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

See also¶

constructor constructs new shared_ptr
(public member function)