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

From cppreference.com

Defined in header <memory> 

template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(1) (since C++11)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(2) (since C++20)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T>
    dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(3) (since C++11)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(4) (since C++20)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(5) (since C++11)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(6) (since C++20)
(constexpr since C++26)		 
template< class T, class U >
std::shared_ptr<T>
    reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
(7) (since C++17) 
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
(8) (since C++20)

Creates a new std::shared_ptr object 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.

3,4) Let expr be the expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()):

  • If expr is ill-formed, the program is ill-formed.
  • If expr does not have well-formed behavior, the behavior is undefined.

If the following expression is ill-formed, the program is ill-formed:

1,2) static_cast<T*>((U*)nullptr)

3,4) dynamic_cast<T*>((U*)nullptr)

5,6) const_cast<T*>((U*)nullptr)

7,8) reinterpret_cast<T*>((U*)nullptr)

Parameters

r - the pointer to convert

Return value

Let Y be typename std::shared_ptr<T>::element_type:

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

2) shared_ptr<T>(std::move(r), static_cast<Y*>(r.get()))

3,4) Let p be dynamic_cast<Y*>(r.get()):

3) p ? shared_ptr<T>(r, p) : shared_ptr<T>()

4) p ? shared_ptr<T>(std::move(r), p) : shared_ptr<T>()

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

6) shared_ptr<T>(std::move(r), const_cast<Y*>(r.get()))

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

8) shared_ptr<T>(std::move(r), reinterpret_cast<Y*>(r.get()))

Notes

The expression std::shared_ptr<T>(/* ***_cast */<T*>(r.get())) might seem to have the same effect, but it will likely result in undefined behavior, attempting to delete the same object twice.

After calling the rvalue overloads (2,4,6,8), r is empty and r.get() == nullptr, except that r is not modified for dynamic_pointer_cast (4) if the dynamic_cast fails.

(since C++20)

Example

#include <iostream>
#include <memory>
 
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<Base>();
    std::cout << "Base pointer says: ";
    basePtr->f();
    
    auto derivedPtr = std::make_shared<Derived>();
    std::cout << "Derived pointer says: ";
    derivedPtr->f();
    
    // static_pointer_cast to go up class hierarchy
    basePtr = std::static_pointer_cast<Base>(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<Derived>(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

See also