using D with C++

[Lars T. Kyllingstad]

Nical wrote:
> Hi, I'm trying to use C/C++ functions in a D program but I cant get
> it to compile.
> I tried the exemple of digital mars's website:
> 
> // ---- file.cpp
> #include <iostream>
> using namespace std;
> int foo(int i, int j, int k)
> {
>     cout << "i = " << i << endl;
>     cout << "j = " << j << endl;
>     cout << "k = " << k << endl;
>     return 7;
> }
> 
> // ---- main.d
> extern (C++) int foo(int i, int j, int k);
> void main()
> {
>     foo(1,2,3);
> }
> 
> and a makefile that might not be correct now that i've experimented
> various things on it:
> 
> OBJ= main.o file.o
> 
> all: ${OBJ}
> 	g++ ${OBJ}
> 
> bin/main.o: file.o
> 	dmd main.d
> 
> bin/file.o:
> 	g++ -c file.cpp -o file.o
> 
> 
> 
> I get this error:
> 
> nico at nical-netbook:~/Programmation/D/test binding$ make
> g++    -c -o file.o file.cpp
> g++ main.o file.o -o test
> /usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/crt1.o: In
> function `_start':
> /build/buildd/eglibc-2.10.1/csu/../sysdeps/i386/elf/start.S:115:
> undefined reference to `main'
> main.o: In function `no symbol':
> main.d:(.text+0x8): undefined reference to `_Dmodule_ref'
> collect2: ld returned 1 exit status
> make: *** [all] Erreur 1
> nico at nical-netbook:~/Programmation/D/test binding$
> 
> thanks in advance
> 
> Nico

The problem is that the linker doesn't find the main() function.  I 
think the following is what happens when you write a D program and 
compile it with DMD:

   1. You write a D-style main() function.

   2. The D compiler automatically includes in the final executable
      a C-style main() function that calls your D main().  This is
      the actual starting point for your program, and the one the
      linker is looking for.

g++ unfortunately doesn't include the C main() function, because it 
doesn't know where to find it.  There are thus two ways to solve your 
problem:

a) Use DMD to link the files.  You then have to explicitly pass it the 
name of the C++ standard library as a linker option:

    g++ -c file.cpp -o file.o
    dmd -L-lstdc++ main.d file.o

b) Use g++ to link the files.  You then have to explicitly pass it the 
name and location of the D standard library and libpthread:

    g++ -c file.cpp -o file.o
    dmd -c main.d
    g++ -L/path/to/D/library -lphobos2 -lpthread main.o file.o

I recommend a).  Less typing.  :)

-Lars