c++ - boost::make_recursive_variant 没有名为 'apply_visitor' 的成员

Question

第一次使用 Boost，并尝试了解如何使用 boost::variant。请在下面找到示例程序和错误消息 (GCC v5.3.0, -std=c++14)

#include <vector>
#include <iostream>
#include <sstream>

#include <boost/variant.hpp>

using int_tree_t =
    boost::make_recursive_variant <
      int,
      std::vector<boost::recursive_variant_>>;

auto operator << (std::ostream& out, const int_tree_t& tree) ->
std::ostream&
{
  struct stringify :
    public boost::static_visitor <std::string>
  {
  public:
    auto operator () (int i) const ->
    std::string
    {
      return std::to_string (i);
    }

    auto operator () (const std::vector<int_tree_t>& vec) const ->
    std::string
    {
      std::stringstream ss;
      ss << "(";

      auto it = std::begin (vec);

      if (!vec.empty ()) {
        ss << *it;
      }

      for (; it != std::end (vec); ++it) {
        ss << " " << *it;
      }

      ss << ")";

      return ss.str ();
    }
  };

  std::stringstream ss;
  ss.flags (out.flags ());

  ss << boost::apply_visitor (stringify (), tree);

  return (out << ss.str ());
}

错误信息:

In file included from /usr/local/include/boost/variant/apply_visitor.hpp:16:0,
                 from /usr/local/include/boost/variant/detail/hash_variant.hpp:23,
                 from /usr/local/include/boost/variant/variant.hpp:34,
                 from /usr/local/include/boost/variant.hpp:17,
                 from test2.cpp:5:
/usr/local/include/boost/variant/detail/apply_visitor_unary.hpp: In instantiation of ‘typename Visitor::result_type boost::apply_visitor(const Visitor&, Visitable&) [with Visitor = operator<<(std::ostream&, const int_tree_t&)::stringify; Visitable = const boost::make_recursive_variant<int, std::vector<boost::recursive_variant_, std::allocator<boost::recursive_variant_> > >; typename Visitor::result_type = std::__cxx11::basic_string<char>]’:
test2.cpp:50:43:   required from here
/usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:84:43: error: ‘const struct boost::make_recursive_variant<int, std::vector<boost::recursive_variant_, std::allocator<boost::recursive_variant_> > >’ has no member named ‘apply_visitor’
     return visitable.apply_visitor(visitor);
                                           ^

如果我理解正确，boost::make_recursive_variant 应该创建一个与 boost::variant 行为相同的类型...但错误消息似乎表明它没有访问所必需的 apply_visitor 函数。我已经阅读了 [this( http://www.boost.org/doc/libs/1_60_0/doc/html/variant/tutorial.html ) 页面，但没有关于必须使用具有递归变体的“特殊”访问模式的内容。

我应该如何编写这个程序？

Answer 1

make_recursive_variant是一个元函数，而不是一个类型本身。所以你必须评估元函数:

using int_tree_t =
    boost::make_recursive_variant <
      int,
      std::vector<boost::recursive_variant_>>::type;
                                             ↑↑↑↑↑↑

否则，您使用的是元函数类型而不是变体类型。