std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast,(3)

NAME
std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast, - std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast,

Synopsis
   Defined in header <memory>
   template< class T, class U >
   std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& (1) (since C++11)
   r ) noexcept;
   template< class T, class U >
   std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) (2) (since C++20)
   noexcept;
   template< class T, class U >
   std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& (3) (since C++11)
   r ) noexcept;
   template< class T, class U >
   std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) (4) (since C++20)
   noexcept;
   template< class T, class U >
   std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r (5) (since C++11)
   ) noexcept;
   template< class T, class U >
   std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) (6) (since C++20)
   noexcept;
   template< class T, class U >
   std::shared_ptr<T> reinterpret_pointer_cast( const (7) (since C++17)
   std::shared_ptr<U>& r ) noexcept;
   template< class T, class U >
   std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& (8) (since C++20)
   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) (since C++20)
   if the dynamic_cast fails.

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

First version
   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};
   }

Second version
   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>{};
       }
   }
                        Third version
   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};
   }
                        Fourth version
   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};
   }

Example
// Run this code
 #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

See also
   constructor constructs new shared_ptr
                 (public member function)