Is there a way to register a value type for an custom integer class?

[david-fong]

I have a class in my library that acts as an integer, with an inclusive upper bound specified via non-type template parameter. The bounds are used to pick appropriate, efficient underlying builtin integer types at compile-time, and provide hints to the compiler that could help it optimize. It's a precondition that the values passed by the user to construct these types (whether implicitly or implicitly) are within-bounds- I.e. the burden is on the user of my library to honour the bounds (though I do as much as I can to provide sanity checks that don't affect performance of release-builds).

My library uses these types for function parameters and function return objects (including getter and setters for container-like types exposed by my library), and within the C++ world, the constructors and conversion functions of these integer wrapper classes are defined such that the user of the library doesn't have to explicitly construct them or unwrap them. They can largely write and read their code as if these classes didn't exist, while getting the space-efficiency, compiler optimization, and debug assertion benefits.

I don't want users of the JS bindings for my library to have to manage memory for these integer-wrapper objects. I would prefer that they stay beneath/behind the JS interface, and that there's automatic conversion between them and JS's Number type. The problem domain for my library is such that upper-bounds are unlikely to go higher than 2^32 (well within Number.MAX_SAFE_INTEGER). I see that there's such a thing as value types, but I only see value_array and value_object.

Is there such a thing as value_integer? Should I raise a feature-request? Is there a workaround along the lines of what I see skimming #18813? (there's a comment there that the workaround may break in the future... Are there any current planned changes that would cause it to break?)

A related question- is there a way to make this throw a JS exception if user input is greater than the integer-wrapper type's upper-bound?

I know I have the option in my bindings file to define overloads for all my library functions that handle this, and I'm guessing there are other bindings-related workarounds, but I'm hoping there's an easier way.

[sbc100]

@brendandahl

[brendandahl]

There's a way to do custom marshalling with embind. It's not really documented and there's been some debate whether we should allow it. If you find it useful though that could be helpful for future plans.

[david-fong]

Thank Brendan :)

This works for a release build, but with my debug build, I'm getting an error when instantiating WASM.

I'm not sure if it's actually related to my custom int wrapper class, or something else. I don't get the following error when I go back to the point in my commit history right before I started using the custom int wrapper type instead of builtin int types, but there were other things I tweaked as well in between... :/ I'll need to do some work to get a minimal repro.

This is what I get during build when emitting .d.ts:

[build] FAILED: build/Debug/my_project.js /my_project/cpp/extras/emscripten/examples/node/build/Debug 
[build] : && /home/me/c/tool/emsdk/upstream/emscripten/em++ -Og -g3 -Xlinker --gc-sections --no-entry -sSTRICT=1 -sASSERTIONS=2 -gsource-map -sMODULARIZE=1 -sEXPORT_NAME=my_project_CREATE_MODULE --emit-tsd /my_project/cpp/out/build/dev.emscripten/build/Debug/my_project.d.ts -sALLOW_UNIMPLEMENTED_SYSCALLS=1 extras/emscripten/CMakeFiles/my_project_js.dir/Debug/my_project.embind.cpp.o -o build/Debug/my_project.js  build/Debug/libmy_project.a  -lembind && cd /my_project/cpp/out/build/dev.emscripten/extras/emscripten && /usr/bin/cmake -E make_directory /my_project/cpp/extras/emscripten/examples/node/build && /usr/bin/cmake -E create_symlink /my_project/cpp/out/build/dev.emscripten/build/Debug /my_project/cpp/extras/emscripten/examples/node/build/Debug
[build] failed to asynchronously prepare wasm: CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744
[build] Aborted(CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744)
[build] /tmp/emscripten_temp_ga_scsve/tsgen.js:546
[build]   var e = new WebAssembly.RuntimeError(what);
[build]           ^
[build] 
[build] RuntimeError: Aborted(CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744)
[build]     at abort (/tmp/emscripten_temp_ga_scsve/tsgen.js:546:11)
[build]     at instantiateArrayBuffer (/tmp/emscripten_temp_ga_scsve/tsgen.js:626:5)
[build]     at async createWasm (/tmp/emscripten_temp_ga_scsve/tsgen.js:704:16)
[build] 
[build] Node.js v22.16.0
[build] em++: error: '/home/me/c/tool/emsdk/node/22.16.0_64bit/bin/node /tmp/emscripten_temp_ga_scsve/tsgen.js /tmp/emscripten_temp_ga_scsve/embind_generated_output.js' failed (returned 1)
[build] ninja: build stopped: subcommand failed.
[proc] The command: /usr/bin/cmake --build /my_project/cpp/out/build/dev.emscripten --config Debug --target all -- exited with code: 1
[driver] Build completed: 00:00:03.241
[build] Build finished with exit code 1

This is what I get when running an example script that uses my JS module:

./extras/emscripten/examples/node/example_node.mjs 
failed to asynchronously prepare wasm: CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744
Aborted(CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744)
/myproject/cpp/out/build/dev.emscripten/build/Debug/myproject.js:502
  var e = new WebAssembly.RuntimeError(what);
          ^

RuntimeError: Aborted(CompileError: WebAssembly.instantiate(): Compiling function #29:"void const* emscripten::internal::getActualType..." failed: expected 1 elements on the stack for fallthru, found 2 @+4744)
    at abort (/myproject/cpp/out/build/dev.emscripten/build/Debug/myproject.js:502:11)
    at instantiateArrayBuffer (/myproject/cpp/out/build/dev.emscripten/build/Debug/myproject.js:583:5)
    at async createWasm (/myproject/cpp/out/build/dev.emscripten/build/Debug/myproject.js:665:16)
    at async Module.<anonymous> (/myproject/cpp/out/build/dev.emscripten/build/Debug/myproject.js:3649:15)
    at async file:///myproject/cpp/extras/emscripten/examples/node/example_node.mjs:9:13

Node.js v24.8.0

(the build is running the NodeJS that comes with Emscripten- I'm on version 4.0.15. when I'm running my example JS script to demo my project, I'm using Node 24.8.0). If you have any troubleshooting tips, it'd be much appreciated!