullptr_t)
      |                  ^~~~~~~~~
In file included from /usr/include/c++/10/ostream:784,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/bits/ostream.tcc:119:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]’
  119 |     basic_ostream<_CharT, _Traits>::
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/10/bits/ostream.tcc:120:34: note:   no known conversion for argument 1 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘std::basic_ostream<char>::__streambuf_type*’ {aka ‘std::basic_streambuf<char>*’}
  120 |     operator<<(__streambuf_type* __sbin)
      |                ~~~~~~~~~~~~~~~~~~^~~~~~
ostest.cpp:66:17: note: candidate: ‘std::ostream& N::operator<<(std::ostream&, const N::A&)’
   66 | ::std::ostream& operator<<(::std::ostream& out, const A& a)
      |                 ^~~~~~~~
ostest.cpp:66:58: note:   no known conversion for argument 2 from ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} to ‘const N::A&’
   66 | ::std::ostream& operator<<(::std::ostream& out, const A& a)
      |                                                 ~~~~~~~~~^
In file included from /usr/include/c++/10/bits/basic_string.h:48,
                 from /usr/include/c++/10/string:55,
                 from /usr/include/c++/10/bits/locale_classes.h:40,
                 from /usr/include/c++/10/bits/ios_base.h:41,
                 from /usr/include/c++/10/ios:42,
                 from /usr/include/c++/10/ostream:38,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/string_view:619:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, std::basic_string_view<_CharT, _Traits>)’
  619 |     operator<<(basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/10/string_view:619:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:48: note:   ‘std::vector<unsigned int>’ is not derived from ‘std::basic_string_view<_CharT, _Traits>’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                                ^
In file included from /usr/include/c++/10/string:55,
                 from /usr/include/c++/10/bits/locale_classes.h:40,
                 from /usr/include/c++/10/bits/ios_base.h:41,
                 from /usr/include/c++/10/ios:42,
                 from /usr/include/c++/10/ostream:38,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/bits/basic_string.h:6468:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>&)’
 6468 |     operator<<(basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/10/bits/basic_string.h:6468:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:48: note:   ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’} is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                                ^
In file included from /usr/include/c++/10/bits/ios_base.h:46,
                 from /usr/include/c++/10/ios:42,
                 from /usr/include/c++/10/ostream:38,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/system_error:262:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)’
  262 |     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
      |     ^~~~~~~~
/usr/include/c++/10/system_error:262:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘const std::error_code&’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:506:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)’
  506 |     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:506:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:48: note:   deduced conflicting types for parameter ‘_CharT’ (‘char’ and ‘std::vector<unsigned int>’)
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                                ^
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:511:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)’
  511 |     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:511:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘char’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:517:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)’
  517 |     operator<<(basic_ostream<char, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:517:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘char’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:523:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)’
  523 |     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:523:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘signed char’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:528:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)’
  528 |     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:528:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘unsigned char’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:589:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)’
  589 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:589:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:48: note:   mismatched types ‘const _CharT*’ and ‘std::vector<unsigned int>’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                                ^
In file included from /usr/include/c++/10/ostream:784,
                 from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/bits/ostream.tcc:321:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)’
  321 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/10/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘const char*’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:606:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)’
  606 |     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:606:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘const char*’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:619:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)’
  619 |     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:619:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘const signed char*’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:624:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)’
  624 |     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/10/ostream:624:5: note:   template argument deduction/substitution failed:
ostest.cpp:68:47: note:   cannot convert ‘N::A::get_uv() const()’ (type ‘N::NN::UV’ {aka ‘std::vector<unsigned int>’}) to type ‘const unsigned char*’
   68 |  out << "A(" << a.get_ev() << ", " << a.get_uv() << ")";
      |                                       ~~~~~~~~^~
In file included from /usr/include/c++/10/iostream:39,
                 from ostest.cpp:1:
/usr/include/c++/10/ostream:773:5: note: candidate: ‘template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&)’
  773 |     operator<<(_Ostream&& __os, const _Tp& __x)
      |     ^~~~~~~~

[snip]

好的，所以我不能像这样专门化向量。让我们这样做：

struct printable_UV
{
    printable_UV(const UV& uv) : ref(uv) {}
    const UV& ref;
};

::std::ostream& operator<<(::std::ostream& out, const printable_UV& uv)
{
    out << "(";
    ::std::copy(uv.ref.begin(), ::std::prev(uv.ref.end()),
            ::std::ostream_iterator<unsigned int>(out, ", "));
    out << uv.ref.back();
    out << ")";

    return out;
}

啪嗒啪嗒的声音仍然存在。但是，如果我使用一个没有这样的名称空间的更简单的示例：

#include <iostream>
#include <vector>
#include <iterator>

using vec = ::std::vector<unsigned int>;

struct printable
{
    const vec& ref;
    printable(const vec& v) : ref(v) {}
};

::std::ostream& operator<<(::std::ostream& out, const printable& v)
{
    out << "(";
    ::std::copy(v.ref.begin(), ::std::prev(v.ref.end()),
            ::std::ostream_iterator<unsigned int>(out, ", "));
    out << v.ref.back();
    out << ")";

    return out;
}

int main(int argc, char **argv)
{
    vec v = { 1, 2, 3 };

    ::std::cout << v << ::std::endl;

    return 0;
}

它工作得很好。这让我认为引入命名空间会使查找失败。我做错了什么？