std::static_pointer_cast, std::dynamic_pointer_cast, std::const_pointer_cast, std::reinterpret_pointer_cast

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C Library
Low level memory management
 
std::shared_ptr
 
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)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
 (2) (since C++20)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
 (3) (since C++11)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
 (4) (since C++20)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
 (5) (since C++11)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
 (6) (since C++20)
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 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.

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)

Parameters

r - The pointer to convert

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!

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);
}
Fourth version 
template< class T, class U > 
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
    auto p = reinterpret_cast<typename std::shared_ptr<T>::element_type*>(r.get());
    return std::shared_ptr<T>(r, p);
}

Example

Run this code
#include <iostream>
#include <memory>
 
struct Base 
{ 
    int a; 
    virtual void f() const { std::cout << "I am base!\n";}
    virtual ~Base(){}
};
 
struct Derived : Base
{
    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

 constructs new shared_ptr
(public member function)
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