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std::to_address

Defined in header `<memory>`
template< class Ptr > constexpr auto to_address(const Ptr& p) noexcept;	(1)	(since C++20)
template< class T > constexpr T* to_address(T* p) noexcept;	(2)	(since C++20)

Obtain the address represented by p without forming a reference to the object pointed to by p.

1) Fancy pointer overload: If the expression std::pointer_traits<Ptr>::to_address(p) is well-formed, returns the result of that expression. Otherwise, returns std::to_address(p.operator->()).

2) Raw pointer overload: If T is a function type, the program is ill-formed. Otherwise, returns p unmodified.

Parameters

fancy or raw pointer

Return value

Raw pointer that represents the same address as p does.

Possible implementation

template<class T>
constexpr T* to_address(T* p) noexcept
{
    static_assert(!std::is_function_v<T>);
    return p;
}
 
template<class T>
constexpr auto to_address(const T& p) noexcept
{
    if constexpr (requires{ std::pointer_traits<T>::to_address(p); }) {
        return std::pointer_traits<T>::to_address(p);
    } else {
        return std::to_address(p.operator->());
    }
}

Notes

std::to_address can be used even when p does not reference storage that has an object constructed in it, in which case std::addressof(*p) cannot be used because there's no valid object for the parameter of std::addressof to bind to.

The fancy pointer overload of to_address inspects the std::pointer_traits<Ptr> specialization. If instantiating that specialization is itself ill-formed (typically because element_type cannot be defined), that results in a hard error outside the immediate context and renders the program ill-formed.

Feature-test macro
`__cpp_lib_to_address`

Example

#include <memory>
 
template<class A>
auto allocator_new(A& a)
{
    auto p = a.allocate(1);
    try {
        std::allocator_traits<A>::construct(a, std::to_address(p));
    } catch (...) {
        a.deallocate(p, 1);
        throw;
    }
    return p;
}
 
template<class A>
void allocator_delete(A& a, typename std::allocator_traits<A>::pointer p)
{
    std::allocator_traits<A>::destroy(a, std::to_address(p));
    a.deallocate(p, 1);
}
 
int main()
{
    std::allocator<int> a;
    auto p = allocator_new(a);
    allocator_delete(a, p);
}