C++ attribute: no_unique_address (since C++20)

Since C++20

Allows this data member to be overlapped with other non-static data members or base class subobjects of its class.

# Notes

[[no_unique_address]] is ignored by MSVC even in C++20 mode; instead, [[msvc::no_unique_address]] is provided.

# Example

#include <iostream>
 
struct Empty {}; // empty class
 
struct X
{
    int i;
    Empty e;
};
 
struct Y
{
    int i;
    [[no_unique_address]] Empty e;
};
 
struct Z
{
    char c;
    [[no_unique_address]] Empty e1, e2;
};
 
struct W
{
    char c[2];
    [[no_unique_address]] Empty e1, e2;
};
 
int main()
{
    // 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 the same address because they have the
    // same type, even though they are marked with [[no_unique_address]]. 
    // However, either may share address with c.
    static_assert(sizeof(Z) >= 2);
 
    // e1 and e2 cannot have the same address, but one of them can share with
    // c[0] and the other with c[1]
    std::cout << "sizeof(W) == 2 is " << (sizeof(W) == 2) << '\n';
}