mirror of git://gcc.gnu.org/git/gcc.git
aaeac1568d
This patch implements the C++2a proposal to allow incomplete types in std::reference_wrapper, which was previously undefined. The change cannot be implemented for earlier standards, because prior to C++2a std::reference_wrapper has a weak result type, so must inspect the template argument to see if it defines a nested result_type member. That is deprecated (but still required) in C++17, and removed from C++2a. The removal of the base class from reference_wrapper is a potential ABI change, as it could alter the layout of a type which derives from reference_wrapper<T> and from an empty type with _Weak_result_type<T> as a base class. Previously the repeated _Weak_result_type<T> base class would have prevented the empty base-class optimization, but if reference_wrapper<T> no longer derives from it, the empty class could be placed at the same address as the reference_wrapper<T> base. In practice, the only types which derive from _Weak_result_type or from _Reference_wrapper_base_memfun or any of its base classes are non-empty types defined in libstdc++: std::reference_wrapper, std::function, and std::_Bind. As they are non-empty types, they are not eligible for EBO anyway. * include/bits/refwrap.h [__cplusplus > 201703L] (_Refwrap_base_arg1, _Refwrap_base_arg2, _Reference_wrapper_base) (_Reference_wrapper_base_memfun): Do not define for C++2a. (reference_wrapper): Do not derive from _Reference_wrapper_base_memfun for C++2a. (reference_wrapper::operator()): Add static assertion. * testsuite/20_util/reference_wrapper/incomplete.cc: New test. From-SVN: r267866 |
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README
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.