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.
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 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
constructs new shared_ptr (public member function) |
