std::static_pointer_cast, std::dynamic_pointer_cast, std::const_pointer_cast, std::reinterpret_pointer_cast - cppreference.com (original) (raw)

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 the initial value of 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 evaluating, respectively:

1,2) static_cast<Y*>(r.get())

3,4) dynamic_cast<Y*>(r.get()). If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty.

5,6) const_cast<Y*>(r.get())

7,8) reinterpret_cast<Y*>(r.get())

The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed:

1,2) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed.

3,4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed.

5,6) The behavior is undefined unless const_cast<T*>((U*)nullptr) is well formed.

7,8) The behavior is undefined unless reinterpret_cast<T*>((U*)nullptr) is well formed.

[edit] Parameters

[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 likely result in undefined behavior, attempting to delete the same object twice!

[edit] Possible implementation

[edit] Example

#include #include   class Base { public: int a; virtual void f() const { std::cout << "I am base!\n"; } virtual ~Base() {} };   class Derived : public Base { public: void f() const override { std::cout << "I am derived!\n"; } ~Derived() {} };   int main() { auto basePtr = std::make_shared(); std::cout << "Base pointer says: "; basePtr->f();   auto derivedPtr = std::make_shared(); std::cout << "Derived pointer says: "; derivedPtr->f();   // static_pointer_cast to go up class hierarchy basePtr = std::static_pointer_cast(derivedPtr); std::cout << "Base pointer to derived says: "; basePtr->f();   // dynamic_pointer_cast to go down/across class hierarchy auto downcastedPtr = std::dynamic_pointer_cast(basePtr); if (downcastedPtr) { std::cout << "Downcasted pointer says: "; downcastedPtr->f(); }   // All pointers to derived share ownership std::cout << "Pointers to underlying derived: " << derivedPtr.use_count() << '\n'; }

Output:

Base pointer says: I am base! Derived pointer says: I am derived! Base pointer to derived says: I am derived! Downcasted pointer says: I am derived! Pointers to underlying derived: 3

[edit] See also