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
