W3cubDocs

/C++

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

Defined in header `<memory>`
template< class T, class U > std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r );	(1)	(since C++11)
template< class T, class U > std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r );	(2)	(since C++11)
template< class T, class U > std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r );	(3)	(since C++11)
template< class T, class U > std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r );	(4)	(since C++17)

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 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 calling (in respective order):

1) static_cast<Y*>(r.get()).

2) dynamic_cast<Y*>(r.get()) (If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty).

3) const_cast<Y*>(r.get()).

4) reinterpret_cast<Y*>(r.get())

The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed:

1) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed.

2) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed.

3) The behavior is undefined unless const_cast<T*>((U*)nullptr) is well formed.

4) The behavior is undefined unless reinterpret_cast<T*>((U*)nullptr) is well formed.

Parameters

The pointer to convert

Exceptions

noexcept specification:

noexcept

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); }

Example

#include <iostream>
#include <memory>
 
struct BaseClass {};
 
struct DerivedClass : BaseClass
{
    void f() const
    {
        std::cout << "Hello World!\n";
    }
    ~DerivedClass(){ // note, it's not virtual
        std::cout << "~DerivedClass\n";
    }
};
 
int main()
{
    std::shared_ptr<BaseClass> ptr_to_base(std::make_shared<DerivedClass>());
 
    // ptr_to_base->f(); // Error won't compile: BaseClass has no member named 'f'
 
    std::static_pointer_cast<DerivedClass>(ptr_to_base)->f(); // OK
    // (constructs a temporary shared_ptr, then calls operator->)
 
    static_cast<DerivedClass*>(ptr_to_base.get())->f(); // also OK
    // (direct cast, does not construct a temporary shared_ptr)
}

Output: