std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast, (3) - Linux Manuals

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

NAME

std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast, - std::static_pointer_cast,std::dynamic_pointer_cast,std::const_pointer_cast,

Synopsis

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

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) (since C++20)
if the dynamic_cast fails.

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