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

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< cpp‎ | memory‎ | shared ptr

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

r | - | 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

Run this code

    #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:

    Hello World!
    Hello World!
    ~DerivedClass

### See also

(constructor) | constructs new `shared_ptr` (public member function)

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