std::static_pointer_cast

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.

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 eventually 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";
    }
};
  
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!

std::static_pointer_cast

Parameters

Exceptions

Notes

Possible implementation

Example

See also