<xstd/misc/type_traits/no_unique_address.hpp>

XSTD_NO_UNIQUE_ADDRESS

#define XSTD_NO_UNIQUE_ADDRESS see below

A portable spelling of the attribute name that gives [[no_unique_address]]'s semantics. It expands to msvc::no_unique_address on MSVC, whose standard spelling is layout-neutral, and to no_unique_address elsewhere.

Notes

An empty member declared with it needs no storage of its own.

Example

Adapted from the example of [[no_unique_address]] on cppreference.com, at revision 174656, by cppreference.com contributors. This example is under CC-BY-SA 3.0 and the GFDL, as cppreference.com's content is; the rest of this page is under the Boost Software License.

#include <xstd/misc/type_traits/empty_member_type.hpp> // empty_member_type
#include <xstd/misc/type_traits/no_unique_address.hpp> // XSTD_NO_UNIQUE_ADDRESS
#include <iostream>                                    // boolalpha, cout

struct Empty
{}; // empty class

struct X
{
        int i;
        Empty e;
};

struct Y
{
        [[XSTD_NO_UNIQUE_ADDRESS]] Empty e;
        int i;
};

struct Z
{
        char c;
        [[XSTD_NO_UNIQUE_ADDRESS]] Empty e1, e2;
};

struct W
{
        [[XSTD_NO_UNIQUE_ADDRESS]] xstd::empty_member_type<struct first> e1;
        [[XSTD_NO_UNIQUE_ADDRESS]] xstd::empty_member_type<struct second> e2;
        char c;
};

auto main() -> int
{
        // the size of any object of empty class type is at least 1
        static_assert(sizeof(Empty) >= 1);

        // at least one more byte is needed to give e a unique address
        static_assert(sizeof(X) >= sizeof(int) + 1);

        // empty member optimized out
        std::cout << "sizeof(Y) == sizeof(int) is " << std::boolalpha << (sizeof(Y) == sizeof(int)) << '\n';

        // e1 and e2 cannot share an address, having the same type, but either may share one with c
        static_assert(sizeof(Z) >= 2);

        // e1 and e2 have different types, so both can share an address with c
        std::cout << "sizeof(W) == 1 is " << (sizeof(W) == 1) << '\n';
}

Output:

sizeof(Y) == sizeof(int) is true
sizeof(W) == 1 is true

See also

Entity	Relation
empty_member_type	the empty type such a member usually has
[[no_unique_address]]	the standard attribute it spells