std:: static_pointer_cast, std:: dynamic_pointer_cast, std:: const_pointer_cast, std:: reinterpret_pointer_cast

创建一个新的 std::shared_ptr 实例，其存储指针通过强制转换表达式从 r 的存储指针获取。

如果 r 为空，则新的 shared_ptr 也为空（但其存储的指针不一定为空）。否则，新的 shared_ptr 将与 r 的初始值共享所有权，除非 dynamic_pointer_cast 执行的 dynamic_cast 返回空指针，此时新指针为空。

令 Y 为 typename std:: shared_ptr < T > :: element_type ，则生成的 std::shared_ptr 的存储指针将分别通过以下方式获取：

这些函数的行为是未定义的，除非从 U* 到 T* 的相应转换是良构的：

参数

注释

表达式 std:: shared_ptr < T > ( static_cast < T * > ( r. get ( ) ) ) 、 std:: shared_ptr < T > ( dynamic_cast < T * > ( r. get ( ) ) ) 和 std:: shared_ptr < T > ( const_cast < T * > ( r. get ( ) ) ) 看似具有相同效果，但实际上均可能导致未定义行为，试图重复删除同一对象！

可能的实现

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};
}

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>{};
}

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};
}

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};
}

示例

#include <iostream>
#include <memory>
class Base
{
public:
    int a;
    virtual void f() const { std::cout << "I am base!\n"; }
    virtual ~Base() {}
};
class Derived : public Base
{
public:
    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 用于向上转换类层次结构
    basePtr = std::static_pointer_cast<Base>(derivedPtr);
    std::cout << "Base pointer to derived says: ";
    basePtr->f();
    // dynamic_pointer_cast 用于向下/跨类层次结构转换
    auto downcastedPtr = std::dynamic_pointer_cast<Derived>(basePtr);
    if (downcastedPtr)
    {
        std::cout << "Downcasted pointer says: ";
        downcastedPtr->f();
    }
    // 所有指向派生类的指针共享所有权
    std::cout << "Pointers to underlying derived: "
              << derivedPtr.use_count()
              << '\n';
}

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

参见