# std::to_address

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class Ptr > 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 pointee.

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

p | - | fancy or raw pointer

### Return value

Raw pointer that represents the same address as `p` does.

### Possible implementation

    template<class>
    struct use_address : std::false_type { };

    template<class T>
        requires requires(const T& p) { std::pointer_traits<T>::to_address(p); }
    struct use_address<T> : std::true_type { };

    template<class T>
    constexpr T* to_address(T* p) noexcept
    {
        static_assert(!std::is_function_v<T>);
        return p;
    }

    template<class T>
    auto to_address(const T& p) noexcept
    {
        if constexpr (use_address<T>::value) {
            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.

### Example

Run this code

    #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);
    }

### See also

pointer_traits (C++11) | provides information about pointer-like types (class template)
to_address [static] (C++20) | obtains a raw pointer from a fancy pointer (inverse of pointer_to) (public static member function of `std::pointer_traits<Ptr>`)

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