std::addressof (3) - Linux Manuals

std::addressof: std::addressof

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 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