無法使用boost :: apply_visitor訪問模板對象

Question

我正在學習boost :: variant並使用apply_visitor訪問其成員。所以，我寫了下面的例子。 我有指向模板對象作爲我的班級成員。我想用apply_visitor訪問它們。

//Example program 
#include <iostream> 
#include <map> 
#include <string> 
#include <boost/shared_ptr.hpp> 
#include <boost/variant.hpp> 
#include <boost/any.hpp> 

class A { 
    template<size_t dim> 
    struct C : boost::static_visitor<C<dim> *>{ 
     int x; 
     C() { 
      x = dim; 
     } 
     template <size_t t> 
     C<t> * operator()(C<t> *s) const { 
      std::cout<<s->x<<std::endl; 
      return s; 
     } 
    }; 
public: 
    C<1>* Aasd; 
    C<2>* Bbsd; 
    std::map<std::string, boost::variant<C<2> *, C<1> * > > matrices;  

    A() { 
     Aasd = new C<1>; 
     Bbsd = new C<2>; 
    matrices.insert(std::make_pair("Bb", Bbsd)); 

    boost::apply_visitor(C<2>(), matrices["Bb"]); 
    // boost::apply_visitor(C<1>(), matrices["Aa"]); 
    } 

    ~A() { 
    delete Aasd; 
    delete Bbsd; 
    }   
}; 

int main() 
{  
    A a; 
    return 0; 
} 

，我從上面的編譯得到的錯誤是

In file included from /usr/include/boost/variant.hpp:17:0, 
       from 8: 
/usr/include/boost/variant/variant.hpp: In instantiation of 'boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = A::C<1ul>*; Visitor = const A::C<2ul>; boost::detail::variant::invoke_visitor<Visitor>::result_type = A::C<2ul>*]': 
/usr/include/boost/variant/detail/visitation_impl.hpp:130:9: 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 A::C<2ul> >; VoidPtrCV = void*; T = A::C<1ul>*; typename Visitor::result_type = A::C<2ul>*; mpl_::true_ = mpl_::bool_<true>]' 
/usr/include/boost/variant/detail/visitation_impl.hpp:173:9: 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 A::C<2ul> >; VoidPtrCV = void*; T = A::C<1ul>*; NoBackupFlag = boost::variant<A::C<2ul>*, A::C<1ul>*>::has_fallback_type_; typename Visitor::result_type = A::C<2ul>*]' 
/usr/include/boost/variant/detail/visitation_impl.hpp:256:5: 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>, A::C<2ul>*, boost::mpl::l_item<mpl_::long_<1l>, A::C<1ul>*, boost::mpl::l_end> > >, boost::mpl::l_iter<boost::mpl::l_end> >; Visitor = boost::detail::variant::invoke_visitor<const A::C<2ul> >; VoidPtrCV = void*; NoBackupFlag = boost::variant<A::C<2ul>*, A::C<1ul>*>::has_fallback_type_; typename Visitor::result_type = A::C<2ul>*; mpl_::false_ = mpl_::bool_<false>]' 
/usr/include/boost/variant/variant.hpp:2367:13: required from 'static typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<const A::C<2ul> >; VoidPtrCV = void*; T0_ = A::C<2ul>*; T1 = A::C<1ul>*; 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_; typename Visitor::result_type = A::C<2ul>*]' 
/usr/include/boost/variant/variant.hpp:2378:13: required from 'typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor(Visitor&) [with Visitor = boost::detail::variant::invoke_visitor<const A::C<2ul> >; T0_ = A::C<2ul>*; T1 = A::C<1ul>*; 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_; typename Visitor::result_type = A::C<2ul>*]' 
/usr/include/boost/variant/variant.hpp:2401:52: required from 'typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::apply_visitor(Visitor&) [with Visitor = const A::C<2ul>; T0_ = A::C<2ul>*; T1 = A::C<1ul>*; 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_; typename Visitor::result_type = A::C<2ul>*]' 
/usr/include/boost/variant/detail/apply_visitor_unary.hpp:76:43: required from 'typename Visitor::result_type boost::apply_visitor(const Visitor&, Visitable&) [with Visitor = A::C<2ul>; Visitable = boost::variant<A::C<2ul>*, A::C<1ul>*>; typename Visitor::result_type = A::C<2ul>*]' 
34:51: required from here 
/usr/include/boost/variant/variant.hpp:1048:32: error: cannot convert 'A::C<1ul>*' to 'boost::detail::variant::invoke_visitor<const A::C<2ul> >::result_type {aka A::C<2ul>*}' in return 
     return visitor_(operand); 

它編譯和工作正常，如果我刪除ç< 1> *從升壓::變種聲明。

任何建議將不勝感激。

Answer 1

所有訪客功能必須遵循由基類定義的方案：boost::static_visitor<C<dim> *>。

這意味着operator()返回值必須始終C<dim> *沒有C<t>* - 看到：

template<size_t dim> 
struct C : boost::static_visitor<C<dim> *> { 

    template <size_t t> 
    C<dim> * operator()(C<t> *s) const { 
    // ^^^ - not t, must be as static_visitor parameter few lines above 
     std::cout<<s->x<<std::endl; 
     return s; // of course this part will have problems to compile 
       // e.g. pointers to C<2> and C<1> are not convertible 
    } 

---

正如我把在return s正確的意見 - 這並不編譯 - 因爲編譯器不能隱蔽C<1>到C<2>。

你可以做些什麼來解決這個問題，主要是介紹基類 - 象下面這樣：

struct CBase { 
    int x; 
}; 

template<size_t dim> 
struct C : CBase, boost::static_visitor<CBase*> { 

    C() : CBase{dim} {} 

    template <size_t t> 
    CBase* operator()(C<t> *s) const { 
     std::cout<<s->x<<std::endl; 
     return s; 
    }