NAME¶

std::addressof - std::addressof

Synopsis¶

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

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 (since C++17)
an lvalue constant subexpression.

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.

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 disallowed by a deleted
address of rvalues overload

See also¶

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