Containers: add max_size()

One more method for STL compatibility.
This one is particularly subtle as it's required by the
`reservable-container` concept:

https://eel.is/c++draft/ranges#range.utility.conv.general-3

Without this concept, ranges::to won't reserve() before copying the
elements (out of a sized range which isn't a common_range).

Implementation notes: there were already a couple of constants denoting
the maximum QByteArray and QString size. Centralize that implementation
in QTypedArrayData, so that QList can use it too.

The maximum allocation size (private constant) needs a even more central
place so that even QVLA can use it. Lacking anything better, I've put it
in qcontainerfwd.h.

Since our containers aren't allocator-aware, I can make max_size() a
static member, and replace the existing constants throughout the rest of
qtbase. (I can't kill them yet as they're used by other submodules.)

[ChangeLog][QtCore][QList] Added max_size().

[ChangeLog][QtCore][QString] Added max_size().

[ChangeLog][QtCore][QByteArray] Added max_size().

[ChangeLog][QtCore][QVarLengthArray] Added max_size().

Change-Id: I176142e31b998f4f787c96333894b8f6653eb70d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/corelib/tools/qlist.qdoc b/src/corelib/tools/qlist.qdoc
index bf0f52eb6b..e07b74cd53 100644
--- a/src/corelib/tools/qlist.qdoc
+++ b/src/corelib/tools/qlist.qdoc
@@ -1329,6 +1329,15 @@
     returns \c false.
 */
 
+/*! \fn template <typename T> qsizetype QList<T>::max_size()
+    \since 6.8
+
+    This function is provided for STL compatibility.
+    It returns the maximum number of elements that the list can
+    theoretically hold. In practice, the number can be much smaller,
+    limited by the amount of memory available to the system.
+*/
+
 /*! \fn template <typename T> QList<T> &QList<T>::operator+=(const QList<T> &other)
 
     Appends the items of the \a other list to this list and