std::addressof

 

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)

 

Defined in header <memory>
template<class T > T* addressof( T& arg )noexcept;	(1)	(since C++11) (constexpr since C++17)
template<class T > const T* addressof(const T&&)= delete;	(2)	(since C++11)

1) Obtains the actual address of the object or function arg, even in presence of overloaded operator&.

2) Rvalue overload is deleted to prevent taking the address of const rvalues.

The expression std::addressof(e) is a constant subexpression, if e is an lvalue constant subexpression.

(since C++17)

Parameters

arg

-

lvalue object or function

Return value

Pointer to arg.

Possible implementation

The implementation below is not constexpr, because reinterpret_cast is not usable in a constant expression. Compiler support is needed (see below).

template<class T>typenamestd::enable_if<std::is_object<T>::value, T*>::type addressof(T& arg)noexcept{returnreinterpret_cast<T*>(&const_cast<char&>(reinterpret_cast<constvolatilechar&>(arg)));}   template<class T>typenamestd::enable_if<!std::is_object<T>::value, T*>::type addressof(T& arg)noexcept{return&arg;}

Correct implementation of this function requires compiler support: GNU libstdc++, LLVM libc++, Microsoft STL.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_addressof_constexpr	201603L	(C++17)	constexprstd::addressof

constexpr for addressof is added by LWG2296, and MSVC STL applies the change to C++14 mode as a defect report.

There are some weird cases where use of built-in operator& is ill-formed due to argument-dependent lookup even if it is not overloaded, and std::addressof can be used instead.

template<class T>struct holder { T t;};   struct incomp;   int main(){ holder<holder<incomp>*> x{};// &x; // error: argument-dependent lookup attempts to instantiate holder<incomp> std::addressof(x);// OK}

Example

operator& may be overloaded for a pointer wrapper class to obtain a pointer to pointer:

Run this code

#include <iostream>#include <memory>   template<class T>struct Ptr { T* pad;// add pad to show difference between 'this' and 'data' T* data; Ptr(T* arg): pad(nullptr), data(arg){std::cout<<"Ctor this = "<< this <<'\n';}   ~Ptr(){ delete data;} T** operator&(){return&data;}};   template<class T>void f(Ptr<T>* p){std::cout<<"Ptr overload called with p = "<< p <<'\n';}   void f(int** p){std::cout<<"int** overload called with p = "<< p <<'\n';}   int main(){ Ptr<int> p(new int(42)); f(&p);// calls int** overload f(std::addressof(p));// calls Ptr<int>* overload, (= this)}

Possible output:

Ctor this = 0x7fff59ae6e88 int** overload called with p = 0x7fff59ae6e90 Ptr overload called with p = 0x7fff59ae6e88

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2598	C++11	std::addressof<const T> could take address of rvalues	disallowed by a deleted overload

See also

allocator	the default allocator (class template)
pointer_to [static]	obtains a dereferenceable pointer to its argument (public static member function of std::pointer_traits<Ptr>)