c++ - 模式匹配 C++ 论文示例不起作用

Question

我在 C++ 中遇到过模式匹配提议，我尝试编译关于表达式树的示例 4.6。

这是提取的代码：

#include <memory>
#include <variant>

struct Expr;
struct Neg {
  std::shared_ptr<Expr> expr;
};

struct Add {
  std::shared_ptr<Expr> lhs, rhs;
};

struct Mul {
  std::shared_ptr<Expr> lhs, rhs;
};

struct Expr : std::variant<int, Neg, Add, Mul> {
  using variant::variant;
};

namespace std {
  template <>
  struct variant_size<Expr> : variant_size<Expr::variant> {};

  template <std::size_t I>
  struct variant_alternative<I, Expr> : variant_alternative<I, Expr::variant> {};
}

int eval(const Expr& expr) {
  struct visitor {

    int operator()(int i) const {return i;}
    int operator()(const Neg& n) const {return -eval(*n.expr);}
    int operator()(const Add& a) const {return eval(*a.lhs) + eval(*a.rhs);}
    int operator()(const Mul& m) const {// Optimize multiplication by 0.
     if (int* i = std::get_if<int>(m.lhs.get()); i && *i == 0) {
       return 0;
     }
     if (int* i = std::get_if<int>(m.rhs.get()); i && *i == 0) {
       return 0;
     }
     return eval(*m.lhs) * eval(*m.rhs);}
  };
  return std::visit(visitor{}, expr);
}

上面的代码给出了 4 个编译错误，其中：

clang++ -std=c++17 main.cpp

In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:778:6: error: implicit instantiation of undefined template 'std::__detail::__variant::_Extra_visit_slot_needed<int, const Expr &>::_Variant_never_valueless<Expr>'
        && !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
            ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:809:44: note: in instantiation of static data member 'std::__detail::__variant::_Extra_visit_slot_needed<int, const Expr &>::value' requested here
        _Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
                                                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:53: note: in instantiation of static data member 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>::__do_cookie' requested here
      _Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
                                                    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
      static constexpr _Array_type _S_vtable
                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
      constexpr auto& __vtable = __detail::__variant::__gen_vtable<
                                                      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
      return __do_visit(std::forward<_Visitor>(__visitor),
             ^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
  return std::visit(visitor{}, expr);
              ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:769:34: note: template is declared here
      template <typename> struct _Variant_never_valueless;
                                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:809:2: error: constexpr variable '__do_cookie' must be initialized by a constant expression
        _Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:53: note: in instantiation of static data member 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>::__do_cookie' requested here
      _Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
                                                    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
      static constexpr _Array_type _S_vtable
                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
      constexpr auto& __vtable = __detail::__variant::__gen_vtable<
                                                      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
      return __do_visit(std::forward<_Visitor>(__visitor),
             ^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
  return std::visit(visitor{}, expr);
              ^
In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:821:36: error: fields must have a constant size: 'variable length array in structure' extension will never be supported
      _Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: note: in instantiation of template class 'std::__detail::__variant::_Multi_array<int (*)(visitor &&, const Expr &), 4>' requested here
      static constexpr _Array_type _S_vtable
                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1638:55: note: in instantiation of template class 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>' requested here
      constexpr auto& __vtable = __detail::__variant::__gen_vtable<
                                                      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
      return __do_visit(std::forward<_Visitor>(__visitor),
             ^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
  return std::visit(visitor{}, expr);
              ^
In file included from main.cpp:6:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1016:36: error: constexpr variable '_S_vtable' must be initialized by a constant expression
      static constexpr _Array_type _S_vtable
                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1640:45: note: in instantiation of static data member 'std::__detail::__variant::__gen_vtable<true, int, visitor &&, const Expr &>::_S_vtable' requested here
        _Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
                                                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/variant:1654:14: note: in instantiation of function template specialization 'std::__do_visit<false, true, visitor, const Expr &>' requested here
      return __do_visit(std::forward<_Visitor>(__visitor),
             ^
main.cpp:48:15: note: in instantiation of function template specialization 'std::visit<visitor, const Expr &>' requested here
  return std::visit(visitor{}, expr);
              ^
4 errors generated.

如何使上面的代码编译？std::variant基本上，问题是如何std::shared_ptr为不完整类型提供递归类型。

该命令clang++ --version给出：

clang version 9.0.0-2 (tags/RELEASE_900/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

标签： c++pattern-matchingexpressionvariant