Registering and binding type across DLL boundaries #758

trexxet · 2025-04-17T21:50:24Z

trexxet
Apr 17, 2025

Hi. I'm trying to figure out if I'm missing something, as I can't register/bind a simple templated struct:

// within namespace Based
template <typename T>
struct Rect2D {
	union { T x, s; };
	union { T y, t; };
	T w, h;

	/* constructors, operators, transform functions etc. */
};

Registration (with excess checks):

// Based::RML::Model::bindBasedTypes()
		bool res = true;
		if (auto rect2di_handle = constructor.RegisterStruct<Rect2D<int>>()) {
			res &= rect2di_handle.RegisterMember("x", &Rect2D<int>::x);
			res &= rect2di_handle.RegisterMember("y", &Rect2D<int>::y);
			res &= rect2di_handle.RegisterMember("w", &Rect2D<int>::w);
			res &= rect2di_handle.RegisterMember("h", &Rect2D<int>::h);
		}
		else {
			log.fatal ("Failed to bind Rect2D<int>");
		}
		if (!res)
			log.fatal ("Failed to bind Rect2D<int> fields");

log.fatal() is not triggered.

And, finally, binding:

Based::Rect2D<int> labelRect = conf["labelRect"]; // getting from Lua config
rmodel = Based::RML::Model::make (rctx, "modelMain", Based::RML::BindTypes::Geometry); // calls the registration code
rmodel->constructor.Bind("labelRect", &labelRect);

Which fails with the message:

Desired data type T not registered with the type register, please use the 'Register...()' functions before binding values, adding members, or registering arrays of non-scalar types.
T: N5Based6Rect2DIiEE
C:/shitcode/basedengine/build/_deps/rmlui-src/Include/RmlUi/Core/DataTypeRegister.h:102
Could not bind variable 'labelRect' to data model, data type not registered.

The Model class is a pretty simple wrapper (some checks omitted):

class Model {
	void bindBasedTypes (BindTypes types);
public:
	std::string name;

	Rml::Context* rctx = nullptr;
	Rml::DataModelConstructor constructor;
	Rml::DataModelHandle model;

	Model (Rml::Context* rctx, const std::string& name, int flags = 0);

	static std::unique_ptr<Model> make (Rml::Context* rctx, const std::string& name, int flags = 0);
};

Model::Model (Rml::Context* rctx, const std::string& name, int flags) : name (name), rctx (rctx) {
	constructor = rctx->CreateDataModel (name);
	bindBasedTypes(flags);
	model = constructor.GetModelHandle();
}

Thanks in advance.

mikke89 · 2025-04-17T23:33:29Z

mikke89
Apr 17, 2025
Maintainer

Hello. I tried to reproduce this, but couldn't do so, this case seems to work for me. I threw together this unit test, and it runs successfully here. Could you test this on your end? Are you able to reproduce the issue in in our testing facilities?

namespace Based {
	static const String document_rect2d_rml = R"(
<rml>
<head>
	<title>Test</title>
	<link type="text/template" href="/assets/window.rml"/>
	<style>
		body.window
		{
			left: 50px;
			right: 50px;
			top: 30px;
			bottom: 30px;
			max-width: -1px;
			max-height: -1px;
		}
	</style>
</head>

<body template="window" data-model="basics">
<p>{{ rect.x }}</p>
<p>{{ rect.y }}</p>
<p>{{ rect.w }}</p>
<p>{{ rect.h }}</p>
</body>
</rml>
)";

	template <typename T>
	struct Rect2D {
		union {
			T x, s;
		};
		union {
			T y, t;
		};
		T w, h;
	};

	bool BindRect2D(Context* context)
	{
		Rml::DataModelConstructor constructor = context->CreateDataModel("basics");
		if (!constructor)
			return false;

		bool res = true;
		if (auto rect2di_handle = constructor.RegisterStruct<Rect2D<int>>())
		{
			res &= rect2di_handle.RegisterMember("x", &Rect2D<int>::x);
			res &= rect2di_handle.RegisterMember("y", &Rect2D<int>::y);
			res &= rect2di_handle.RegisterMember("w", &Rect2D<int>::w);
			res &= rect2di_handle.RegisterMember("h", &Rect2D<int>::h);
		}
		else
		{
			FAIL("Failed to bind Rect2D<int>");
		}
		if (!res)
		{
			FAIL("Failed to bind Rect2D<int> fields");
		}

		constructor.Bind("rect", new Based::Rect2D<int>{1, 2, 3, 4});
		return true;
	}
} // namespace Based

TEST_CASE("data_binding.templated_struct")
{
	Context* context = TestsShell::GetContext();
	REQUIRE(context);

	REQUIRE(Based::BindRect2D(context));

	ElementDocument* document = context->LoadDocumentFromMemory(Based::document_rect2d_rml);
	REQUIRE(document);
	document->Show();

	TestsShell::RenderLoop();

	CHECK(document->QuerySelector("p:nth-child(1)")->GetInnerRML() == "1");
	CHECK(document->QuerySelector("p:nth-child(2)")->GetInnerRML() == "2");
	CHECK(document->QuerySelector("p:nth-child(3)")->GetInnerRML() == "3");
	CHECK(document->QuerySelector("p:nth-child(4)")->GetInnerRML() == "4");

	document->Close();

	TestsShell::ShutdownShell();
}

4 replies

trexxet Apr 18, 2025
Author

Hm, works fine. I tried making this test more complicated and close to my real case, and it still works fine.

So, time to gdb this. And turns out that:

Here, Rect2D<int> gets id = 0:

RmlUi/Include/RmlUi/Core/DataModelHandle.h

Line 182 in b1541f5

const FamilyId id = Family<T>::Id();
And here, Rect2D<int> gets id = 2:

RmlUi/Include/RmlUi/Core/DataTypeRegister.h

Line 98 in b1541f5

FamilyId id = Family<T>::Id();
And...

RmlUi/Include/RmlUi/Core/Traits.h

Line 84 in b1541f5

// Note: An ID for a given type may not match across DLL-boundaries.

Oh no, that's my case. Registration occurs in the engine dll, and the binding happens in the application exe.

mikke89 Apr 18, 2025
Maintainer

Oh, I see, you're doing this from different dynamic libraries. See if it might help to pass a custom DataTypeRegister when creating the data model, you can construct that in a different DLL than the one used for the RmlUi context. Generally, I would recommend to create the type register and do all the data binding in the same DLL. See also some discussion around this in #409.

trexxet Apr 18, 2025
Author

I tried making using self-provided data type register, and, unfortunately, no - it changes nothing.

Registering the type and binding variable in the same executable is a solution, but for my case that means that the application must manually register some basic types from the engine. The desired workflow is that application asks engine to register it's own basic types, and then application binds variables of that types.

So, at this point:

It has nothing to do with Rect2D being templated
It has nothing to do with Rect2D being in a namespace (I made such assumption because the mangled type name contained the namespace)

Going further, FamilyBase::GetNewId() gets called 3 times:

Registering the Rect2D type
Registering the member of Rect2D
Binding the Rect2D variable

The FamilyBase::GetNewId() itself works as expected. However, because DataModelConstructor::RegisterStruct() and DataTypeRegister::GetDefinitionDetail() are templated and located in headers, they get placed in different executables.

So, the 3rd call occurs because it's triggered by

RmlUi/Include/RmlUi/Core/DataTypeRegister.h

Line 98 in b1541f5

FamilyId id = Family<T>::Id();

As I understand, it's supposed that the compiler would instantiate a template of Family<T> when registering T, and then use it. However, template instances are unique for each executable, so the Family<Rect2D> in engine dll is different from the one in the application exe.

At this stage, I think that the ID generation should be reworked. I'll think on the possible solutions and try to implement and test them. The first thing coming up is making DTR to use typeid names instead of ids as keys.

trexxet Apr 18, 2025
Author

Well, using typeid name as family id solves this and passes unit tests on my setup. I'll make a PR, but it requires careful consideration and testing, as typeid is compiler specific.

trexxet · 2025-04-26T22:05:08Z

trexxet
Apr 26, 2025
Author

Solved, see #759 discussion and https://mikke89.github.io/RmlUiDoc/pages/data_bindings/model.html#registering-types-across-library-boundaries

0 replies

trexxet · 2025-07-16T21:46:45Z

trexxet
Jul 16, 2025
Author

Well, this story got an unexpected continuation.

By default MinGW exports all symbols in dll, regardless of dllexport attribute you use. This can be changed by setting -fvisibility=hidden -Wl,--exclude-all-symbols, so only dllexport symbols are exported.

So, here we go. Header:

extern template class BASED_API Rml::Family<Based::Rect2D<int>>;

Cpp:

template class Rml::Family<Based::Rect2D<int>>;

And... undefined reference to Rml::Family<Based::Rect2D<int> >::Id(). What?
Spending several hours to find a solution, I stumbled upon this MinGW bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109380.

Let's look into the Family class:

template <typename T>
class Family : FamilyBase {
public:
	static FamilyId Id()
	{
		static int id = GetNewId();
		return static_cast<FamilyId>(id);
	}
};

Matches the bug exactly. The Id() is implicitly inlined (because it's defined within the class definition), we instantiate the template, but MinGW ignores dllexport for inline methods of template instance.

So, I guess I'd stick to keeping all symbols being exported, until this bug gets fixed, if it ever would - the first report was 8 years ago and it's still in an unconfirmed state.

2 replies

mikke89 Jul 17, 2025
Maintainer

This makes me sad :\ Maybe worth testing with the API macro in the cpp file as well, if you didn't already do this?

But nice work tracing down that bug. I guess there is not much to do from our end, so we'll have to accept that workaround until it is fixed.

trexxet Jul 18, 2025
Author

Yes, I tested it. Unfortunately, MinGW ignores dllexport for definitions and warns about it, as they are required only for declarations.

Registering and binding type across DLL boundaries #758

Uh oh!

trexxet Apr 17, 2025

Replies: 3 comments · 6 replies

Uh oh!

mikke89 Apr 17, 2025 Maintainer

Uh oh!

trexxet Apr 18, 2025 Author

Uh oh!

mikke89 Apr 18, 2025 Maintainer

Uh oh!

Uh oh!

trexxet Apr 18, 2025 Author

Uh oh!

trexxet Apr 18, 2025 Author

Uh oh!

trexxet Apr 26, 2025 Author

Uh oh!

Uh oh!

trexxet Jul 16, 2025 Author

Uh oh!

mikke89 Jul 17, 2025 Maintainer

Uh oh!

Uh oh!

trexxet Jul 18, 2025 Author

trexxet
Apr 17, 2025

Replies: 3 comments 6 replies

mikke89
Apr 17, 2025
Maintainer

trexxet Apr 18, 2025
Author

mikke89 Apr 18, 2025
Maintainer

trexxet Apr 18, 2025
Author

trexxet Apr 18, 2025
Author

trexxet
Apr 26, 2025
Author

trexxet
Jul 16, 2025
Author

mikke89 Jul 17, 2025
Maintainer

trexxet Jul 18, 2025
Author