Calypso: Direct and full interfacing to C++

[Elie Morisse via Digitalmars-d-announce]

On Wednesday, 13 May 2015 at 15:54:47 UTC, John Colvin wrote:
> That's great! I'm looking forward to being able to easily make 
> direct use of some of the great C++ code out there.
>
> Are there are performance pitfalls to watch out for that are 
> unique to the way calypso interfaces between D and C++? E.g. 
> sneaky copies, implicit callbacks to keep things synced etc.

There are a few:

- Many simple non-POD C++ classes are being mapped to D classes 
instead of structs, for example QFlags<AlignmentFlag> (qt5demo.d 
uses its alias Qt::Alignment) which is a wrapper around a single 
int but contains an overloaded assignment operator so is 
considered non-POD in C++. Same story for QString.
string, vector, almost all STL types are non-POD too because they 
have base classes.
So this may be inconvenient when trying to avoid heap 
allocations.. For QFlags and QString we could do more than just 
checking if the class/struct is POD according to the C++ 
definition (i.e we could ignore overloaded operators, which are 
allowed in D structs), but STL types will most likely stay that 
way.

- For the time being when calling a C++ function taking a class 
value argument with FunctionB(new ClassA(...)), the ClassA 
instance is 1/GC-allocated 2/ctor is called 3/copied to the 
stack-allocated memory for the call. Whereas in C++ 
FunctionB(ClassA(...)) the ClassA constructor is directly called 
over the stack-allocated memory. In D there's no way to avoid the 
copy yet, I think we could mimic the C++ way but that'd involve 
another small alteration to the language.

That's about it, I can't think of anything else, what's cool is 
that almost all symbols Calypso exposes are the C++ symbols 
themselves with no wrapper layer in-between, with a few 
exceptions such as overloaded operators opBinary!"+" etc. which 
dispatch the call to the C++ operator mapped to a non-templated 
function (because the C++ overloaded operator might be virtual, 
it can't be mapped directly to a template).