C++:“……不是多态类型"；使用 boost::dynamic_pointer_cast 时

2021-12-31 00:00:00 compiler-errors c++ shared-ptr dynamic-cast

为什么我会收到以下代码的以下错误?

Why do I receive the following error for the following code?

1>C:Libsoost_1_44oost/smart_ptr/shared_ptr.hpp(259): error C2683: 'dynamic_cast' : 'my_namespace::A' is not a polymorphic type
1>          D:[location][header_filename].h(35) : see declaration of 'my_namespace::A'
1>          C:Libsoost_1_44oost/smart_ptr/shared_ptr.hpp(522) : see reference to function template instantiation 'boost::shared_ptr<T>::shared_ptr<my_namespace::A>(const boost::shared_ptr<my_namespace::A> &,boost::detail::dynamic_cast_tag)' being compiled
1>          with
1>          [
1>              T=my_namespace::B
1>          ]
1>          [location][source_filename].cpp(217) : see reference to function template instantiation 'boost::shared_ptr<T> boost::dynamic_pointer_cast<my_namespace::B,striker::A>(const boost::shared_ptr<my_namespace::A> &)' being compiled
1>          with
1>          [
1>              T=my_namespace::B
1>          ]
1>C:Libsoost_1_44oost/smart_ptr/shared_ptr.hpp(260): fatal error C1903: unable to recover from previous error(s); stopping compilation

C++ 代码大致如下:

The C++ code is more or less the following:

#include <list>
#include "boost/pointer_cast.hpp"
#include "boost/shared_ptr.hpp"

struct A
{
public:
    A(const MyEnum an_enum_, const int an_int_) :
        an_enum(an_enum_),
        an_int(an_int_)
    {}

    const MyEnum an_enum;
    const int an_int;
};

struct B : public A {
public:
    B(const int some_int_, const MyStruct &a_struct_) :
        A(ENUM_OPTION_A, an_int_),
        a_struct(a_struct_)
    {}

    const MyStruct a_struct;
};


// Ussage in some function:
// ...
boost::shared_ptr<A> a_ptr = boost::shared_ptr<A>( new B() );
std::list<boost::shared_ptr<A>> a_list;
a_list.push_back(a_ptr);
// ...
boost::shared_ptr<A> a_ptr2 = a_list.front();
boost::shared_ptr<B> b_ptr = boost::dynamic_pointer_cast<B>(a_ptr2); // <-- error here
// ...

推荐答案

dynamic_cast 仅适用于多态类.而polymorphic类是至少有一个virtual函数的类，即使是析构函数.

dynamic_cast works ONLY with polymorphic class. And polymorphic class is that which has atleast one virtual function, even be it the destructor.

//polymorphic classes
struct A
{
   virtual ~A(); //even virtual destructor makes a class polymorphic!
};
struct B : A
{
   void f();
};

//non-polymorphic classes    
struct C
{
   ~C(); //not virtual
};

struct D : C
{
   void f(); //not virtual either
};

在上面的代码中，A和B是多态类，而C和D不是.

In the above code, A and B are polymorphic classes, but C and D are not.

A *pA = new B();
B *pB = dynamic_cast<B*>(pA); //okay

C *pC = new D();
D *pD = dynamic_cast<D*>(pC);  //error -  not polymorphic class

注意在dynamic_cast中，只有源类型需要多态才能编译.如果目标不是多态，则dynamic_cast 将返回空指针.

Note that in dynamic_cast, only the source type need to be polymorphic in order to compile. If the destination isn't polymorphic, then dynamic_cast will return null pointer.

D *pD = dynamic_cast<D*>(pA);  //okay - source (pA) is polymorphic

if ( pD )  
      cout << "pD is not null" ;
else 
      cout << "pD is null";

输出:

pD is null

在线演示:https://web.archive.org/web/20000000000000/http://www.ideone.com/Yesxc

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