Introduce platform API abstraction for QOpenGLContext

The API is available by including qopenglcontext.h as usual,
but scoped in the QPlatformInterface namespace. The namespace
exposes platform specific type-safe interfaces that provide:

 a) Factory functions for adopting native contexts, e.g.

    QCocoaGLContext::fromNative(nsContext, shareContext);

 b) Access to underlying native handles, e.g.

    openGLContext->platformInterface<QCocoaGLContext>->nativeContext()

 c) Platform specific functionality, e.g.

    static QWGLContext::openGLModuleHandle()

    openGLContext->platformInterface<QEGLContext>->doSomething();

The platform interfaces live close to the classes they extend,
removing the need for complex indirection and plumbing, and
avoids kitchen-sink modules and APIs such as the extras modules,
QPlatformFunctions, or QPlatformNativeInterface.

In the case of QOpenGLContext these platform APIs are backed
by the platform plugin, so dynamic_cast is used to ensure the
platform plugin supports the requested interface, but this is
and implementation detail. The interface APIs are agnostic
to where the implementation lives, while still being available
to the user as part of the APIs they extend/augment.

The documentation will be restored when the dust settles.

Task-number: QTBUG-80233
Change-Id: Iac612403383991c4b24064332542a6e4bcbb3293
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 26 insertions, 0 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 08e631568d..ff36c0f8e5 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -1292,6 +1292,32 @@ template <bool B, typename T = void> struct QEnableIf;
 template <typename T> struct QEnableIf<true, T> { typedef T Type; };
 }
 
+#define QT_VA_ARGS_CHOOSE(_1, _2, _3, _4, _5, _6, _7, _8, _9, N, ...) N
+#define QT_VA_ARGS_EXPAND(...) __VA_ARGS__ // Needed for MSVC
+#define QT_VA_ARGS_COUNT(...) QT_VA_ARGS_EXPAND(QT_VA_ARGS_CHOOSE(__VA_ARGS__, 9, 8, 7, 6, 5, 4, 3, 2, 1))
+#define QT_OVERLOADED_MACRO_EXPAND(MACRO, ARGC) MACRO##ARGC
+#define QT_OVERLOADED_MACRO_IMP(MACRO, ARGC) QT_OVERLOADED_MACRO_EXPAND(MACRO, ARGC)
+#define QT_OVERLOADED_MACRO(MACRO, ...) QT_VA_ARGS_EXPAND(QT_OVERLOADED_MACRO_IMP(MACRO, QT_VA_ARGS_COUNT(__VA_ARGS__))(__VA_ARGS__))
+
+// Ensures that the interface's typeinfo is exported so that
+// dynamic casts work reliably, and protects the destructor
+// so that pointers to the interface can't be deleted.
+#define QT_DECLARE_PLATFORM_INTERFACE(InterfaceClass) \
+    protected: virtual ~InterfaceClass(); public:
+
+// Provides a definition for the interface destructor
+#define QT_DEFINE_PLATFORM_INTERFACE2(Namespace, InterfaceClass) \
+    QT_PREPEND_NAMESPACE(Namespace)::InterfaceClass::~InterfaceClass() = default
+
+// Provides a definition for the destructor, and an explicit
+// template instantiation of the platform interface accessor.
+#define QT_DEFINE_PLATFORM_INTERFACE3(Namespace, InterfaceClass, PublicClass) \
+    QT_DEFINE_PLATFORM_INTERFACE2(Namespace, InterfaceClass); \
+    template Q_DECL_EXPORT QT_PREPEND_NAMESPACE(Namespace)::InterfaceClass *PublicClass::platformInterface() const
+
+#define QT_DEFINE_PLATFORM_INTERFACE(...) QT_OVERLOADED_MACRO(QT_DEFINE_PLATFORM_INTERFACE, QPlatformInterface, __VA_ARGS__)
+#define QT_DEFINE_PRIVATE_PLATFORM_INTERFACE(...) QT_OVERLOADED_MACRO(QT_DEFINE_PLATFORM_INTERFACE, QPlatformInterface::Private, __VA_ARGS__)
+
 QT_END_NAMESPACE
 
 // We need to keep QTypeInfo, QSysInfo, QFlags, qDebug & family in qglobal.h for compatibility with Qt 4.