使用自定义访问者时自定义类型的 Boost 变体失败（源自 boost::static_visitor）

Question

我正在尝试使用 boost::variant 并使用如下标准类型进行打印。

typedef boost::variant<int, std::string> myVariant;

myVariant my;
my = "Hello";
std::cout << my << endl;

但是当我引入自定义类型时

struct MyStructTag
{
   uint32_t ver;
   double kind;
   std::string type;
};

typedef boost::variant<MyStructTag, std::string> myVariant;

它抛出了以下错误，但无法完全理解。

main.cpp: In instantiation of ‘std::ostream& test_variant::custom_ostream::operator()(const T&) const [with T = test_variant::MyStructTag; std::ostream = std::basic_ostream<char>]’:
v_1_55_0/boost/variant/variant.hpp:938:32:   required from ‘boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = const test_variant::MyStructTag; Visitor = const test_variant::custom_ostream; boost::detail::variant::invoke_visitor<Visitor>::result_type = std::basic_ostream<char>&]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:113:9:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::true_) [with Visitor = boost::detail::variant::invoke_visitor<const test_variant::custom_ostream>; VoidPtrCV = const void*; T = test_variant::MyStructTag; typename Visitor::result_type = std::basic_ostream<char>&; mpl_::true_ = mpl_::bool_<true>]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:156:9:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke(int, Visitor&, VoidPtrCV, T*, NoBackupFlag, int) [with Visitor = boost::detail::variant::invoke_visitor<const test_variant::custom_ostream>; VoidPtrCV = const void*; T = test_variant::MyStructTag; NoBackupFlag = boost::variant<test_variant::MyStructTag, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::has_fallback_type_; typename Visitor::result_type = std::basic_ostream<char>&]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:237:5:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl(int, int, Visitor&, VoidPtrCV, mpl_::false_, NoBackupFlag, Which*, step0*) [with Which = mpl_::int_<0>; step0 = boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, test_variant::MyStructTag, boost::mpl::l_item<mpl_::long_<1l>, std::basic_string<char>, boost::mpl::l_end> > >, boost::mpl::l_iter<boost::mpl::l_end> >; Visitor = boost::detail::variant::invoke_visitor<const test_variant::custom_ostream>; VoidPtrCV = const void*; NoBackupFlag = boost::variant<test_variant::MyStructTag, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::has_fallback_type_; typename Visitor::result_type = std::basic_ostream<char>&; mpl_::false_ = mpl_::bool_<false>]’
v_1_55_0/boost/variant/variant.hpp:2237:13:   required from ‘static typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<const test_variant::custom_ostream>; VoidPtrCV = const void*; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = std::basic_ostream<char>&]’
v_1_55_0/boost/variant/variant.hpp:2259:13:   required from ‘typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::invoke_visitor<const test_variant::custom_ostream>; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = std::basic_ostream<char>&]’
v_1_55_0/boost/variant/variant.hpp:2281:52:   required from ‘typename Visitor::result_type boost::variant<T0, TN>::apply_visitor(Visitor&) const [with Visitor = const test_variant::custom_ostream; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = std::basic_ostream<char>&]’
v_1_55_0/boost/variant/detail/apply_visitor_unary.hpp:74:43:   required from ‘typename Visitor::result_type boost::apply_visitor(const Visitor&, Visitable&) [with Visitor = test_variant::custom_ostream; Visitable = const boost::variant<test_variant::MyStructTag, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >; typename Visitor::result_type = std::basic_ostream<char>&]’
main.cpp:34:59:   required from here
main.cpp:23:21: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
          return os_ << val;
                     ^
In file included from /usr/include/c++/4.8.2/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/4.8.2/ostream:602:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = test_variant::MyStructTag]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^
In file included from v_1_55_0/boost/variant/variant.hpp:2325:0,
                 from v_1_55_0/boost/variant.hpp:17,
                 from main.cpp:2:
v_1_55_0/boost/variant/detail/variant_io.hpp: In instantiation of ‘void boost::detail::variant::printer<OStream>::operator()(const T&) const [with T = test_variant::MyStructTag; OStream = std::basic_ostream<char>]’:
v_1_55_0/boost/variant/variant.hpp:938:32:   required from ‘boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = const test_variant::MyStructTag; Visitor = boost::detail::variant::printer<std::basic_ostream<char> >; boost::detail::variant::invoke_visitor<Visitor>::result_type = void]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:113:9:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::true_) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >; VoidPtrCV = const void*; T = test_variant::MyStructTag; typename Visitor::result_type = void; mpl_::true_ = mpl_::bool_<true>]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:156:9:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke(int, Visitor&, VoidPtrCV, T*, NoBackupFlag, int) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >; VoidPtrCV = const void*; T = test_variant::MyStructTag; NoBackupFlag = boost::variant<test_variant::MyStructTag, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::has_fallback_type_; typename Visitor::result_type = void]’
v_1_55_0/boost/variant/detail/visitation_impl.hpp:237:5:   required from ‘typename Visitor::result_type boost::detail::variant::visitation_impl(int, int, Visitor&, VoidPtrCV, mpl_::false_, NoBackupFlag, Which*, step0*) [with Which = mpl_::int_<0>; step0 = boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, test_variant::MyStructTag, boost::mpl::l_item<mpl_::long_<1l>, std::basic_string<char>, boost::mpl::l_end> > >, boost::mpl::l_iter<boost::mpl::l_end> >; Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >; VoidPtrCV = const void*; NoBackupFlag = boost::variant<test_variant::MyStructTag, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::has_fallback_type_; typename Visitor::result_type = void; mpl_::false_ = mpl_::bool_<false>]’
v_1_55_0/boost/variant/variant.hpp:2237:13:   required from ‘static typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >; VoidPtrCV = const void*; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = void]’
v_1_55_0/boost/variant/variant.hpp:2259:13:   required from ‘typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = void]’
v_1_55_0/boost/variant/variant.hpp:2281:52:   required from ‘typename Visitor::result_type boost::variant<T0, TN>::apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::printer<std::basic_ostream<char> >; T0_ = test_variant::MyStructTag; TN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; typename Visitor::result_type = void]’
v_1_55_0/boost/variant/detail/variant_io.hpp:88:5:   required from ‘std::basic_ostream<_CharT, _Traits>& boost::operator<<(std::basic_ostream<_CharT, _Traits>&, const boost::variant<U0, UN ...>&) [with E = char; T = std::char_traits<char>; U0 = test_variant::MyStructTag; UN = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >}]’
main.cpp:47:17:   required from here
v_1_55_0/boost/variant/detail/variant_io.hpp:64:14: error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’
         out_ << operand;

完整代码

#include <iostream>
#include <boost/variant.hpp>
#include <cstdint>
namespace test_variant {
   struct MyStructTag
   {
     uint32_t ver;
     double kind;
     std::string type;
   };

   typedef boost::variant<MyStructTag, std::string> myVariant;

   struct custom_ostream : public boost::static_visitor<std::ostream&>
   {
      std::ostream& os_;

      custom_ostream(std::ostream& os) : os_(os) {}
#if 1
      template <typename T>
      std::ostream& operator()(T const& val) const
      {
         return os_ << val;
      }
#else
      std::ostream& operator()(MyStructTag const& val) const
      {
         return os_ << val.type;
      }
#endif

      friend std::ostream& operator<<(std::ostream& os, myVariant const& v)
      {
         return boost::apply_visitor(custom_ostream{os}, v);
      }
  };

using namespace test_variant;

int main(int argc, char* agrv[])
{
   myVariant my;

   my = "test";
   std::cout << my << std::endl;

   return 0;
}

代码编译为：

g++ --std=c++11 -o main -Iv_1_55_0/ main.cpp

尝试挑选出访问自定义有界类型，但没有成功。

任何输入都会有所帮助。

Answer 1

错误原因是通过ADL找不到operator<<(std::ostream& os, myVariant const& v)。

依赖于参数的查找是一个过程，它涉及扩展在名称查找中检查的名称空间集以查找不合格的函数调用，std::cout << my 就是一个例子。也就是说，你不拼写std::cout.operator<<(my)，也不拼写std::operator<<(std::cout, my)。

ADL 构建了一组关联的命名空间，其中可能会找到函数的名称（此处为 operator<<）。这些命名空间可以是，例如：

• 参数类型的命名空间，
• 参数类型的模板参数类型的命名空间。

operator<< 的参数是：

• std::cout，位于 std 命名空间中，
• boost::variant，位于 boost 命名空间中。

除了这些命名空间之外，还考虑了模板参数的命名空间，它们是：

• MyStructTag，位于 test_variant 命名空间中，
• std::string，位于 std 命名空间中。

最重要的是，ADL 会考虑关联类作为类模板实例化的参数，这里是：

• std::string,
• MyStructTag.

此时，operator<<(std::ostream& os, myVariant const& v) 可以通过 ADL 找到，如果它在内部声明，例如 test_variant 命名空间本身。但是，该函数是在 custom_ostream 中声明和定义的，对于在类中声明的 friend 函数，它们对 ADL 不可见，除非该类是关联类。由于custom_ostream 不在关联类的集合中，因此查找无法找到与调用参数匹配的operator<< 重载。

为了解决这个问题，请在 test_variant 命名空间本身中定义 operator<<，并在 custom_ostream 中只保留一个朋友声明。

DEMO