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
| r | - | The pointer to convert |
Exceptions
noexcept specification: noexceptNotes
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!
See also
constructs new shared_ptr (public member function) |
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