<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">As software using c++17 is upon us, we have a slight problem with std::optional.<div class=""><br class=""></div><div class="">clang has supported std::optional for some time, and clang-9.0, at least, appears to support std::optional right back to the ancient systems.</div><div class=""><br class=""></div><div class="">however, there is one small part of std::optional, std::optional::value, that needs a callback function in case of an exception being thrown, called “bad_optional_access”. That symbol is compiled into libc++.dylib as of 10.13.</div><div class=""><br class=""></div><div class="">So clang-9.0 supports 99% of std::optional all the way back, but that bad_optional_access function limits that one part of it, std::optional::value, to 10.13. </div><div class=""><br class=""></div><div class="">There are availability tests build into the libc++ headers based on system version to turn that off.</div><div class=""><br class=""></div><div class="">However, on 10.6, where I build and install libc++ 5.0, that symbol exists in libc++.dylib, so I disable clang’s availability tests.</div><div class=""><br class=""></div><div class="">It is quite simple (I think) to inline a function implementing bad_optional_access into the <optional> header. That could be used for 10.7 to 10.12, and then all those systems could comfortably implement c++17, and no problem. Otherwise we have to install a newer libc++.dylib on older systems, which is a simple but big problem, as it were.</div><div class=""><br class=""></div><div class="">Here’s the <optional> patch. I only speak basic c++, so improve this if you can.</div><div class=""><br class=""></div><div class="">So I am proposing to use this patch, and disable Apple’s availability test in <__config> for bad_optional_access.</div><div class=""><br class=""></div><div class="">Then all systems from 10.5 to current can use c++17 std::optional, and disaster averted until the next crisis.</div><div class=""><br class=""></div><div class="">Warts?</div><div class=""><br class=""></div><div class="">Ken</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><optional> patch:</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; font-size: 11.899999618530273px; margin-top: 0px; margin-bottom: 16px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-top-left-radius: 6px; border-top-right-radius: 6px; border-bottom-right-radius: 6px; border-bottom-left-radius: 6px; caret-color: rgb(36, 41, 46); color: rgb(36, 41, 46);" class=""><code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; padding: 0px; margin: 0px; border-top-left-radius: 6px; border-top-right-radius: 6px; border-bottom-right-radius: 6px; border-bottom-left-radius: 6px; word-break: normal; border: 0px; display: inline; overflow: visible; line-height: inherit; word-wrap: normal;" class="">$ diff -u optional.orig optional
--- optional.orig 2020-07-04 18:51:05.000000000 -0700
+++ optional 2020-07-04 18:51:13.000000000 -0700
@@ -168,13 +168,10 @@
namespace std // purposefully not using versioning namespace
{
-class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
- : public exception
-{
+// 20.5.8, class bad_optional_access
+class bad_optional_access : public logic_error {
public:
- // Get the key function ~bad_optional_access() into the dylib
- virtual ~bad_optional_access() _NOEXCEPT;
- virtual const char* what() const _NOEXCEPT;
+ explicit bad_optional_access() : logic_error{"bad optional access"} {}
};
} // std</code></pre><div class=""><br class=""></div></div></body></html>