std::static_pointer_cast, std::dynamic_pointer_cast, std::const_pointer_cast - cppreference.com

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

From cppreference.com
< cpp‎ | memory‎ | shared ptr

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)

Creates a new instance of std::shared_ptr whose managed object type is obtained from the `r`'s managed object type using a cast expression. Both smart pointers will share the ownership of the managed object.

The resulting std::shared_ptr's managed object will be obtained by calling (in respective order):

1) `static_cast<T*>(r.get())`.
2) `dynamic_cast<T*>(r.get())` (If the result of the `dynamic_cast` is a null pointer value, the returned `shared_ptr` will be empty).
3) `const_cast<T*>(r.get())`.

In any case, if the parameter `r` is an empty std::shared_ptr the result will be a new empty std::shared_ptr.

### [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 eventually 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

Run this code

    #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

(constructor) | constructs new `shared_ptr` (public member function) [edit]

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=cpp/memory/shared_ptr/pointer_cast&oldid=80880"
Category:

- unconditionally noexcept