Using C++ class in D

Question

I am trying to find a way to use C++ classes in D.

http://www.digitalmars.com/d/2.0/cpp_interface.html

D cannot call C++ special member functions, and vice versa. These include constructors, destructors, conversion operators, operator overloading, and allocators.

So, I am attempting to dumb down these C++ functions to C style function calls. Here is the proof I am working with.

helper.h

class someClass {
    public:
        someClass();
        char *whatSayYou();
};

extern "C"
{
    someClass *hearMeOut();
}

helper.cpp

#include "helper.h"

someClass::someClass()
{

}

char *someClass::whatSayYou()
{
    return "Everything is gravy";
}


someClass *hearMeOut()
{
    return new someClass;
}

main.d

import std.stdio;

int main(string[] args)
{
    someClass *awesomeExample = hearMeOut();
    char *shoutoutToTheWorld = awesomeExample.whatSayYou();
    writefln(std.string.toString(shoutoutToTheWorld));
    return 0;
}


extern (C++)
{
    interface someClass
    {
        char *whatSayYou();
    }

    someClass *hearMeOut();
}

And here is how I complied it.

g++-4.3 -c -I code/dg3d_helper -I /usr/local/include/ -o code/dg3d_helper/helper.o code/dg3d_helper/helper.cpp
code/dg3d_helper/helper.cpp: In member function ‘char* someClass::whatSayYou()’:
code/dg3d_helper/helper.cpp:19: warning: deprecated conversion from string constant to ‘char*’
gdc-4.3 -g -c -I code/ -o code/main.o code/main.d
gdc-4.3 -g -I code/ -o main code/dg3d_helper/helper.o code/main.o -lstdc++

And I get a segmentation fault as soon as the method is called.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000402fa0 in _Dmain (args=...) at code/main.d:7
7       char *shoutoutToTheWorld = awesomeExample.whatSayYou();
(gdb) bt
#0  0x0000000000402fa0 in _Dmain (args=...) at code/main.d:7
#1  0x000000000041b1aa in _D9dgccmain211_d_run_mainUiPPaPUAAaZiZi2goMFZv ()
#2  0x000000000041b235 in _d_run_main ()
#3  0x00002aaaab8cfc4d in __libc_start_main () from /lib/libc.so.6
#4  0x0000000000402d59 in _start ()

Answer 1

Your C++ version returns by value.

Your D version expects it to return by reference.

Essentially, your C++ version sticks a copy of someClass on the stack. D thinks that C++ will have put a pointer on the stack. It tries to interpret the copy of someClass as a pointer, and bad things happen.

The problem is that in D, classes and interfaces are always returned by reference. C++ returns everything by value unless you indicate that its either a reference or a pointer.

Thus you need this:

someClass * hearMeOut() { return new someClass; }

Don't forget to delete it later.