Dynamically binding to D code using extern(D)

[Hipreme]

I write this post as both a learning tool, a question and an 
inquiry.

There are just a lot of drawbacks in trying to do function 
exporting while using D.

That interface is absurdly confuse and that is probably why I've 
never seen a project here which made an use of extern(D) while 
using a DLL.

While I'm making my DLL's generation, there are a lot of pitfalls 
that I can feel into.

**Simple Function**

```

module something;
extern(D) export int sum(int a, int b){return a + b;}
```

The correct way to bind to that function would be:

```

module app;
import core.demangle

int function(int a, int b) sum;

void main()
{
     sum = cast(typeof(sum))GetProcAddress(someDll, 
mangleFunc!(typeof(sum)("something.sum");
}

```

And that should be it for loading a simple function.

Now, lets make our case a bit more complicated:

**Overloaded function**


```

module something;

extern(D) export int add(int a, int b)
{
     return a + b;
}

extern(D) export float add(float a, float b)
{
    return a+b;
}
```


For loading those functions, the correct way would be

```

module app;
import core.demangle;


int function(int a, int b) sumInt;
float function(float a, float b) sumFloat;

int sum(int a, int b){return sumInt(a, b);}
float sum(float a, float b){return sumFloat(a,b);}

void main()
{
     sumInt = cast(typeof(sumInt))GetProcAddress(dll, 
mangleFunc!(typeof(sumInt))("something.sum"));
     sumFloat = cast(typeof(sumFloat))GetProcAddress(dll, 
mangleFunc!(typeof(sumFloat))("something.sum"));
}
```

Notice how much the overall complexity starts to increase as 
there seems to be no way to put get the overloads and there 
doesn't seem to be any advantage in using extern(D).


**Static Methods**

The only difference from the default functions is that we need to 
pass the class name as a module name.


**Static Methods returning user data**

That is mainly the reason I'm writing that post. It made me 
really wonder if I should really use extern(D).


This section will use 3 files because after all, there is really 
a (consistency?) problem


```
module supertest;
import ultratest;

class SuperTest
{
    extern(D) export static SuperTest getter(){return new 
SuperTest();}
    extern(D) export static UltraTest ultraGetter(){return new 
UltraTest();}

    import core.demangle;

    pragma(msg, 
mangleFunc!(typeof(&SuperTest.getter))("supertest.SuperTest.getter"));
    //Prints _D9supertest9SuperTest6getterFZCQBeQx
    pragma(msg, 
mangleFunc!(typeof(&SuperTest.ultraGetter))("supertest.SuperTest.ultraGetter"));
    //Prints 
_D9supertest9SuperTest11ultraGetterFZC9ultratest9UltraTest

}
```

```
module ultratest;
class UltraTest{}
```

```
module app;
import core.demangle;

void main()
{
    //???

}
```

As you can see at module supertest, the pattern seems to break 
when returning user data
for another module. From my knowledge, I don't know how could I 
get this function, specially because you will need to know: the 
module that you're importing the function + the module that where 
the userdata is defined for getting it.


It seems pretty insane to work with that.


extern(D) advantages:

-

extern(D) disadvantages:

- Code only callable in D(probably no other language as a 
demangler)
- I don't remember seeing any other code before in that post 
doing that, so, no documentation at all
- You will need to call the demangler for binding to a symbol, 
which in my project,  it could make each call to a unique type 
from the demangler costs 15KB
- You will need to know the module which you imported your 
function
- If your function returns userdata from another function, there 
doesn't seem to be any workaround
- Doesn't provide any overloading binding support though the 
language has support to overloading


extern(C) advantages:

- Code callable from any language as it is absolutely intuitive
- Well documented

extern(C) disadvantages:

- You will need to declare your function pointer as extern(C) or 
it will swap the arguments order.



I have not even entered in the case where I tried overloading 
static methods, which I think it would need to declarate aliases 
to the static methods typings for actually generating a mangled 
name.

I want to know if extern(D) is actually meant to not be touched. 
adr said that his use for that was actually when doing


extern(C):
//Funcs defined here


extern(D): //Resets the linkage to the default one


So, there are just too many disadvantages for doing extern(D) for 
binding it to any code, I would like to know where we can get 
more documentation than what I posted here right now (really, 
I've never saw any code binding to an extern(D) code). And I do 
believe that is the main reason why people usually don't use 
dynamic libs in D, it is just inviable as you would need to 
regenerate all the API yourself