std::addressof

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>
typename std::enable_if<std::is_object<T>::value, T*>::type addressof(T& arg) noexcept
{
    return reinterpret_cast<T*>(
               &const_cast<char&>(
                   reinterpret_cast<const volatile char&>(arg)));
}
 
template<class T>
typename std::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)	constexpr `std::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

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External libraries

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>`)