Short live q20::to_address!

An implementation of C++20 std::to_address, a prerequesite for
QSpan<>.

The test cases are inspired by libstdc++'s test suite, just to avoid
missing some cases, but the to_address implementation is white-room.

Fixes: QTBUG-108430
Change-Id: I4c092fdd7a56c0b279068e341bbf91a725ca3b1f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/q20memory.h

@@ -1,4 +1,5 @@
 // Copyright (C) 2023 The Qt Company Ltd.
+// Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #ifndef Q20MEMORY_H
@@ -7,9 +8,9 @@
 #include <QtCore/qtconfigmacros.h>
 
 #include <memory>
-#include <utility>
 
 #include <type_traits>
+#include <utility>
 
 //
 //  W A R N I N G
@@ -45,6 +46,48 @@ T *construct_at(T *ptr, Args && ... args)
 #endif // __cpp_lib_constexpr_dynamic_alloc
 } // namespace q20
 
+
+namespace q20 {
+// like std::to_address
+#ifdef __cpp_lib_to_address
+using std::to_address;
+#else
+// http://eel.is/c++draft/pointer.conversion
+template <typename T>
+constexpr T *to_address(T *p) noexcept {
+    // http://eel.is/c++draft/pointer.conversion#1:
+    //    Mandates: T is not a function type.
+    static_assert(!std::is_function_v<T>, "to_address must not be used on function types");
+    return p;
+}
+
+template <typename Ptr, typename std::enable_if_t<!std::is_pointer_v<Ptr>, bool> = true>
+constexpr auto to_address(const Ptr &ptr) noexcept; // fwd declared
+
+namespace detail {
+    // http://eel.is/c++draft/pointer.conversion#3
+    template <typename Ptr, typename = void>
+    struct to_address_helper {
+        static auto get(const Ptr &ptr) noexcept
+        { return q20::to_address(ptr.operator->()); }
+    };
+    template <typename Ptr>
+    struct to_address_helper<Ptr, std::void_t<
+            decltype(std::pointer_traits<Ptr>::to_address(std::declval<const Ptr&>()))
+        >>
+    {
+        static auto get(const Ptr &ptr) noexcept
+        { return std::pointer_traits<Ptr>::to_address(ptr); }
+    };
+} // namespace detail
+
+template <typename Ptr, typename std::enable_if_t<!std::is_pointer_v<Ptr>, bool>>
+constexpr auto to_address(const Ptr &ptr) noexcept
+{ return detail::to_address_helper<Ptr>::get(ptr); }
+
+#endif // __cpp_lib_to_address
+} // namespace q20
+
 QT_END_NAMESPACE
 
 #endif /* Q20MEMORY_H */

tests/auto/corelib/global/CMakeLists.txt

@@ -21,3 +21,4 @@ add_subdirectory(qglobalstatic)
 add_subdirectory(qhooks)
 add_subdirectory(qoperatingsystemversion)
 add_subdirectory(qxp)
+add_subdirectory(q20)

tests/auto/corelib/global/q20/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(memory)

tests/auto/corelib/global/q20/memory/CMakeLists.txt

@@ -0,0 +1,10 @@
+# Copyright (C) 2022 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+qt_internal_add_test(tst_q20_memory
+    EXCEPTIONS
+    SOURCES
+        tst_q20_memory.cpp
+    LIBRARIES
+        Qt::Core
+)

tests/auto/corelib/global/q20/memory/tst_q20_memory.cpp

@@ -0,0 +1,161 @@
+// Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+
+#include <q20memory.h>
+#include <QtCore/q20memory.h>
+
+#include <QTest>
+#include <QObject>
+#include <QExplicitlySharedDataPointer>
+#include <QSharedDataPointer>
+
+struct Private : QSharedData {};
+
+class tst_q20_memory : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void raw();
+    void smart();
+    void recursion();
+    void usesPointerTraits();
+    void prefersPointerTraits();
+
+private Q_SLOTS:
+    void to_address_broken_const_propagation_QExplicitlySharedDataPointer()
+    { to_address_broken_const_propagation<QExplicitlySharedDataPointer<Private>>(); }
+    void to_address_broken_const_propagation_QSharedDataPointer()
+    { to_address_broken_const_propagation<QSharedDataPointer<Private>>(); }
+    void to_address_broken_const_propagation_shared_ptr()
+    { to_address_broken_const_propagation<std::shared_ptr<Private>>(); }
+    void to_address_broken_const_propagation_unique_ptr()
+    { to_address_broken_const_propagation<std::unique_ptr<Private>>(); }
+
+private:
+    template <typename Pointer>
+    void to_address_broken_const_propagation();
+};
+
+void tst_q20_memory::raw()
+{
+    auto i = 0;
+    auto p = &i;
+    QVERIFY(q20::to_address(p) == &i);
+}
+
+template <typename T>
+class MinimalPtr {
+public:
+    using element_type = T;
+
+    explicit MinimalPtr(T *d) : d(d) {}
+
+    T *operator->() const noexcept { return d; }
+
+private:
+    T *d;
+};
+
+void tst_q20_memory::smart()
+{
+    int i;
+    MinimalPtr ptr(&i);
+    QCOMPARE_EQ(q20::to_address(ptr), &i);
+}
+
+template <typename T>
+class RecursivePtr {
+public:
+    using element_type = T;
+
+    explicit RecursivePtr(T *d) : d(d) {}
+
+    MinimalPtr<T> operator->() const noexcept { return d; }
+
+private:
+    MinimalPtr<T> d;
+};
+
+void tst_q20_memory::recursion()
+{
+    int i;
+    RecursivePtr ptr(&i);
+    QCOMPARE_EQ(q20::to_address(ptr), &i);
+}
+
+template <typename T>
+class NoDerefOperatorPtr {
+public:
+    using element_type = T;
+
+    explicit NoDerefOperatorPtr(T *d) : d(d) {}
+
+    T *get() const noexcept { return d; }
+
+private:
+    T *d;
+};
+
+namespace std {
+template <typename T>
+struct pointer_traits<NoDerefOperatorPtr<T>>
+{
+    static T *to_address(const NoDerefOperatorPtr<T> &ptr) noexcept { return ptr.get(); }
+};
+} // namespace std
+
+void tst_q20_memory::usesPointerTraits()
+{
+    int i;
+    NoDerefOperatorPtr ptr(&i);
+    QCOMPARE_EQ(q20::to_address(ptr), &i);
+}
+
+template <typename T>
+class PrefersPointerTraitsPtr
+{
+public:
+    using element_type = T;
+
+    explicit PrefersPointerTraitsPtr(T *d) : d(d) {}
+
+    T *operator->() const noexcept { return nullptr; }
+
+    T *get() const noexcept { return d; }
+
+private:
+    T *d;
+};
+
+namespace std {
+template <typename T>
+struct pointer_traits<PrefersPointerTraitsPtr<T>>
+{
+    static T *to_address(const PrefersPointerTraitsPtr<T> &p) noexcept { return p.get(); }
+};
+} // namespace std
+
+void tst_q20_memory::prefersPointerTraits()
+{
+    int i;
+    PrefersPointerTraitsPtr ptr(&i);
+    QCOMPARE_EQ(q20::to_address(ptr), &i);
+}
+
+template <typename Pointer>
+void tst_q20_memory::to_address_broken_const_propagation()
+{
+    Pointer p(nullptr);
+    QCOMPARE_EQ(q20::to_address(p), nullptr);
+    p = Pointer{new Private()};
+    QCOMPARE_EQ(q20::to_address(p), p.operator->());
+    static_assert(std::is_same_v<decltype(q20::to_address(p)),
+                                 decltype(std::as_const(p).operator->())>);
+}
+
+QTEST_GUILESS_MAIN(tst_q20_memory)
+#include "tst_q20_memory.moc"
+