qnativeinterface.h

Go to the documentation of this file.

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#ifndef QNATIVEINTERFACE_H
#define QNATIVEINTERFACE_H
 
#include <QtCore/qglobal.h>
 
QT_BEGIN_NAMESPACE
 
// We declare a virtual non-inline function in the form
// of the destructor, making it the key function. This
// ensures that the typeinfo of the class is exported.
// By being protected, we also ensure that pointers to
// the interface can't be deleted.
#define QT_DECLARE_NATIVE_INTERFACE_3(NativeInterface, Revision, BaseType)
    protected:
        virtual ~NativeInterface();
 
        struct TypeInfo {
            using baseType = BaseType;
            static constexpr char const *name = QT_STRINGIFY(NativeInterface);
            static constexpr int revision = Revision;
        };
 
        template <typename, typename>
        friend struct QNativeInterface::Private::has_type_info;
 
        template <typename>
        friend bool constexpr QNativeInterface::Private::hasTypeInfo();
 
        template <typename>
        friend struct QNativeInterface::Private::TypeInfo;
    public:
        NativeInterface() = default;
        Q_DISABLE_COPY_MOVE(NativeInterface)
 
// Revisioned interfaces only make sense when exposed through a base
// type via QT_DECLARE_NATIVE_INTERFACE_ACCESSOR, as the revision
// checks happen at that level (and not for normal dynamic_casts).
#define QT_DECLARE_NATIVE_INTERFACE_2(NativeInterface, Revision)
    static_assert(false, "Must provide a base type when specifying revision");
 
#define QT_DECLARE_NATIVE_INTERFACE_1(NativeInterface)
    QT_DECLARE_NATIVE_INTERFACE_3(NativeInterface, 0, void)
 
#define QT_DECLARE_NATIVE_INTERFACE(...)
    QT_OVERLOADED_MACRO(QT_DECLARE_NATIVE_INTERFACE, __VA_ARGS__)
 
namespace QNativeInterface::Private {
 
    // Basic type-trait to verify that a given native interface has
    // all the required type information for us to evaluate it.
    template <typename NativeInterface, typename = void>
    struct has_type_info : std::false_type {};
 
    // The type-trait is friended by TypeInfo, so that we can
    // evaluate TypeInfo in the template arguments.
    template <typename NativeInterface>
    struct has_type_info<NativeInterface, std::void_t<
        typename NativeInterface::TypeInfo,
        typename NativeInterface::TypeInfo::baseType,
        decltype(&NativeInterface::TypeInfo::name),
        decltype(&NativeInterface::TypeInfo::revision)
        >> : std::true_type {};
 
    // We need to wrap the instantiation of has_type_info in a
    // function friended by TypeInfo, otherwise MSVC will not
    // let us evaluate TypeInfo in the template arguments.
    template <typename NativeInterface>
    bool constexpr hasTypeInfo()
    {
        return has_type_info<NativeInterface>::value;
    }
 
    template <typename NativeInterface>
    struct TypeInfo
    {
        // To ensure SFINAE works for hasTypeInfo we can't use it in a constexpr
        // variable that also includes an expression that relies on the type
        // info. This helper variable is okey, as it it self contained.
        static constexpr bool haveTypeInfo = hasTypeInfo<NativeInterface>();
 
        // We can then use the helper variable in a constexpr condition in a
        // function, which does not break SFINAE if haveTypeInfo is false.
        template <typename BaseType>
        static constexpr bool isCompatibleHelper()
        {
            if constexpr (haveTypeInfo)
                return std::is_base_of<typename NativeInterface::TypeInfo::baseType, BaseType>::value;
            else
                return false;
        }
 
        // MSVC however doesn't like constexpr functions in enable_if_t conditions,
        // so we need to wrap it yet again in a constexpr variable. This is fine,
        // as all the SFINAE magic has been resolved at this point.
        template <typename BaseType>
        static constexpr bool isCompatibleWith = isCompatibleHelper<BaseType>();
 
        // The revision and name accessors are not used in enable_if_t conditions,
        // so we can leave them as constexpr functions. As this class template is
        // friended by TypeInfo we can access the protected members of TypeInfo.
        static constexpr int revision()
        {
            if constexpr (haveTypeInfo)
                return NativeInterface::TypeInfo::revision;
            else
                return 0;
        }
 
        static constexpr char const *name()
        {
            if constexpr (haveTypeInfo)
                return NativeInterface::TypeInfo::name;
            else
                return nullptr;
        }
    };
 
    // Wrapper type to make the error message in case
    // of incompatible interface types read better.
    template <typename I>
    struct NativeInterface : TypeInfo<I> {};
} // QNativeInterface::Private
 
// Declares an accessor for the native interface
#ifdef Q_QDOC
#define QT_DECLARE_NATIVE_INTERFACE_ACCESSOR(T)
    template <typename QNativeInterface>
    QNativeInterface *nativeInterface() const;
#else
#define QT_DECLARE_NATIVE_INTERFACE_ACCESSOR(T)
    template <typename NativeInterface, typename TypeInfo = QNativeInterface::Private::NativeInterface<NativeInterface>,
    typename BaseType = T, std::enable_if_t<TypeInfo::template isCompatibleWith<T>, bool> = true>
    NativeInterface *nativeInterface() const
    {
        return static_cast<NativeInterface*>(resolveInterface(
            TypeInfo::name(), TypeInfo::revision()));
    }
    protected:
        void *resolveInterface(const char *name, int revision) const;
    public:
#endif
 
QT_END_NAMESPACE
 
#endif // QNATIVEINTERFACE_H