当我们用“裸”指针进行类层次上的上下行转换时，可以使用dynamic_cast。当然我们也可以使用static_cast，只是dynamic_cast在进行下行转换的时候（即基类到派生类）具有类型检查功能，而static_cast没有。因此存在安全问题。
当我们使用智能指针时，如果需要进行类层次上的上下行转换时，可以使用std::static_pointer_cast()、std::dynamic_pointer_cast、std::const_pointer_cast()和std::reinterpret_pointer_cast()。它们的功能和std::static_cast()、std::dynamic_cast、std::const_cast()和std::reinterpret_cast()类似，只不过转换的是智能指针std::shared_ptr，返回的也是std::shared_ptr类型。
1、std::static_pointer_cast()：当指针是智能指针时候，向上转换，用static_cast 则转换不了，此时需要使用static_pointer_cast。
2、std::dynamic_pointer_cast()：当指针是智能指针时候，向下转换，用dynamic_cast 则转换不了，此时需要使用dynamic_pointer_cast（此处注意：base基类需要至少有一个virtual成员函数（即多态类型）才能允许动态强制转换，否则编译报错）。
3、std::const_pointer_cast()：功能与std::const_cast()类似
4、std::reinterpret_pointer_cast()：功能与std::reinterpret_cast()类似
Defined in header <memory>
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(1) (since C++11)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(2) (since C++20)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(3) (since C++11)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(4) (since C++20)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(5) (since C++11)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(6) (since C++20)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(7) (since C++17)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(8) (since C++20)
基类和派生类的智能指针转换要使用std::dynamic_pointer_cast和std::static_pointer_cast。由于std::dynamic_pointer_cast和dynamic_cast原理一样，std::static_pointer_cast和static_cast原理一样
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.
(since C++20)
Parameters
r - The pointer to convert
Notes
The expressions std::shared_ptr<T>(static_cast<T*>(r.get())), std::shared_ptr<T>(dynamic_cast<T*>(r.get())) and std::shared_ptr<T>(const_cast<T*>(r.get())) might seem to have the same effect, but they all will likely result in undefined behavior, attempting to delete the same object twice!
Possible implementation
1、std::static_pointer_cast():
template< class T, class U > std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept { auto p = static_cast<typename std::shared_ptr<T>::element_type*>(r.get()); return std::shared_ptr<T>(r, p); }
2、std::dynamic_pointer_cast()
template< class T, class U > std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept { if (auto p = dynamic_cast<typename std::shared_ptr<T>::element_type*>(r.get())) { return std::shared_ptr<T>(r, p); } else { return std::shared_ptr<T>(); } }
3、std::const_pointer_cast()
template< class T, class U > std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept { auto p = const_cast<typename std::shared_ptr<T>::element_type*>(r.get()); return std::shared_ptr<T>(r, p); }
4、std::reinterpret_pointer_cast()
template< class T, class U > std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept { auto p = reinterpret_cast<typename std::shared_ptr<T>::element_type*>(r.get()); return std::shared_ptr<T>(r, p); }
使用示例：
#include <iostream> #include <memory> struct Base { int a; virtual void f() const { std::cout << "I am base!\n";} virtual ~Base(){} }; struct Derived : Base { void f() const override { std::cout << "I am derived!\n"; } ~Derived(){} }; int main(){ auto basePtr = std::make_shared<Base>(); std::cout << "Base pointer says: "; basePtr->f(); auto derivedPtr = std::make_shared<Derived>(); std::cout << "Derived pointer says: "; derivedPtr->f(); // static_pointer_cast to go up class hierarchy basePtr = std::static_pointer_cast<Base>(derivedPtr); std::cout << "Base pointer to derived says: "; basePtr->f(); // dynamic_pointer_cast to go down/across class hierarchy auto downcastedPtr = std::dynamic_pointer_cast<Derived>(basePtr); if(downcastedPtr) { std::cout << "Downcasted pointer says: "; downcastedPtr->f(); } // All pointers to derived share ownership std::cout << "Pointers to underlying derived: " << derivedPtr.use_count() << "\n"; } Output: Base pointer says: I am base! Derived pointer says: I am derived! Base pointer to derived says: I am derived! Downcasted pointer says: I am derived! Pointers to underlying derived: 3 示例2 #include <iostream> // std::cout std::endl #include <memory> // std::shared_ptr std::dynamic_pointer_cast std::static_pointer_cast class base { public: virtual ~base(void) = default; }; class derived : public base { }; class test : public base { }; int main(void) { std::cout << std::boolalpha; // 两个不同的派生类对象 auto derivedobj = std::make_shared<derived>(); auto testobj = std::make_shared<test>(); // 隐式转换 derived->base std::shared_ptr<base> pointer1 = derivedobj; // static_pointer_cast derived->base auto pointer2 = std::static_pointer_cast<base>(derivedobj); // dynamic_pointer_cast base->derived auto pointer3 = std::dynamic_pointer_cast<derived>(pointer1); std::cout << (pointer3 == nullptr) << std::endl; // dynamic_pointer_cast base->derived auto pointer4 = std::dynamic_pointer_cast<test>(pointer1); std::cout << (pointer4 == nullptr) << std::endl; return 0; }
输出结果：
false
true
std::reinterpret_pointer_cast()和std::const_pointer_cast()示例代码：
#include <memory> #include <cassert> #include <cstdint> int main() { std::shared_ptr<int> foo; std::shared_ptr<const int> bar; foo = std::make_shared<int>(10); bar = std::const_pointer_cast<const int>(foo); std::cout << "*bar: " << *bar << std::endl; *foo = 20; std::cout << "*bar: " << *bar << std::endl; std::shared_ptr<std::int8_t[]> p(new std::int8_t[4]{1, 1, 1, 1}); std::shared_ptr<std::int32_t[]> q = std::reinterpret_pointer_cast<std::int32_t[]>(p); std::int32_t r = q[0]; std::int32_t x = (1 << 8) | (1 << 16) | (1 << 24) | 1; assert(r == x); return 0; }
输出：
*bar: 10
*bar: 20
