C++ 尝试使用 dynamic_cast 时获取“源类型不是多态的”
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Getting "source type is not polymorphic" when trying to use dynamic_cast
提问by Andrew Tomazos
struct A {};
struct B : A {};
int main()
{
A* a = new B();
B* b = dynamic_cast<B*>(a);
}
gives:
给出：
cannot dynamic_cast 'a' (of type 'struct A*') to type 'struct B*' (source type is not polymorphic)
How can I make Apolymorphic? I want to safely cast it to B.
我怎样才能使A多态？我想安全地将它投射到 B。
(One way is to add a dummy virtual function, but is there a better way?)
（一种方法是添加一个虚拟的虚函数，但有没有更好的方法？）
回答by juanchopanza
You need to make Apolymorphic, which you can do by adding a virtualdestructor or any virtual function:
您需要实现A多态，您可以通过添加virtual析构函数或任何虚函数来实现：
struct A {
virtual ~A() = default;
};
or, before C++11,
或者，在 C++11 之前，
struct A {
virtual ~A() {}
};
Note that a polymorphic type should have a virtual destructor anyway, if you intend to safely call delete on instances of a derived type via a pointer to the base.
请注意，如果您打算通过指向基类的指针安全地对派生类型的实例调用 delete，那么无论如何多态类型都应该具有虚拟析构函数。
回答by Luchian Grigore
You need at least a virtualfunction - typically, if no others are suitable, the destructor:
您至少需要一个virtual函数 - 通常，如果没有其他合适的函数，则析构函数：
struct A {
virtual ~A() {}
};
回答by Andy Prowl
As your compiler says, your type Ais not polymorphic. You should add a virtualfunction to it. For instance, a virtualdestructor could be a good choice:
正如您的编译器所说，您的类型A不是多态的。你应该virtual给它添加一个函数。例如，virtual析构函数可能是一个不错的选择：
struct A { virtual ~A() { } };
// ^^^^^^^ This makes A a polymorphic type
struct B : A {};
int main()
{
A* a = new B();
B* b = dynamic_cast<B*>(a); // Should work now
}
