Std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast, (3) Linux Manual Page

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

std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast, (3) Linux Manual Page