std::static_pointer_cast, std::dynamic_pointer_cast, std::const_pointer_cast, std::reinterpret_pointer_cast
Defined in header <memory>
.
Declarations
- C++20
- C++17
- C++11
// 1)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 2)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
// 3)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 4)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
// 5)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 6)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
// 7)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 8)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
// 1)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 3)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 5)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 7)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 1)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 3)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
// 5)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& 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) if the dynamic_cast fails (Since C++20).
Parameters
r
- 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
static_pointer_cast
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};
}
dynamic_pointer_cast
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>{};
}
const_pointer_cast
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};
}
reinterpret_pointer_cast
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
#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';
}
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