DLF September 2023 Monthly Meeting Summary

[RazvanN]

On Monday, 13 November 2023 at 16:52:06 UTC, Paul Backus wrote:
> On Monday, 13 November 2023 at 16:23:05 UTC, Tim wrote:
>> The visitor can already be `extern(D)`. Only member functions 
>> overriding those in the base class need to be `extern(C++)`. 
>> Other member functions can then use paratemers, which would be 
>> incompatible with `extern(C++)`.
>
> Wow, it really was that simple all along. Thanks for the tip!

That's not a complete solution. If you template the visitor and 
have some member functions that override the extern(C++) base 
class methods, then you will still end up with errors if you use 
any types that use non-mapable C++ types because of the mangling. 
For example:

```d
extern(C++) class DMDVisitor
{
     void visit(Expression e) {}
}

class MyVisitor(T) : DMDVisitor
{
     T field;
     extern(C++) override void visit(Expression e) {}   // 
troubled line
}
```

Instantiating MyVisitor with a string type will issue an error at 
the troubled line because the mangler needs to take into account 
the template instance. Since C++ has no dynamic array 
correspondent you essentially cannot call MyVisitor.visit from 
C++ and therefore the compiler issues an error.

However, if you don't use templated visitors you are fine, you 
can just mark your introducing functions/fields as extern(D) and 
even if the class is extern(C++) it will work.