Provide API for "placement new" construction of meta-types

By making it possible to specify the place in memory where a
type should be constructed, any meta-type can be allocated on
the stack, for example. In the QML/JS QObject binding, this
makes it possible to call slots and access properties from
JavaScript without having to perform any mallocs (e.g. due to
QVariant creation) in the C++ <--> JS value conversion, in
the best case.

In addition to QMetaType::construct() and QMetaType::destruct(),
this change introduces QMetaType::typeSize(), which returns the
size of a type in bytes. This can be used to prepare a suitable
buffer for constructing a type using construct().

Benchmarks indicate that in-place construction is 2-5x faster
than normal construction for core and GUI types on linux-g++.

Note that there is already a QMetaType::construct() function
in Qt 4, which has been renamed to QMetaType::create() in Qt 5.
In order to avoid existing usages of construct() in user code
to call the Qt 5 construct() (when they really meant to call
create()), the third argument ("copy") of construct() is made
mandatory. Hence, calls to QMetaType::construct() written for
Qt 4 will cause a compile error when compiled with Qt 5, and
the user must adapt his code.

Task-number: QTBUG-12574
Change-Id: I836f06f6ee1c1c3edbd199a03424c78c942bdd3e
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>

1 files changed, 21 insertions, 3 deletions

diff --git a/src/widgets/kernel/qwidgetsvariant.cpp b/src/widgets/kernel/qwidgetsvariant.cpp
index a349e161e3..1a21d91829 100644
--- a/src/widgets/kernel/qwidgetsvariant.cpp
+++ b/src/widgets/kernel/qwidgetsvariant.cpp
@@ -141,6 +141,9 @@ struct QMetaTypeGuiHelper
     QMetaType::SaveOperator saveOp;
     QMetaType::LoadOperator loadOp;
 #endif
+    QMetaType::Constructor constructor;
+    QMetaType::Destructor destructor;
+    int size;
 };
 
 extern Q_CORE_EXPORT const QMetaTypeGuiHelper *qMetaTypeWidgetsHelper;
@@ -151,13 +154,21 @@ extern Q_CORE_EXPORT const QMetaTypeGuiHelper *qMetaTypeWidgetsHelper;
      typedef void *(*QCreate##TYPE)(const TYPE *); \
      static const QCreate##TYPE qCreate##TYPE = qMetaTypeCreateHelper<TYPE>; \
      typedef void (*QDelete##TYPE)(TYPE *); \
-     static const QDelete##TYPE qDelete##TYPE = qMetaTypeDeleteHelper<TYPE>;
+     static const QDelete##TYPE qDelete##TYPE = qMetaTypeDeleteHelper<TYPE>; \
+     typedef void *(*QConstruct##TYPE)(void *, const TYPE *); \
+     static const QConstruct##TYPE qConstruct##TYPE = qMetaTypeConstructHelper<TYPE>; \
+     typedef void (*QDestruct##TYPE)(TYPE *); \
+     static const QDestruct##TYPE qDestruct##TYPE = qMetaTypeDestructHelper<TYPE>;
 #else
 #  define Q_DECL_METATYPE_HELPER(TYPE) \
      typedef void *(*QCreate##TYPE)(const TYPE *); \
      static const QCreate##TYPE qCreate##TYPE = qMetaTypeCreateHelper<TYPE>; \
      typedef void (*QDelete##TYPE)(TYPE *); \
      static const QDelete##TYPE qDelete##TYPE = qMetaTypeDeleteHelper<TYPE>; \
+     typedef void *(*QConstruct##TYPE)(void *, const TYPE *); \
+     static const QConstruct##TYPE qConstruct##TYPE = qMetaTypeConstructHelper<TYPE>; \
+     typedef void (*QDestruct##TYPE)(TYPE *); \
+     static const QDestruct##TYPE qDestruct##TYPE = qMetaTypeDestructHelper<TYPE>; \
      typedef void (*QSave##TYPE)(QDataStream &, const TYPE *); \
      static const QSave##TYPE qSave##TYPE = qMetaTypeSaveHelper<TYPE>; \
      typedef void (*QLoad##TYPE)(QDataStream &, TYPE *); \
@@ -172,13 +183,20 @@ Q_DECL_METATYPE_HELPER(QSizePolicy)
 #ifdef QT_NO_DATASTREAM
 #  define Q_IMPL_METATYPE_HELPER(TYPE) \
      { reinterpret_cast<QMetaType::Creator>(qCreate##TYPE), \
-       reinterpret_cast<QMetaType::Deleter>(qDelete##TYPE) }
+       reinterpret_cast<QMetaType::Deleter>(qDelete##TYPE), \
+       reinterpret_cast<QMetaType::Constructor>(qConstruct##TYPE), \
+       reinterpret_cast<QMetaType::Destructor>(qDestruct##TYPE), \
+       sizeof(TYPE) \
+     }
 #else
 #  define Q_IMPL_METATYPE_HELPER(TYPE) \
      { reinterpret_cast<QMetaType::Creator>(qCreate##TYPE), \
        reinterpret_cast<QMetaType::Deleter>(qDelete##TYPE), \
        reinterpret_cast<QMetaType::SaveOperator>(qSave##TYPE), \
-       reinterpret_cast<QMetaType::LoadOperator>(qLoad##TYPE) \
+       reinterpret_cast<QMetaType::LoadOperator>(qLoad##TYPE), \
+       reinterpret_cast<QMetaType::Constructor>(qConstruct##TYPE), \
+       reinterpret_cast<QMetaType::Destructor>(qDestruct##TYPE), \
+       sizeof(TYPE) \
      }
 #endif