Custom allocators

One common issue when using the genericjs section is the lack of support for custom allocators that return void pointers.

Let’s take this code for example:

void* myalloc(size_t s) {
  return malloc(s);
}

Cheerp will emit a compile time error:

Cheerp: Legacy C memory allocation must be immediately casted to the actual type. Any other use of the return value is invalid

The problem is that allocations in cheerp are translated to the creation of a JS object with the right set of properties. This code:

class Foo {
    int var1;
    int var2;
};
...
Foo foo* = (Foo*)malloc(sizeof(Foo));

translates to the following JS code:

tmp0 = { i0: 0, i1: 0 };

As you can see, the type information provided by the cast operator is needed to provide the right object layout. An equivalent (and better) solution is to use the C++ new operator whenever possible.

If for some reason you really need the custom allocator function, consider using a template:

template<class T>
T* myalloc(size_t size)
{
        return (T*)malloc(size);
}
...
Foo* foo = myalloc<Foo>(sizeof(Foo));