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std::addressof

Defined in header `<memory>`
template< class T > T* addressof(T& arg);		(since C++11) (until C++17)
template< class T > constexpr T* addressof(T& arg);		(since C++17)

Obtains the actual address of the object or function arg, even in presence of overloaded operator&

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.

Exceptions

noexcept specification:

noexcept

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:

#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