QArrayDataPointer: add a C++20 ranges-style optional projection to assign()

This will be useful for implementing QString::assign(), which
otherwise has the problem that it's d_ptr is based on char16_t, but
it's assign() is supposed to be able to deal with iterators whose
value_type returns QChar.

Task-number: QTBUG-106198
Change-Id: I87882bf749b4e21b7b32391167962d3e6bae9983
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/corelib/tools/qarraydatapointer.h b/src/corelib/tools/qarraydatapointer.h
index b458dc3626..3839baefcf 100644
--- a/src/corelib/tools/qarraydatapointer.h
+++ b/src/corelib/tools/qarraydatapointer.h
@@ -7,6 +7,8 @@
 #include <QtCore/qarraydataops.h>
 #include <QtCore/qcontainertools_impl.h>
 
+#include <QtCore/q20functional.h>
+
 QT_BEGIN_NAMESPACE
 
 template <class T>
@@ -305,8 +307,8 @@ public:
         this->ptr = res;
     }
 
-    template <typename InputIterator>
-    void assign(InputIterator first, InputIterator last)
+    template <typename InputIterator, typename Projection = q20::identity>
+    void assign(InputIterator first, InputIterator last, Projection proj = {})
     {
         // This function only provides the basic exception guarantee.
         constexpr bool IsFwdIt = std::is_convertible_v<
@@ -340,12 +342,12 @@ public:
                     break;
                 } else {
                     do {
-                        (*this)->emplace(size, *first);
+                        (*this)->emplace(size, proj(*first));
                     } while (++first != last);
                     return;         // size() is already correct (and dst invalidated)!
                 }
             }
-            *dst = *first;          // overwrite existing element
+            *dst = proj(*first);    // overwrite existing element
             ++dst;
             ++first;
         }