Calypso

Calypso is an experimental LDC fork that offers alternative interfacing to C++ based on Clang instead of implementing it "from scratch" as is currently being done in the official compiler. It attempts to widen and facilitate C++ support while minimizing intrusions into the code of the DMD frontend and LDC.

It is designed so that when the day comes splitting Calypso from LDC will be easy and Calypso could then exist as a LDC plugin in the form of a shared library.

Usage

modmap (C++) "<vector>"; // parses <vector>, doesn't import anything
modmap (C++) "<algorithm>";

import (C++) std.vector; // imports the std::vector class template
import (C++) std._ : find; // selectively imports the std::find function template, 
           // special modules named "_" group symbols from a namespace other than 
           // classes, structs, and enums

void main()
{
    import std.stdio : writeln;

    vector!char v; // both C++ classes and structs are value types, 
                   // like D structs and unlike D classes
    foreach (c; "XYZABC")
        v.push_back(c);
    writeln("v.size = ", v.size);

    auto it = find(v.begin, v.end, 'Z');
    it++;

    writeln(*it); // print 'A'
}

Advanced features

Calypso's features include:

• C++ class and struct construction and destruction
• Support of C++ classes making use of multiple inheritance
• D classes inheriting from C++ classes, and overriding C++ virtual methods by D methods (with the generation of C++ vtables)
• Template support, instantiation of C++ class and function templates by D code
• Overloaded operators, conversion operators
• Constant and empty preprocessor macros (mapped to enums)
• C++ exception catching (any type, not just std::exception)

Current status

Importing and using C++03 libraries is fully implemented (not C++0x/C++11 yet). However due to sometimes subtle differences between D and C++, to remaining bugs, or because importing a template-heavy library can be very testing for the DMD frontend (resulting in forward referencing errors), hitting a bug that makes the semantic pass fail is still a common occurrence for untested libraries. They are most of the time quickly fixable after investigating.

Calypso has been tested primarily on Linux. OS X has been barely tested but some examples have been reported to work, and MSVC support is still incomplete.

Libraries known to work well are:

• Qt 5 (both Widgets and Quick, see below)
• OpenCV 3.x
• Tesseract
• Ogre3D 1.x

This is by no means a exhaustive list of working libraries, only the ones which have been successfully used in a (commercial) project. Others have been reported to work in basic examples (Boost, GDAL, FastFlow).

Qt 5 tools

Using the C++ API of Qt 5 isn't straightforward even with Calypso, because Qt's meta-object system relies on code produced by the Moc code generator. Without Moc, classes cannot have new signals, slots and properties, cannot be constructed or used from QML, etc.

Another smaller issue is that Qt Designer for Widgets' .ui files are translated by uic into C++ headers. Although the generated C++ header could be loaded by Calypso, it's not possible to use custom widgets implemented in D.

To solve these two problems, a moc package that replicates the C++ Moc's functionality entirely with D's metaprogramming capabilities and a D generator for uic were created :

 https://github.com/Syniurge/Calypso-Qt

Missing features

• Register the destructor of C++ classes and structs while allocating a C++ class through the GC (as is being done for D structs)
• Automatically call copy constructors on function arguments
• Currently Calypso has only been truly tested on Linux

MSVC TODOs

• vtable generation for the C++ part of "hybrid" D classes deriving from a C++ class
• exception handling
• linker support in runtime/link_autogen_cpp.cmake

How to help

More testing is needed. If your code fails to compile and you don't intend to fix the code of Calypso yourself a good way to help is to open a Github issue, copy the failing code and provide the name and version of the library used, so investigating can begin right away.

Links

• Source code (GitHub)
• Catching C++ exceptions in D with Calypso