如何实现boost :: variant派生类？

Question

我已经尝试了几个小时来编写一个派生自的类boost::variant.但我不明白是什么问题(我不明白编译错误意味着什么).

实现干净boost::variant派生类的规则是什么？

#include <boost/variant.hpp>

class MyVariant : public boost::variant<char,bool>
{
public:    
       MyVariant ()          : boost::variant<char,bool>( ) {}

       template <typename T>
       MyVariant(      T& v) : boost::variant<char,bool>(v) {}

       template <typename T>
       MyVariant(const T& v) : boost::variant<char,bool>(v) {}
};

int main ()
{
      MyVariant a;
      MyVariant b = a;        //compilation error
  //  MyVariant c = MyVariant();
  //  MyVariant d (true);
  //  MyVariant e ('E');
}

为什么我要使用继承？(编辑给@zaufi更多细节)

• 我想要一个空状态
• 我想接受const char*为string
• 我想接受int为long
• 我想给出enum类型

例如,在伪C++代码中,我希望:

class MyVariant : public boost::variant<char,bool,long,std::string>
{
  typedef boost::variant<char,bool,long,std::string> super;

public:    

  // I know here I should specialize templeted constructors
  // but I is more clear like that, isn't it?    
  MyVariant()              : super('e')             {} //empty -> char
  MyVariant(char        c) : super(std::string(1,c)){} //char  -> string
  MyVariant(const char* s) : super(std::string(s) ) {} //char* -> string
  MyVariant(int         v) : super(long       (v) ) {} //TODO boundaries    
  /* other constructors ... */

  enum Type
  {
    NONE,  //my empty state = char type
    BOOL,
    LONG,
    STRING
  };

  Type type() const { return (Type) which(); }    
};

基本代码段(在问题的顶部)已在不同平台上进行了测试

• boost v1.33 + GCC 4.1(Linux)
• 提升v1.52 + GCC 4.7(MinGW)
• boost v1.52 + Visual C++ 2010(v10)

下面我的两个版本的GCC的错误(如果它困扰某人,我可以删除其中一个......)

---

$ g++ --version
g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ myVariant.cpp
/usr/include/boost/variant/variant.hpp: In constructor 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(T&) [with T = MyVariant, T0_ = char, T1 = bool, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]':
myVariant.cpp:10:   instantiated from 'MyVariant::MyVariant(T&) [with T = MyVariant]'
myVariant.cpp:19:   instantiated from here
/usr/include/boost/variant/variant.hpp:1348: error: call of overloaded 'convert_construct(MyVariant&, long int)' is ambiguous
/usr/include/boost/variant/variant.hpp:1262: note: candidates are: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = MyVariant, T0_ = char, T1 = bool, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]
/usr/include/boost/variant/variant.hpp:1321: note:                 void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char, U1 = bool, U2 = boost::detail::variant::void_, U3 = boost::detail::variant::void_, U4 = boost::detail::variant::void_, U5 = boost::detail::variant::void_, U6 = boost::detail::variant::void_, U7 = boost::detail::variant::void_, U8 = boost::detail::variant::void_, U9 = boost::detail::variant::void_, U10 = boost::detail::variant::void_, U11 = boost::detail::variant::void_, U12 = boost::detail::variant::void_, U13 = boost::detail::variant::void_, U14 = boost::detail::variant::void_, U15 = boost::detail::variant::void_, U16 = boost::detail::variant::void_, U17 = boost::detail::variant::void_, U18 = boost::detail::variant::void_, U19 = boost::detail::variant::void_, T0_ = char, T1 = bool, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]
/usr/include/boost/variant/variant.hpp:1330: note:                 void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char, U1 = bool, U2 = boost::detail::variant::void_, U3 = boost::detail::variant::void_, U4 = boost::detail::variant::void_, U5 = boost::detail::variant::void_, U6 = boost::detail::variant::void_, U7 = boost::detail::variant::void_, U8 = boost::detail::variant::void_, U9 = boost::detail::variant::void_, U10 = boost::detail::variant::void_, U11 = boost::detail::variant::void_, U12 = boost::detail::variant::void_, U13 = boost::detail::variant::void_, U14 = boost::detail::variant::void_, U15 = boost::detail::variant::void_, U16 = boost::detail::variant::void_, U17 = boost::detail::variant::void_, U18 = boost::detail::variant::void_, U19 = boost::detail::variant::void_, T0_ = char, T1 = bool, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]

---

$ g++ --version
g++.exe (GCC) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ myVariant.cpp -I /c/.../include/
In file included from c:/.../include/boost/variant.hpp:17:0,
                 from myVariant.cpp:1:
c:/.../include/boost/variant/variant.hpp: In instantiation of 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(T&) [with T = MyVariant; T0_ = char; T1 = bool; T2 = boost::detail::variant::void_; T3 = boost::detail::variant::void_; T4 = boost::detail::variant::void_; T5 = boost::detail::variant::void_; T6 = boost::detail::variant::void_; T7 = boost::detail::variant::void_; T8 = boost::detail::variant::void_; T9 = boost::detail::variant::void_; T10 = boost::detail::variant::void_; T11 = boost::detail::variant::void_; T12 = boost::detail::variant::void_; T13 = boost::detail::variant::void_; T14 = boost::detail::variant::void_; T15 = boost::detail::variant::void_; T16 = boost::detail::variant::void_; T17 = boost::detail::variant::void_; T18 = boost::detail::variant::void_; T19 = boost::detail::variant::void_]':
myVariant.cpp:9:66:   required from 'MyVariant::MyVariant(T&) [with T = MyVariant]'
myVariant.cpp:18:25:   required from here
c:/.../include/boost/variant/variant.hpp:1406:9: error: call of overloaded convert_construct(MyVariant&, long int)' is ambiguous
c:/.../include/boost/variant/variant.hpp:1406:9: note: candidates are:
c:/.../include/boost/variant/variant.hpp:1316:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = MyVariant; T0_ = char; T1 = bool; T2 = boost::detail::variant::void_; T3 = boost::detail::variant::void_; T4 = boost::detail::variant::void_; T5 = boost::detail::variant::void_; T6 = boost::detail::variant::void_; T7 = boost::detail::variant::void_; T8 = boost::detail::variant::void_; T9 = boost::detail::variant::void_; T10 = boost::detail::variant::void_; T11 = boost::detail::variant::void_; T12 = boost::detail::variant::void_; T13 = boost::detail::variant::void_; T14 = boost::detail::variant::void_; T15 = boost::detail::variant::void_; T16 = boost::detail::variant::void_; T17 = boost::detail::variant::void_; T18 = boost::detail::variant::void_; T19 = boost::detail::variant::void_; mpl_::false_ = mpl_::bool_<false>]
c:/.../include/boost/variant/variant.hpp:1376:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char; U1 = bool; U2 = boost::detail::variant::void_; U3 = boost::detail::variant::void_; U4 = boost::detail::variant::void_; U5 = boost::detail::variant::void_; U6 = boost::detail::variant::void_; U7 = boost::detail::variant::void_; U8 = boost::detail::variant::void_; U9 = boost::detail::variant::void_; U10 = boost::detail::variant::void_; U11 = boost::detail::variant::void_; U12 = boost::detail::variant::void_; U13 = boost::detail::variant::void_; U14 = boost::detail::variant::void_; U15 = boost::detail::variant::void_; U16 = boost::detail::variant::void_; U17 = boost::detail::variant::void_; U18 = boost::detail::variant::void_; U19 = boost::detail::variant::void_; T0_ = char; T1 = bool; T2 = boost::detail::variant::void_; T3 = boost::detail::variant::void_; T4 = boost::detail::variant::void_; T5 = boost::detail::variant::void_; T6 = boost::detail::variant::void_; T7 = boost::detail::variant::void_; T8 = boost::detail::variant::void_; T9 = boost::detail::variant::void_; T10 = boost::detail::variant::void_; T11 = boost::detail::variant::void_; T12 = boost::detail::variant::void_; T13 = boost::detail::variant::void_; T14 = boost::detail::variant::void_; T15 = boost::detail::variant::void_; T16 = boost::detail::variant::void_; T17 = boost::detail::variant::void_; T18 = boost::detail::variant::void_; T19 = boost::detail::variant::void_]
c:/.../include/boost/variant/variant.hpp:1385:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char; U1 = bool; U2 = boost::detail::variant::void_; U3 = boost::detail::variant::void_; U4 = boost::detail::variant::void_; U5 = boost::detail::variant::void_; U6 = boost::detail::variant::void_; U7 = boost::detail::variant::void_; U8 = boost::detail::variant::void_; U9 = boost::detail::variant::void_; U10 = boost::detail::variant::void_; U11 = boost::detail::variant::void_; U12 = boost::detail::variant::void_; U13 = boost::detail::variant::void_; U14 = boost::detail::variant::void_; U15 = boost::detail::variant::void_; U16 = boost::detail::variant::void_; U17 = boost::detail::variant::void_; U18 = boost::detail::variant::void_; U19 = boost::detail::variant::void_; T0_ = char; T1 = bool; T2 = boost::detail::variant::void_; T3 = boost::detail::variant::void_; T4 = boost::detail::variant::void_; T5 = boost::detail::variant::void_; T6 = boost::detail::variant::void_; T7 = boost::detail::variant::void_; T8 = boost::detail::variant::void_; T9 = boost::detail::variant::void_; T10 = boost::detail::variant::void_; T11 = boost::detail::variant::void_; T12 = boost::detail::variant::void_; T13 = boost::detail::variant::void_; T14 = boost::detail::variant::void_; T15 = boost::detail::variant::void_; T16 = boost::detail::variant::void_; T17 = boost::detail::variant::void_; T18 = boost::detail::variant::void_; T19 = boost::detail::variant::void_]

Answer 1

接受的答案和其他答案都没有回答这个问题.有很好的理由从boost :: variant派生出来.

• 您希望将方法添加到变体中,以使其行为更具多态性
• 你想继续使用boost::apply_visitor(visitor(), variant)而不跳过篮球

以下是MyVariant继承自的类型的工作示例boost::variant.该代码要求C++ 11继承(复杂)构造函数boost::variant.

编辑:实现的相等比较等运算符boost::variant不会自动为派生类工作.这是如何boost::variant实现的(非预期的)结果,它明确地阻止了使用模板与"外来"类型的比较.可以使用显式重载启用运算符,如示例中所示.

#include<iostream>
#include<boost/variant.hpp>

struct type_size_visitor : public boost::static_visitor<unsigned> {
  template <typename T>
  unsigned operator()(const T&) const { return sizeof(T); }
};

template <typename... Types>
class MyVariant : public boost::variant<Types...>
{
  using base_type = boost::variant<Types...>;
public:
  // inherit constructors
  using base_type::base_type;

  // add a new method
  unsigned type_size() { return boost::apply_visitor(type_size_visitor(), *this); }

  /* Inheriting operators from boost::variant does not
     work as it normally would. We have to add explicit
     overloads for the operators we want to use.
  */
  bool operator==(const MyVariant& rhs) const {
    // cast rhs to base and delegate to operator in base
    return base_type::operator==(static_cast<const base_type&>(rhs));
  }
};

int main() {
  MyVariant<std::string, char> v;
  v = 1;
  std::cout << v.type_size() << " " << sizeof(char) << std::endl;
  v = std::string("foo");
  std::cout << v.type_size() << " " << sizeof(std::string) << std::endl;

  // comparison operators need workaround shown above
  MyVariant<std::string, char> a, b;
  a = 1; b = 1;
  assert(a == b);
}