try_capture and try_create_instance (#663)

Author: oldnewthing

strings/base_activation.h

@@ -487,6 +487,12 @@ WINRT_EXPORT namespace winrt
         impl::get_factory_cache().clear();
     }
 
+    template <typename Interface>
+    auto try_create_instance(guid const& clsid, uint32_t context = 0x1 /*CLSCTX_INPROC_SERVER*/, void* outer = nullptr)
+    {
+        return try_capture<Interface>(WINRT_IMPL_CoCreateInstance, clsid, outer, context);
+    }
+
     template <typename Interface>
     auto create_instance(guid const& clsid, uint32_t context = 0x1 /*CLSCTX_INPROC_SERVER*/, void* outer = nullptr)
     {

strings/base_com_ptr.h

@@ -153,6 +153,18 @@ WINRT_EXPORT namespace winrt
             *other = m_ptr;
         }
 
+        template <typename F, typename...Args>
+        bool try_capture(F function, Args&&...args)
+        {
+            return function(args..., guid_of<T>(), put_void()) >= 0;
+        }
+
+        template <typename O, typename M, typename...Args>
+        bool try_capture(com_ptr<O> const& object, M method, Args&&...args)
+        {
+            return (object.get()->*(method))(args..., guid_of<T>(), put_void()) >= 0;
+        }
+
         template <typename F, typename...Args>
         void capture(F function, Args&&...args)
         {
@@ -204,6 +216,21 @@ WINRT_EXPORT namespace winrt
         type* m_ptr{};
     };
 
+    template <typename T, typename F, typename...Args>
+    impl::com_ref<T> try_capture(F function, Args&& ...args)
+    {
+        void* result{};
+        function(args..., guid_of<T>(), &result);
+        return { result, take_ownership_from_abi };
+    }
+
+    template <typename T, typename O, typename M, typename...Args>
+    impl::com_ref<T> try_capture(com_ptr<O> const& object, M method, Args&& ...args)
+    {
+        void* result{};
+        (object.get()->*(method))(args..., guid_of<T>(), &result);
+        return { result, take_ownership_from_abi };
+    }
     template <typename T, typename F, typename...Args>
     impl::com_ref<T> capture(F function, Args&& ...args)
     {

test/old_tests/UnitTests/capture.cpp

@@ -58,3 +58,27 @@ TEST_CASE("capture")
     REQUIRE_THROWS_AS(capture<IDispatch>(a, &ICapture::CreateMemberCapture, 0), hresult_no_interface);
     REQUIRE_THROWS_AS(d.capture(a, &ICapture::CreateMemberCapture, 0), hresult_no_interface);
 }
+
+TEST_CASE("try_capture")
+{
+    // Identical to the "capture" test above, just with different
+    // error handling.
+    com_ptr<ICapture> a = try_capture<ICapture>(CreateCapture, 10);
+    REQUIRE(a->GetValue() == 10);
+    a = nullptr;
+    REQUIRE(a.try_capture(CreateCapture, 20));
+    REQUIRE(a->GetValue() == 20);
+
+    auto b = try_capture<ICapture>(a, &ICapture::CreateMemberCapture, 30);
+    REQUIRE(b->GetValue() == 30);
+    b = nullptr;
+    REQUIRE(b.try_capture(a, &ICapture::CreateMemberCapture, 40));
+    REQUIRE(b->GetValue() == 40);
+
+    com_ptr<IDispatch> d;
+
+    REQUIRE(!try_capture<IDispatch>(CreateCapture, 0));
+    REQUIRE(!d.try_capture(CreateCapture, 0));
+    REQUIRE(!try_capture<IDispatch>(a, &ICapture::CreateMemberCapture, 0));
+    REQUIRE(!d.try_capture(a, &ICapture::CreateMemberCapture, 0));
+}

test/old_tests/UnitTests/create_instance.cpp

@@ -9,3 +9,12 @@ TEST_CASE("create_instance")
     com_ptr<IFileOpenDialog> dialog = create_instance<IFileOpenDialog>(guid_of<FileOpenDialog>());
     REQUIRE(dialog);
 }
+
+TEST_CASE("try_create_instance")
+{
+    com_ptr<IFileOpenDialog> dialog = try_create_instance<IFileOpenDialog>(guid_of<FileOpenDialog>());
+    REQUIRE(dialog);
+
+    dialog = try_create_instance<IFileOpenDialog>(CLSID_NULL);
+    REQUIRE(!dialog);
+}