to_message() function extracts exception text from catch(...) (#857)

Author: oldnewthing

strings/base_error.h

@@ -546,6 +546,31 @@ WINRT_EXPORT namespace winrt
         }
     }
 
+    inline __declspec(noinline) hstring to_message()
+    {
+        if (winrt_to_message_handler)
+        {
+            return winrt_to_message_handler(WINRT_IMPL_RETURNADDRESS());
+        }
+
+        try
+        {
+            throw;
+        }
+        catch (hresult_error const& e)
+        {
+            return e.message();
+        }
+        catch (std::exception const& ex)
+        {
+            return to_hstring(ex.what());
+        }
+        catch (...)
+        {
+            std::terminate();
+        }
+    }
+
     [[noreturn]] inline void throw_last_error()
     {
         throw_hresult(impl::hresult_from_win32(WINRT_IMPL_GetLastError()));

strings/base_extern.h

@@ -1,5 +1,6 @@
 
 __declspec(selectany) int32_t(__stdcall* winrt_to_hresult_handler)(void* address) noexcept {};
+__declspec(selectany) winrt::hstring(__stdcall* winrt_to_message_handler)(void* address) {};
 __declspec(selectany) void(__stdcall* winrt_throw_hresult_handler)(uint32_t lineNumber, char const* fileName, char const* functionName, void* returnAddress, winrt::hresult const result) noexcept {};
 __declspec(selectany) void(__stdcall* winrt_suspend_handler)(void const* token) noexcept {};
 __declspec(selectany) void(__stdcall* winrt_resume_handler)(void const* token) noexcept {};

test/old_tests/UnitTests/hresult_error.cpp

@@ -556,3 +556,23 @@ TEST_CASE("hresult, std abi support")
     }
 }
 
+TEST_CASE("hresult, to_message")
+{
+    try
+    {
+        throw std::out_of_range("oh no, out of range");
+    }
+    catch (...)
+    {
+        REQUIRE(to_message() == L"oh no, out of range");
+    }
+
+    try
+    {
+        throw hresult_error(E_HANDLE, L"oh no, invalid handle");
+    }
+    catch (...)
+    {
+        REQUIRE(to_message() == L"oh no, invalid handle");
+    }
+}

test/test/custom_error.cpp

@@ -37,6 +37,24 @@ namespace
         return 0;
     }
 
+    // Global handler to translate custom exception to message
+    hstring __stdcall message_handler(void* address)
+    {
+        REQUIRE(address);
+
+        try
+        {
+            throw;
+        }
+        catch (CustomError)
+        {
+            return L"a custom error";
+        }
+
+        REQUIRE(false);
+        return {};
+    }
+
     static bool s_loggerCalled = false;
 
     void __stdcall logger(uint32_t lineNumber, char const* fileName, char const* functionName, void* returnAddress, winrt::hresult const result) noexcept
@@ -77,3 +95,22 @@ TEST_CASE("custom_error_logger")
     winrt_throw_hresult_handler = nullptr;
     s_loggerCalled = false;
 }
+
+TEST_CASE("custom_error_message")
+{
+    // Set up global handler
+    REQUIRE(!winrt_to_message_handler);
+    winrt_to_message_handler = message_handler;
+
+    try
+    {
+        throw CustomError();
+    }
+    catch (...)
+    {
+        REQUIRE(to_message() == L"a custom error");
+    }
+
+    // Remove global handler
+    winrt_to_message_handler = nullptr;
+}