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, 7 insertions, 0 deletions

diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index 2900b0178e..da83fc1a21 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -8,6 +8,7 @@
 #include <QtCore/qpair.h>
 #include <QtCore/qatomic.h>
 #include <QtCore/qflags.h>
+#include <QtCore/qcontainerfwd.h>
 #include <string.h>
 
 QT_BEGIN_NAMESPACE
@@ -169,6 +170,12 @@ struct QTypedArrayData
             (quintptr(data) + sizeof(QArrayData) + alignment - 1) & ~(alignment - 1));
         return static_cast<T *>(start);
     }
+
+    constexpr static qsizetype max_size() noexcept
+    {
+        // -1 to deal with the pointer one-past-the-end
+        return (QtPrivate::MaxAllocSize - sizeof(QtPrivate::AlignedQArrayData) - 1) / sizeof(T);
+    }
 };
 
 namespace QtPrivate {