minimal object.d implementation that allows classes

[Steven Schveighoffer]

On 12/7/17 9:45 AM, Jean-Louis Leroy wrote:
> On Thursday, 7 December 2017 at 13:08:09 UTC, Luís Marques wrote:
>> On Thursday, 7 December 2017 at 08:59:08 UTC, Wild wrote:
>>> You could modify the one I use for PowerNex[1]. It is a hacked 
>>> version of Adam D. Ruppes
>>> minimal.zip. There are only a few imports for strlen, 
>>> mem{set,move,cpy}, etc.
>>
>> This one seems to be the one for me. For instance, typeid(ClassName) 
>> seems to work, which I need.
>>
>> For completeness, I had looked into minlibd but it didn't seemed 
>> compatible with LDC and difficult for me to fix, IIRC. Regarding 
>> Mike's question of what my use case is, I wanted my program to run in 
>> webassembly/asmjs. Since druntime/phobos haven't been ported to that 
>> target, I was being careful with my design to not depend on the 
>> runtime, but then I found out I love the openmethods package. So I 
>> decided to check if I could implement just enough of druntime to allow 
>> a forked version of openmethods to work. As far as I can  tell that's 
>> feasible.
> 
> I am currently trying to understand how the compiler and the runtime 
> interact. ClassInfo contains a vtbl array whose ptr property is the same 
> as the vptr found in an instance of that class. However, when I modify 
> that pointer (e.g. by reserving 1000 more entries) and create a new 
> instance, it still contains the old pointer. So it looks like the 
> compiler sets the vptr without consulting the ClassInfo but somehow 
> reflects it there. I'd appreciate if anybody can explain how it works, 
> or sends me links to relevant info.
> 
> Example:
> 
> import std.stdio;
> 
> class Foo {
>    void foo() {}
> }
> 
> void main() {
>    auto oldPtr = Foo.classinfo.vtbl.ptr;
>    Foo.classinfo.vtbl.reserve(1000);
>    writeln(oldPtr != Foo.classinfo.vtbl.ptr); // true
>    Object foo = new Foo();
>    writeln(oldPtr == *cast(void***)foo); // true, alas
> }
> 

The object is constructed here:

https://github.com/dlang/druntime/blob/master/src/rt/lifetime.d#L71

specifically, the vtbl is set when you blit the initializer array 
(another piece of the TypeInfo) into the class object here: 
https://github.com/dlang/druntime/blob/master/src/rt/lifetime.d#L115

So basically, the reason why it happens is because the pointer you are 
changing is not the piece that actually is used to intialize the block.

On a side note however, you really shouldn't change data in a ClassInfo 
at all, and probably the compiler shouldn't let you! This is static 
readonly data, and changing this is I think even disallowed on some OSes.

Relevant source of how vtbls in D work:

https://dlang.org/spec/abi.html#classes

-Steve