模板上的begin()和end()自由函数重载

Question

我有一个模板类,Iterable; 我想要重载begin()和end()自由功能.它存储的数据作为vector的unique_ptr,但接口使用boost::indirect_iterator的便利性.

我的代码构建和运行CLang-3.5,但我试过g++-4.9,它没有.但我不知道为什么？(以及哪种编译器具有正确的行为).

template<typename T>
using SimpleVec = std::vector<T, std::allocator<T>>;

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
class Iterable
{
        template <typename friendT,
                  template <typename> class friendContainer,
                  class friendString>
        friend boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT>>::iterator>
            begin(Iterable<friendT, friendContainer, friendString>& i);

        template <typename friendT,
                  template <typename> class friendContainer,
                  class friendString>
        friend boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT>>::iterator>
            end(Iterable<friendT, friendContainer, friendString>& i);
};

和免费功能:

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
    begin(Iterable<T, Container, String>& i)
{
    return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(begin(i._c));
}

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
    end(Iterable<T, Container, String>& i)
{
    return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(end(i._c));
}

在g++,错误是:

../../API/net/session/ClientSession.h:83:29: error: call of overloaded 'begin(GroupManager&)' is ambiguous
      for(auto& grp : groups())
                             ^
../../API/net/session/ClientSession.h:83:29: note: candidates are:
In file included from ../../API/net/session/../permission/full/PermissionManager.h:5:0,
                 from ../../API/net/session/Session.h:3,
                 from ../../API/net/session/ClientSession.h:2,
                 from ../../API/net/session/ClientSessionBuilder.h:2,
                 from ../client/main.cpp:2:
../../API/net/session/../permission/full/../../Iterable.h:22:4: note: boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT> >::iterator> begin(Iterable<friendT, friendContainer, friendString>&) [with friendT = Group; friendContainer = SimpleVec; friendString = std::basic_string<char>; T = Permission; Container = SimpleVec; String = std::basic_string<char>; typename friendContainer<std::unique_ptr<friendT> >::iterator = __gnu_cxx::__normal_iterator<std::unique_ptr<Group, std::default_delete<Group> >*, std::vector<std::unique_ptr<Group, std::default_delete<Group> >, std::allocator<std::unique_ptr<Group, std::default_delete<Group> > > > >]
    begin(Iterable<friendT, friendContainer, friendString>& i);
    ^
../../API/net/session/../permission/full/../../Iterable.h:142:2: note: boost::indirect_iterator<typename Container<std::unique_ptr<_Tp> >::iterator> begin(Iterable<T, Container, String>&) [with T = Group; Container = SimpleVec; String = std::basic_string<char>; typename Container<std::unique_ptr<_Tp> >::iterator = __gnu_cxx::__normal_iterator<std::unique_ptr<Group, std::default_delete<Group> >*, std::vector<std::unique_ptr<Group, std::default_delete<Group> >, std::allocator<std::unique_ptr<Group, std::default_delete<Group> > > > >]
  begin(Iterable<T, Container, String>& i)
  ^

所以它看起来像g++看到friend声明的另一个函数？

Answer 1

感谢 @sehe 的回答和这个 cppreference page ，我终于找到了一个答案（更具可读性）。

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
class Iterable
{
        friend boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
            begin(Iterable& i)
        {
            return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(begin(i._c));
        }

        friend boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
            end(Iterable& i)
        {
            return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(end(i._c));
        }
};