dynamic_pointer_cast 文档

文档来源：https://en.cppreference.com/w/cpp/memory/shared_ptr/pointer_cast

template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
...

Creates a new instance of std::shared_ptr whose stored pointer is obtained from r's stored pointer using a cast expression.

If r is empty, so is the new shared_ptr (but its stored pointer is not necessarily null). Otherwise, the new shared_ptr will share ownership with the initial value of r, except that it is empty if the dynamic_cast performed by dynamic_pointer_cast returns a null pointer. Let Y be typename std::shared_ptr<T>::element_type, then the resulting std::shared_ptr's stored pointer will be obtained by evaluating, respectively: 1,2) static_cast<Y*>(r.get()) 3,4) dynamic_cast<Y*>(r.get()). If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty. 5,6) const_cast<Y*>(r.get()) 7,8) reinterpret_cast<Y*>(r.get())

The behavior of these functions is undefined unless the corresponding cast from U to T is well formed: 1,2) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed. 3,4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. 5,6) The behavior is undefined unless const_cast<T*>((U*)nullptr) is well formed. 7,8) The behavior is undefined unless reinterpret_cast<T*>((U*)nullptr) is well formed.

After calling the rvalue overloads (2,4,6,8), r is empty and r.get() == nullptr, except that r is not modified for dynamic_pointer_cast (4) if the dynamic_cast fails.

动态转换

来源：https://stackoverflow.com/questions/23795265/dynamic-cast-across-a-shared-ptr

If you just want to call a function from B you can use one of these:

std::shared_ptr<A> ap = ...;
dynamic_cast<B&>(*ap).b_function();
if (B* bp = dynamic_cast<B*>(ap.get()) {
    ...
}

In case you actually want to get a std::shared_ptr<B> from the std::shared_ptr<A>, you can use use

std::shared_ptr<B> bp = std::dynamic_pointer_cast<B>(ap);

方法二 std::dynamic_cast

class A {
};

class B: public A {
}

int main() {
    std::shared_ptr<A> pb = std::make_shared<B>();
    auto x = dynamic_cast<B*>(pb.get());
    if(nullptr != x) {
        //
    }
}

方法三 std::dynamic_pointer_cast 转换为 shared_ptr

If you just want to call a function from B you can use one of these:

std::shared_ptr<A> ap = ...;
dynamic_cast<B&>(*ap).b_function();
if (B* bp = dynamic_cast<B*>(ap.get()) {
    ...
}

In case you actually want to get a std::shared_ptr from the std::shared_ptr, you can use use

std::shared_ptr<B> bp = std::dynamic_pointer_cast<B>(ap);