Linking to proprietary C++ library in old ABI format using gc c-3. 2

[Eljay Love-Jensen]

Hi Nic,

>How would this thunk layer work ?

The C-ABI thunk layer acts as a bridge from the disparate C++-ABI's of 2.95 and 3.2.

The effect:
[1] <--> [2] <--> [3] <--> [4]

1) C++ 2.95 library
2) C thunk library
3) C++ 3.2 facade library
4) your application

>The first thing our product architect asked me is what would happen if their were any static initializers in the library being thunked ?

The static initializers should still get called when the library is loaded.  Like normal.

>How does one even get at a C++ object from C ?

namespace Database
{

class Foo
{
  int m;
public:
  Foo() : m(0) { }
  void reset(int i) { m = i; }
 ... };

}

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// C-ABI thunk layer knows about the Database namespace, but does not expose the Database namespace.

extern "C" void* NewFoo();
extern "C" void FooReset(void* p, int i);
extern "C" int GetError();

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static int gError = 0;

// Foo factory.
void* NewFoo()
{
  try
  {
    Foo* foo = new Foo;
  }
  catch(...)
  {
    // You'd want something a little more robust than this.
    gError = 1;
  }
}

// Thunk for Foo's reset method.
void FooReset(void* p, int i)
{
  Foo* foo = (Foo*)p;
  foo->reset(i);
}

// Exception helper.
int GetError()
{
  return gError;
}

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namespace Facade
{

struct ThunkError { };

class Foo
{
  void* foo;
public:
  Foo();
  void reset(int i);
};

Foo::Foo()
: foo(NewFoo())
{
  if(GetError()) throw ThunkError;
}

void Foo::reset(int i)
{
  FooReset(foo, i);
  if(GetError()) throw ThunkError;
}

}

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Each database class would need it's own Facade (or proxy) counterpart.

Each method and constructor (that you utilize) would need a C-ABI thunk routine.

It's not actually very hard to do; but it is a lot of rote work.

The jagged edge is making sure you handle exceptions correctly (usefully).  And remember:  exceptions CANNOT pass through a C-ABI barrier!

HTH,
--Eljay