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

From cppreference.com
< cpp‎ | memory‎ | shared ptr
 
 
 
Dynamic memory management
Uninitialized storage
(C++17)
(deprecated since c++17)
(deprecated since c++17)
(deprecated since c++17)
Garbage collection support
Miscellaneous
(C++11)
(C++11)
C Library
Low level memory management
 
 
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.

Contents

[edit] Parameters

r - The pointer to convert

[edit] Exceptions

noexcept specification:  
noexcept
  

[edit] 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!

[edit] 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);
}

[edit] Example

#include <iostream>
#include <memory>
 
struct BaseClass {};
 
struct DerivedClass : BaseClass
{
    void f() const
    {
        std::cout << "Hello World!\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:

Hello World!
Hello World!

[edit] See also

constructs new shared_ptr
(public member function) [edit]