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std::addressof

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

Contents

[edit] Parameters

arg - lvalue object or function

[edit] Return value

Pointer to arg.

[edit] Possible implementation

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;
}

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

[edit] 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

[edit] See also

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