C++：使用 boost::dynamic_pointer_cast 时“...不是多态类型”
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/5831765/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
C++: "... is not a polymorphic type" while using boost::dynamic_pointer_cast
提问by Jonathan
Why do I receive the following error for the following code?
为什么我收到以下代码的以下错误？
1>C:\Libs\boost_1_44\boost/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:\Libs\boost_1_44\boost/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:\Libs\boost_1_44\boost/smart_ptr/shared_ptr.hpp(260): fatal error C1903: unable to recover from previous error(s); stopping compilation
The C++ code is more or less the following:
C++ 代码或多或少如下：
#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
// ...
回答by Nawaz
dynamic_castworks ONLY with polymorphic class. And polymorphicclass is that which has atleast one virtualfunction, even be it the destructor.
dynamic_cast仅适用于多态类。而polymorphic类是具有ATLEAST一个virtual功能，即使是它的析构函数。
//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
};
In the above code, Aand Bare polymorphic classes, but Cand Dare not.
在上面的代码中，A和B是多态类，但C和D不是。
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
Note that in dynamic_cast, only the source type need to be polymorphicin order to compile. If the destination isn't polymorphic, then dynamic_castwill return null pointer.
请注意，在 中dynamic_cast，只有源类型需要多态才能编译。如果目标不是polymorphic，dynamic_cast则将返回空指针。
D *pD = dynamic_cast<D*>(pA); //okay - source (pA) is polymorphic
if ( pD )
cout << "pD is not null" ;
else
cout << "pD is null";
Output:
输出：
pD is null
Online demo: https://web.archive.org/web/20000000000000/http://www.ideone.com/Yesxc
在线演示：https://web.archive.org/web/20000000000000/http: //www.ideone.com/Yesxc
回答by Alexander Gessler
'dynamic_cast' : 'my_namespace::A' is not a polymorphic typebecause it doesn't define or inherit a single virtualfunction. Just add a virtual destructor and you'll be fine.
'dynamic_cast' : 'my_namespace::A' is not a polymorphic type因为它没有定义或继承单个virtual函数。只需添加一个虚拟析构函数就可以了。
dynamic_castworks only for such 'polymorphic' types.
dynamic_cast仅适用于此类“多态”类型。
回答by sharptooth
struct Ahas no virtual methods (not even a destructor), so you can't dynamic_castfrom A*- only pointers to types with at least one virtual member function can be used dynamic_caston. boost::dynamic_pointer_castdoes dynamic_castinside, to it's subject to same requirements.
struct A不具有虚拟方法（甚至没有析构函数），所以不能dynamic_cast从A*-仅指针类型的具有至少一个虚拟成员函数，可以使用dynamic_cast上。boost::dynamic_pointer_cast在dynamic_cast里面，它受到相同的要求。
