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

 

C++

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Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) 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) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) 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::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

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) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler (C++11) set_new_handler launder (C++17) bad_alloc bad_array_new_length (C++11) nothrow_t align_val_t (C++17) destroying_delete_t (C++20) new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

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)

 

template< class T, class U > std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r );	(1)	(since C++11)
template< class T, class U > std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r );	(2)	(since C++11)
template< class T, class U > std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r );	(3)	(since C++11)
template< class T, class U > std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r );	(4)	(since C++17)

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 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 calling (in respective order):

1) static_cast<Y*>(r.get()).

2) dynamic_cast<Y*>(r.get()) (If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty).

3) const_cast<Y*>(r.get()).

4) reinterpret_cast<Y*>(r.get())

The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed:

1) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed.

2) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed.

3) The behavior is undefined unless const_cast<T*>((U*)nullptr) is well formed.

4) The behavior is undefined unless reinterpret_cast<T*>((U*)nullptr) is well formed.

Parameters

r

-

The pointer to convert

Exceptions

noexcept specification:  

noexcept

  

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); }

Example

Run this code

#include <iostream>
#include <memory>
 
struct BaseClass {};
 
struct DerivedClass : BaseClass
{
    void f() const
    {
        std::cout << "Hello World!\n";
    }
    ~DerivedClass(){ // note, it's not virtual
        std::cout << "~DerivedClass\n";
    }
};
 
int main()
{
    std::shared_ptr<BaseClass> ptr_to_base(std::make_shared<DerivedClass>());
 
    // ptr_to_base->f(); // Error won't compile: BaseClass has no member named 'f'
 
    std::static_pointer_cast<DerivedClass>(ptr_to_base)->f(); // OK
    // (constructs a temporary shared_ptr, then calls operator->)
 
    static_cast<DerivedClass*>(ptr_to_base.get())->f(); // also OK
    // (direct cast, does not construct a temporary shared_ptr)
}

Output:

Hello World!
Hello World!
~DerivedClass

See also

(constructor)

constructs new shared_ptr (public member function)