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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Memory management library

voidify (exposition only*) Allocators allocator allocator_traits (C++11) allocation_result (C++23) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) construct_at (C++20) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) destroy_at (C++17) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::uninitialized_copy_n (C++20)  ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::construct_at (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::destroy (C++20) ranges::uninitialized_default_construct_n (C++20)  ranges::uninitialized_value_construct_n (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::pool_options (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17) pmr::monotonic_buffer_resource (C++17) Explicit lifetime management start_lifetime_as (C++23) start_lifetime_as_array (C++23) Types for composite class design indirect (C++26) polymorphic (C++26) Uninitialized storage(until C++20) raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Garbage collector support(until C++23) declare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) get_pointer_safety (C++11)(until C++23)

Low level memory management operator newoperator new[] operator deleteoperator delete[] nothrow_t nothrow new_handler set_new_handler get_new_handler (C++11) bad_alloc bad_array_new_length (C++11) align_val_t (C++17) destroying_delete_t (C++20) launder (C++17) Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) owner_hash (C++26) owner_equal (C++26) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) is_sufficiently_aligned (C++26) C Library malloc calloc realloc free aligned_alloc (C++17)

 

std::shared_ptr

Member functions

shared_ptr::shared_ptr

shared_ptr::~shared_ptr

shared_ptr::operator=

Modifiers

shared_ptr::reset

shared_ptr::swap

Observers

shared_ptr::get

shared_ptr::operator*shared_ptr::operator->

shared_ptr::operator[] (C++17)

shared_ptr::use_count

shared_ptr::unique (until C++20*)

shared_ptr::operator bool

shared_ptr::owner_before

shared_ptr::owner_hash (C++26)

shared_ptr::owner_equal (C++26)

Non-member functions

swap(std::shared_ptr)

make_sharedmake_shared_for_overwrite (C++20)

allocate_sharedallocate_shared_for_overwrite (C++20)

static_pointer_castdynamic_pointer_castconst_pointer_castreinterpret_pointer_cast (C++17)

get_deleter

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

operator<<

atomic_xxx functions (until C++26*)

Helper classes

atomic<std::shared_ptr> (C++20)

hash<std::shared_ptr>

Deduction guides(C++17)

 

Defined in header <memory>
template<class T, class U > std::shared_ptr<T> static_pointer_cast(conststd::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(conststd::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(conststd::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(conststd::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 typenamestd::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

static_pointer_cast
template<class T, class U>std::shared_ptr<T> static_pointer_cast(conststd::shared_ptr<U>& r)noexcept{auto p =static_cast<typenamestd::shared_ptr<T>::element_type*>(r.get());returnstd::shared_ptr<T>{r, p};}
dynamic_pointer_cast
template<class T, class U>std::shared_ptr<T> dynamic_pointer_cast(conststd::shared_ptr<U>& r)noexcept{if(auto p =dynamic_cast<typenamestd::shared_ptr<T>::element_type*>(r.get()))returnstd::shared_ptr<T>{r, p};elsereturnstd::shared_ptr<T>{};}
const_pointer_cast
template<class T, class U>std::shared_ptr<T> const_pointer_cast(conststd::shared_ptr<U>& r)noexcept{auto p =const_cast<typenamestd::shared_ptr<T>::element_type*>(r.get());returnstd::shared_ptr<T>{r, p};}
reinterpret_pointer_cast
template<class T, class U>std::shared_ptr<T> reinterpret_pointer_cast(conststd::shared_ptr<U>& r)noexcept{auto p =reinterpret_cast<typenamestd::shared_ptr<T>::element_type*>(r.get());returnstd::shared_ptr<T>{r, p};}

Example

Run this code

#include <iostream>#include <memory>   class Base {public:int a;virtualvoid 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 hierarchyauto downcastedPtr = std::dynamic_pointer_cast<Derived>(basePtr);if(downcastedPtr){std::cout<<"Downcasted pointer says: "; downcastedPtr->f();}   // All pointers to derived share ownershipstd::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)