Use [[no_unique_address]] when supported for data member of view adaptor?

[patrickroberts]

Maybe for compatibility it can be a conditionally defined preprocessor macro such as BOOST_STL_INTERFACES_NO_UNIQUE_ADDRESS?

stl_interfaces/include/boost/stl_interfaces/view_adaptor.hpp

Line 312 in 510eb43

F f_;

So something like:

// <boost/stl_interfaces/config.hpp>

#if defined(__has_cpp_attribute)
#    if __has_cpp_attribute(no_unique_address)
#        define BOOST_STL_INTERFACES_NO_UNIQUE_ADDRESS [[no_unique_address]]
#    endif
#endif

#if !defined(BOOST_STL_INTERFACES_NO_UNIQUE_ADDRESS)
#    define BOOST_STL_INTERFACES_NO_UNIQUE_ADDRESS
#endif

and then change the line to

BOOST_STL_INTERFACES_NO_UNIQUE_ADDRESS F f_;

The advantage of this is that if std::is_empty_v<F> then std::is_empty_v<boost::stl_interfaces::adaptor<F>> will also be true, which allows it in turn to be used as a potentially empty data member in another class and not occupy additional address space in the layout.

[patrickroberts]

If there is a desire to still get EBO when [[no_unique_address]] is not supported, then adaptor<F> can inherit from F when std::is_empty_v<F> and also !std::is_final_v<F>, otherwise EBO isn't possible. But that would require a partial class template specialization, which would be a bit more involved change than the suggestion above.