c++ - 对 std/boost 移动的模糊调用

Question

遇到无法编译的代码:

#include <boost/move/utility.hpp>
#include <utility>
#include <deque>
#include <map>
#include <vector>
#include <boost/date_time/posix_time/posix_time_types.hpp>

using namespace std;

int main() {
    typedef std::pair<int, std::deque<int>> FirstPair;
    typedef std::vector<FirstPair> VectorFirstPair;
    typedef std::pair<boost::posix_time::time_duration, VectorFirstPair> SecondPair;
    typedef std::map<boost::posix_time::time_duration, SecondPair> Map;
    Map mapInstance;
    SecondPair newElement = make_pair(boost::posix_time::not_a_date_time, VectorFirstPair());
    mapInstance.insert(make_pair(boost::posix_time::seconds(10), move(newElement))).first;
}

这在使用 boost 1.55 的 gcc 4.8.2 上失败(不是在 boost 1.54 上)并出现以下错误(ideone here):

test.cpp: In function ‘int main()’:
test.cpp:17:81: error: call of overloaded ‘move(SecondPair&)’ is ambiguous
     mapInstance.insert(make_pair(boost::posix_time::seconds(10), move(newElement))).first;
                                                                                 ^
test.cpp:17:81: note: candidates are:
In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
                 from /usr/include/c++/4.8/utility:70,
                 from /usr/include/boost/config/no_tr1/utility.hpp:21,
                 from /usr/include/boost/config/select_stdlib_config.hpp:37,
                 from /usr/include/boost/config.hpp:40,
                 from /usr/include/boost/move/detail/config_begin.hpp:10,
                 from /usr/include/boost/move/utility.hpp:17,
                 from test.cpp:1:
/usr/include/c++/4.8/bits/move.h:101:5: note: constexpr typename std::remove_reference< <template-parameter-1-1> >::type&& std::move(_Tp&&) [with _Tp = std::pair<boost::posix_time::time_duration, std::vector<std::pair<int, std::deque<int> > > >&; typename std::remove_reference< <template-parameter-1-1> >::type = std::pair<boost::posix_time::time_duration, std::vector<std::pair<int, std::deque<int> > > >]
     move(_Tp&& __t) noexcept
     ^
In file included from test.cpp:1:0:
/usr/include/boost/move/utility.hpp:138:55: note: typename boost::remove_reference<T>::type&& boost::move(T&&) [with T = std::pair<boost::posix_time::time_duration, std::vector<std::pair<int, std::deque<int> > > >&; typename boost::remove_reference<T>::type = std::pair<boost::posix_time::time_duration, std::vector<std::pair<int, std::deque<int> > > >]
          inline typename remove_reference<T>::type && move(T&& t) BOOST_NOEXCEPT

这不应该编译吗？ using namespace 子句不应该使这变得明确吗？为什么编译器在这里选择 boost::move 作为可行的候选者？

请注意，如果我删除定义的类型中的 boost 类型(用例如 int 替换它们)，这不会导致任何错误。

Answer 1

这是因为 ADL - 参数依赖查找(请参阅此 answer，它也与 boost 相关)。

问题是命名空间 boost 被认为是因为:

§ 3.4.2 Argument-dependent name lookup

2. ... The sets of namespaces and classes is determined entirely by the types of the function arguments (and the namespace of any template template argument). ...

因此，因为某些与 boost 相关的类型是 std::pair 的模板参数，所以也考虑了 boost::move。 (恕我直言，这不应该，因为我无法弄清楚如何围绕这种歧义进行编程)。