Std::addressof (3) Linux Manual Page

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

Parameters

arg – lvalue object or function

Return value

Pointer to arg.

Possible implementation

  template< class T >
  T* addressof(T& arg)
  {
      return reinterpret_cast<T*>(
                 &const_cast<char&>(
                    reinterpret_cast<const volatile char&>(arg)));
  }

Note: the above implementation is_oversimplified and is not constexpr (which requires compiler support).

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

std::addressof (3) Linux Manual Page