leaningtech/cheerp

Cheerp - C/C++ to WebAssembly and JavaScript

Alessandro Pignotti

@alexp-sssup

Well, no, you could do something like

[[cheerp::genericjs]] client::EventListener* getNewCallback()
{
C* a = new C(); 
return cheerp::Callback([a](float x, float y) { a->method(x, y); }) 
}

In this way for every call to getNewCallback you will also get an independently allocated class

Or you can allocate 1 instance of the class during initialization and then store the callback in the global JS namespace, for example

Eugene Mikhailovski

@Eugenexz

@alexp-sssup I need "a" to be instantiated only once elsewhere

Alessandro Pignotti

@alexp-sssup

namespace [[cheerp::genericjs]] client
{
client::EventListener* wasmCallback;
}
[[cheerp::genericjs]] void webMain()
{
...
C* a = new C();
client::wasmCallback = cheerp::Callback([a](float x, float y)...);
...
}

And now from JS code you can use the wasmCallback(x,y) function

No, as usual you will need a [[cheerp::genericjs]] function, I tagged webMain in the example before.

Eugene Mikhailovski

@Eugenexz

@alexp-sssup but now webMain is not wasm

Alessandro Pignotti

@alexp-sssup

No indeed, if you have heavy computation code in webMain you may create a smaller function with [[cheerp::genericjs]] just to allocate the callback you need

There is most often no negative effect in having some code tagged with [[cheerp::genericjs]], it is fairly performant anyway

Eugene Mikhailovski

@Eugenexz

@alexp-sssup If I have a smaller function with [[cheerp::genericjs]] just to allocate the callback then I will still need the globally allocated wasm class.

Alessandro Pignotti

@alexp-sssup

@Eugenexz I am not following

You can allocate the instance they way you prefer and then pass it an argument to the small genericjs function (both pointer and reference arguments will work)

In case you have bugs with using the C++ lambda as in the example you should also try the std::bind method which returns an std::function

Eugene Mikhailovski

@Eugenexz

Ultimately, I just don't see where the separation is between wasm and JS in your examples if I want to instantiate my wasm class in the wasm section

Alessandro Pignotti

@alexp-sssup

The separation is well defined, cheerp::genericjs]] functions will be in JS, the rest will be in wasm

You can allocate wasm classes also in genericjs function, they will still be fully wasm

Eugene Mikhailovski

@Eugenexz

@alexp-sssup I can pass [[cheerp::genericjs]] or [[cheerp::jsexport]] arguments into wasm functions?

Alessandro Pignotti

@alexp-sssup

[[cheerp::jsexport]] cannot be applied to arguments, what do you mean?

wasm functions cannot manipulate genericjs objects, but the other way around is fully supported

Eugene Mikhailovski

@Eugenexz

Then what did you mean by "You can allocate the instance they way you prefer and then pass it an argument to the small genericjs function"?

Alessandro Pignotti

@alexp-sssup

You can pass wasm objects into genericjs code

not the other way around

So you can allocate your wasm instance the way you like

and then pass it to genericjs only to forge the callback

Eugene Mikhailovski

@Eugenexz

Ok I understand now

@alexp-sssup Thanks

Alessandro Pignotti

@alexp-sssup

You are welcome :-)

Eugene Mikhailovski

@Eugenexz

@alexp-sssup But again, I can't use cheerp 2.0 unless I know how to properly import functions into wasm

Alessandro Pignotti

@alexp-sssup

@Eugenexz Can you make an example of what you want to achieve?

Eugene Mikhailovski

@Eugenexz

C++: extern "C" { int createBuffer(); }; JS: WebAssembly.instantiate(r, {_createBuffer: () => {...}});

Alessandro Pignotti

@alexp-sssup

The easiest way is to declare the functions in the client namespace and call those from a small genericjs function

Eugene Mikhailovski

@Eugenexz

How do I declare them in the client namespace? And I will have to in turn call these genericjs functions from the wasm section, right?

Alessandro Pignotti

@alexp-sssup

namespace [[cheerp::genericjs]] client
{
int createBuffer();
}

And as you said you will need to call the genericjs from wasm

Eugene Mikhailovski

@Eugenexz

@alexp-sssup great, that works, thanks!

Eugene Mikhailovski

@Eugenexz

@alexp-sssup although now I have to go through two function calls instead of one

Alessandro Pignotti

@alexp-sssup

Yeah, that's not great, in the next release calling directly client functions from wasm will be supported in some cases

This one should be fine

Eugene Mikhailovski

@Eugenexz

Sounds good

Carlo Piovesan

@carlopi

I wrote an article on how to port a C library to JS, it may be of interest: https://medium.com/leaningtech/a-fast-bigint-js-in-an-evening-compiling-c-to-javascript-db61ae733512

Eugene Mikhailovski

@Eugenexz

Hello, inside a constructor of a class, I have two calls with different parameters to a member method of another class and everything works, but when I remove the second call, I get RuntimeError: function signature mismatch

Alessandro Pignotti

@alexp-sssup

@Eugenexz There is nothing wrong in what you described. There may be a compiler bug but we need a test case to be able to understand it. It is also likely that the problem is already fixed in current master, do you have the possibility of testing using Linux + our nightly PPA, for example on a virtual machine?

Eugene Mikhailovski

@Eugenexz

@alexp-sssup How is there nothing wrong? Yes, I may try

Alessandro Pignotti

@alexp-sssup

I meant that you described something which is supposed to work, the error you received is obviously not normal

Eugene Mikhailovski

@Eugenexz

@alexp-sssup that makes more sense haha

Timothée Jeannin

@TimotheeJeannin

Hi, is there anybody here?

Alessandro Pignotti

@alexp-sssup

Sure

Alessandro Pignotti

@alexp-sssup

@TimotheeJeannin what's your question?

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