NAME¶

std::addressof - std::addressof

Synopsis¶

Defined in header <memory>
template< class T > (since C++11)
T* addressof( T& arg ) noexcept; (until C++17)
template< class T > (1) (since C++17)
constexpr T* addressof( T& arg ) noexcept;
template <class T> (2) (since C++17)
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 (which requires compiler support).

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: __cpp_lib_addressof_constexpr

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 << std::endl;
}

~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

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>)