On Monday, 13 December 2021 at 09:21:26 UTC, Jan wrote:
On Monday, 13 December 2021 at 07:48:34 UTC, evilrat wrote:
On Sunday, 12 December 2021 at 21:24:39 UTC, Jan wrote:
In D I have an extern(C++) class:
```cpp
extern(C++) class A
{
~this();
// other stuff
}
```
An a function that takes A by const reference:
```cpp
void CppFunc(const A& arg);
```
But how do I bind this in D ?
```cpp
extern(C++) void CppFunc(A arg); // tries to pass as 'A*'
extern(C++) void CppFunc(ref const(A) arg); // tries to pass
as 'A const * const &'
```
I have solved similar problems with other classes by
declaring them as struct in D, but that only works for
classes that have no virtual functions. I now have a class
where I do need to use a class on the D side, and now I have
problems passing these objects to C++.
You can tell compiler to mangle it as struct/class using
extern(C++, struct).
```d
extern (C++, struct) // will use struct mangling even though
it's a class
class SomeDClass
{
...
}
```
I tried this, but it doesn't work, because it seems D decides
how to pass the object by whether it is a class or struct in D,
not in C++. So even with the change as you suggested it, it
still tries to pass the object as a pointer to begin with.
You'll have to use something called a
[shim](https://en.wikipedia.org/wiki/Shim_(computing)), it seems.
For example:
`main.d` :
```d
extern(C++) class A{}
extern(C++) void cppFunc_shim(A arg);
void main(){
A a = new A();
cppFunc_shim(a);
}
```
`cppShim.cpp` :
```c++
class A{};
extern void cppFunc(A const &arg);
void cppFunc_shim(A *param){
const A forwardingVar = A(*param);
cppFunc(forwardingVar);
}
```
`cppFunc.cpp` :
```c++
#include "iostream"
class A{};
void cppFunc(A const &arg){
//std::cout << arg << std::endl;
std::cout << "Called cppFunc :D" << std::endl;
}
```
Then pass the following on the command line(assuming all files
are in the same directory):
`ldmd2 main.d cppFunc.o cppShim.o -L-lstdc++`
That's what it took to make it work for me, dunno if more
convenient methods exist.
Hope it helps :D
