How can I use class and wasm?

Jack jckj33 at
Mon Oct 26 21:38:39 UTC 2020

On Friday, 16 October 2020 at 03:42:22 UTC, Adam D. Ruppe wrote:
> On Friday, 16 October 2020 at 03:04:25 UTC, Jack wrote:
>> How can I allocate memory for this class?
> It is possible but not easy without druntime.
> If you are using -betterC, you can use extern(C++) classes with 
> extern(D) members. The compiler will let you declare that. But 
> then you need to allocate it. `__traits(classInstanceSize, 
> Whatever)` will tell you the size to malloc, but you also need 
> to copy an initializer over before you call the constructor.
> I have a technique here that works on dmd...

I did consider do something like this but the class would only 
live in the function's lifetime, right? that wouldn't work if I 
need to pass the class around

> but ldc is more strict about the type definition and I don't 
> know the magic it expects there... like it should be doable but 
> idk how so this might not be of much use.

Which type definition are you refering to? you mean the class 
members/memory layout of a class?

> Personally, I prefer to just not use betterC and make my own 
> mini runtime:

I was thinking the same, this seems the way to go.

> in particular

This is great! I'll try write a small druntime too for my case. 
Do you have links/resources where things like __heap_base, 
__data_end are defined? assuming there's such documentation 
somewhere, if you did into the compiler to get those, let me know 
where you can it too. I didn't understand the interation of 
webassembly-core.js with D. for example, you have this:

// placeholder to be filled in by the loader
var memory;

How did you put the compiler/linker to set memory's location 
properly so you can do things like:

memorySize: function() { return memory.buffer.byteLength; },


in the same way that ldc defines __headp_base and __data_end, 
doesn't it define some variables where the implementation of 
memorySize() and growMemory() could be done in D? also, where can 
I find this Sebastiaan's project that you mentioned in the 
article? it's a druntime like yours?


this is great, good enough to put me in the direction on how to 
do that. Also, I found C++ has this EMSCRIPTEN_BINDINGS() from 
emscripten/bind.h header, see[1] for example. I haven't tried 
yet. But maybe marking the class as extern(C) that would work 
with D's class too, right? I don't know about the C++'s or 
compiler's dependences that would prevent it from working with D. 
In fact, that seems quite a hacky. Do you think we are better off 
write our own small druntime? if I got an working nice one, I'll 
put on github for those who would like to use this.


Thanks for your answer, that was really helpful.

> But that's also not easy, lots of unfinished cases in my thing, 
> but I did manage to make it work... for my specific case.

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