Add get_iids_tearoff() hook.

Add a `get_iids_tearoff()` hook to complement the `query_interface_tearoff()`
hook.

`query_interface_tearoff()` allows dynamically extending types with
additional interfaces that can be accessed with the `QueryInterface()`
method. However, these interfaces were not discoverable by the `GetIids()`
method (and therefore not by the `winrt::get_interfaces()` function
either).

The `get_iids_tearoff()` hook allows adding the GUIDs of interfaces
to the array returned by `GetIids()`.

This also fixes a bug in the implementation of the
`root_implements_type::is_composing` branch of `NonDelegatingGetIids()`
where `*array` was updated causing the local iids to be unreachable by
the caller and risking the caller reading past the end of the array.

strings/base_implements.h

@@ -912,6 +912,11 @@ namespace winrt::impl
             return error_no_interface;
         }
 
+        virtual std::vector<guid> get_iids_tearoff() const noexcept
+        {
+            return {};
+        }
+
         root_implements() noexcept
         {
         }
@@ -1021,7 +1026,8 @@ namespace winrt::impl
         int32_t __stdcall NonDelegatingGetIids(uint32_t* count, guid** array) noexcept
         {
             auto const& local_iids = static_cast<D*>(this)->get_local_iids();
-            uint32_t const& local_count = local_iids.first;
+            auto tearoff_iids = get_iids_tearoff();
+            auto local_count = local_iids.first + tearoff_iids.size();
             if constexpr (root_implements_type::is_composing)
             {
                 if (local_count > 0)
@@ -1033,8 +1039,10 @@ namespace winrt::impl
                     {
                         return error_bad_alloc;
                     }
-                    *array = std::copy(local_iids.second, local_iids.second + local_count, *array);
-                    std::copy(inner_iids.cbegin(), inner_iids.cend(), *array);
+                    auto _array = *array;
+                    _array = std::copy(local_iids.second, local_iids.second + local_iids.first, _array);
+                    _array = std::copy(tearoff_iids.cbegin(), tearoff_iids.cend(), _array);
+                    std::copy(inner_iids.cbegin(), inner_iids.cend(), _array);
                 }
                 else
                 {
@@ -1051,7 +1059,9 @@ namespace winrt::impl
                     {
                         return error_bad_alloc;
                     }
-                    std::copy(local_iids.second, local_iids.second + local_count, *array);
+                    auto _array = *array;
+                    _array = std::copy(local_iids.second, local_iids.second + local_iids.first, _array);
+                    std::copy(tearoff_iids.cbegin(), tearoff_iids.cend(), _array);
                 }
                 else
                 {

test/old_tests/UnitTests/Composable.cpp

@@ -1,12 +1,14 @@
 #include "pch.h"
 #include "catch.hpp"
 #include "winrt/Composable.h"
+#include <windows.foundation.h>
 
 using namespace winrt;
 using namespace Windows::Foundation;
 using namespace Composable;
 
 using namespace std::string_view_literals;
+using hstring = ::winrt::hstring;
 
 namespace
 {
@@ -167,4 +169,118 @@ TEST_CASE("Composable conversions")
 {
     TestCalls(*make_self<Foo>());
     TestCalls(*make_self<Bar>());
-}
+}
+
+namespace
+{
+    // Creates an implementation of IStringable as a tearoff.
+    HRESULT make_stringable(winrt::Windows::Foundation::IInspectable const& object, hstring const& value, void** result) noexcept
+    {
+        struct stringable final : ABI::Windows::Foundation::IStringable
+        {
+            stringable(winrt::Windows::Foundation::IInspectable const& object, hstring const& value) :
+                m_object(object.as<::IInspectable>()),
+                m_value(value)
+            {
+            }
+
+            HRESULT __stdcall ToString(HSTRING* result) noexcept final
+            {
+                return WindowsDuplicateString(static_cast<HSTRING>(get_abi(m_value)), result);
+            }
+
+            HRESULT __stdcall QueryInterface(GUID const& id, void** result) noexcept final
+            {
+                if (is_guid_of<IStringable>(id))
+                {
+                    *result = static_cast<ABI::Windows::Foundation::IStringable*>(this);
+                    AddRef();
+                    return S_OK;
+                }
+
+                return m_object->QueryInterface(id, result);
+            }
+
+            ULONG __stdcall AddRef() noexcept final
+            {
+                return 1 + m_references.fetch_add(1, std::memory_order_relaxed);
+            }
+
+            ULONG __stdcall Release() noexcept final
+            {
+                uint32_t const remaining = m_references.fetch_sub(1, std::memory_order_relaxed) - 1;
+
+                if (remaining == 0)
+                {
+                    delete this;
+                }
+
+                return remaining;
+            }
+
+            HRESULT __stdcall GetIids(ULONG* count, GUID** iids) noexcept final
+            {
+                return m_object->GetIids(count, iids);
+            }
+
+            HRESULT __stdcall GetRuntimeClassName(HSTRING* result) noexcept final
+            {
+                return m_object->GetRuntimeClassName(result);
+            }
+
+            HRESULT __stdcall GetTrustLevel(::TrustLevel* result) noexcept final
+            {
+                return m_object->GetTrustLevel(result);
+            }
+
+        private:
+
+            com_ptr<::IInspectable> m_object;
+            hstring m_value;
+            std::atomic<uint32_t> m_references{ 1 };
+        };
+
+        *result = new (std::nothrow) stringable(object, value);
+        return *result ? S_OK : E_OUTOFMEMORY;
+    }
+}
+
+TEST_CASE("Composable tearoff")
+{
+    struct Tearoff : DerivedT<Tearoff, IClosable>
+    {
+        void Close()
+        {
+        }
+
+        int32_t query_interface_tearoff(winrt::guid const& id, void** result) const noexcept final
+        {
+            if (is_guid_of<IStringable>(id))
+            {
+                return make_stringable(*this, L"ToString", result);
+            }
+
+            *result = nullptr;
+            return E_NOINTERFACE;
+        }
+
+        std::vector<winrt::guid> get_iids_tearoff() const noexcept final
+        {
+            return {winrt::guid_of<IStringable>()};
+        }
+    };
+
+    auto object = make<Tearoff>();
+    auto ifaces = get_interfaces(object);
+
+    REQUIRE(object.as<IClosable>());
+    REQUIRE(object.as<IStringable>());
+    // IBaseOverrides is repeated twice for some reason, so actual size is 7 but there are only 6 unique interfaces
+    REQUIRE(ifaces.size() >= 6);
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<Base>()) != ifaces.end());
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<IBaseProtected>()) != ifaces.end());
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<IBaseOverrides>()) != ifaces.end());
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<Derived>()) != ifaces.end());
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<IClosable>()) != ifaces.end());
+    REQUIRE(std::find(ifaces.begin(), ifaces.end(), winrt::guid_of<IStringable>()) != ifaces.end());
+}

test/test/tearoff.cpp

@@ -164,6 +164,11 @@ namespace
             *result = nullptr;
             return E_NOINTERFACE;
         }
+
+        std::vector<winrt::guid> get_iids_tearoff() const noexcept final
+        {
+            return {winrt::guid_of<IPersist>()};
+        }
     };
 
     struct RuntimeType : winrt::implements<RuntimeType, winrt::IClosable>
@@ -196,6 +201,11 @@ namespace
             *result = nullptr;
             return E_NOINTERFACE;
         }
+
+        std::vector<winrt::guid> get_iids_tearoff() const noexcept final
+        {
+            return {winrt::guid_of<winrt::IStringable>()};
+        }
     };
 }
 
@@ -215,6 +225,11 @@ TEST_CASE("tearoff")
         REQUIRE(S_OK == persist->GetClassID(&result));
         REQUIRE(winrt::is_guid_of<IPersist>(result));
 
+        winrt::com_array<winrt::guid> iids = winrt::get_interfaces(closable);
+        REQUIRE(iids.size() == 2);
+        REQUIRE(std::find(iids.begin(), iids.end(), winrt::guid_of<winrt::IClosable>()) != iids.end());
+        REQUIRE(std::find(iids.begin(), iids.end(), winrt::guid_of<IPersist>()) != iids.end());
+
         // query_interface_tearoff happily ignores any other queries.
         REQUIRE(closable.try_as<winrt::IActivationFactory>() == nullptr);
 
@@ -249,6 +264,11 @@ TEST_CASE("tearoff")
         winrt::IStringable stringable = closable.as<winrt::IStringable>();
         REQUIRE(stringable.ToString() == L"ToString");
 
+        winrt::com_array<winrt::guid> iids = winrt::get_interfaces(closable);
+        REQUIRE(iids.size() == 2);
+        REQUIRE(std::find(iids.begin(), iids.end(), winrt::guid_of<winrt::IClosable>()) != iids.end());
+        REQUIRE(std::find(iids.begin(), iids.end(), winrt::guid_of<winrt::IStringable>()) != iids.end());
+
         // Calling an IInspectable function on the tearoff forwards to the object.
         REQUIRE(winrt::get_class_name(stringable) == L"RuntimeClassName");