[PATCH] libstdc++: Add Doxygen comment for string::resize_and_overwite

[Jonathan Wakely <jwakely@redhat.com>]

Reviews of the resize_and_overwite description welcome. I've tried to
strike a balance between pedantic precision and user-friendliness.

-- >8 --

This is a complicated API that should be clearly documented.

Also improve the comment on basic_ios::_M_setstate.

libstdc++-v3/ChangeLog:

	* include/bits/basic_ios.h (basic_ios::_M_setstate): Add
	caveat to comment.
	* include/bits/basic_string.h (resize_and_overwrite): Add
	doxygen comment.
---
 libstdc++-v3/include/bits/basic_ios.h    |  2 +-
 libstdc++-v3/include/bits/basic_string.h | 28 ++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/basic_ios.h b/libstdc++-v3/include/bits/basic_ios.h
index e0667b7d049..d0a4e7d3dfd 100644
--- a/libstdc++-v3/include/bits/basic_ios.h
+++ b/libstdc++-v3/include/bits/basic_ios.h
@@ -159,7 +159,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // Flip the internal state on for the proper state bits, then
       // rethrows the propagated exception if bit also set in
-      // exceptions().
+      // exceptions(). Must only be called within a catch handler.
       void
       _M_setstate(iostate __state)
       {
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index c81dc0d425a..1abac655fd1 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -1117,6 +1117,34 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 #if __cplusplus > 202002L
 #define __cpp_lib_string_resize_and_overwrite 202110L
+      /** Resize the string and call a function to fill it.
+       *
+       * @param __n   The maximum size requested.
+       * @param __op  A callable object that writes characters to the string.
+       *
+       * This is a low-level function that is easy to misuse, be careful.
+       *
+       * Calling `str.resize_and_overwrite(n, op)` will reserve at least `n`
+       * characters in `str`, evaluate `n2 = std::move(op)(str.data(), n)`,
+       * and finally set the string length to `n2` (adding a null terminator
+       * at the end). The function object `op` is allowed to write to the
+       * extra capacity added by the initial reserve operation, which is not
+       * allowed if you just call `str.reserve(n)` yourself.
+       *
+       * This can be used to efficiently fill a `string` buffer without the
+       * overhead of zero-initializing characters that will be overwritten
+       * anyway.
+       *
+       * The callable `op` not access the string directly (only through the
+       * pointer passed as its first argument), must not write more than `n`
+       * characters to the string; must return a value no greater than `n`;
+       * and must ensure that all characters up to the returned length are
+       * valid after it returns (i.e. there must be no uninitialized values
+       * left in the string after the call, because accessing them would
+       * have undefined behaviour).
+       *
+       * @since C++23
+       */
       template<typename _Operation>
 	constexpr void
 	resize_and_overwrite(size_type __n, _Operation __op);
-- 
2.39.2