Wrapping a C++ object in extern "C"

Question

consider a simple example class:

class BankAccount {
public:
   BankAccount() { balance =0.0; };
  ~BankAccount() {};
   void deposit(double amount) {
      balance += amount;
   }
   private:
      double balance;
};

Now say I want to wrap this in extern "C" so that I can call it from many different programming languages such as C# and Java. I tried the following which seemed to work:

// cbankAccount.h:
extern "C" unsigned long createBackAccount(); 
extern "C" void deposit(unsigned long bankAccount, double amount);
// cbankAccount.cpp
unsigned long createBackAccount() {
  BankAccount *b = new BankAccount();
  return (unsigned long) b;
}
void deposit(unsigned long bankAccount, double amount) {
  BankAccount *b = (BankAccount*) bankAccount;
  b->deposit(amount);
} 

Is this portable? Is the type unsigned "unsigned long" large enough for an object pointer? Any other problems with this approach?

Thank in advance for any answers!

Answer 1

Yeah. It's bad. Really bad. unsigned long- just no. Return a properly typed BankAccount*- the other languages will see it on the other end as a generic pointer (such as System.IntPtr) and there's no need to return an untyped pointer when the binary interface doesn't type pointers anyway.

extern "C" BankAccount* CreateBankAccount() {
    return new BankAccount;
}
extern "C" void deposit(BankAccount* account, double amount) {
    account->deposit(amount);
}

Answer 2

That basically looks fine with one proviso. Trying to use an integer type to hold a pointer is not a great idea—much better to use void* since that, by definition, is the width of a pointer.

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Actually, I think @DeadMG's answer is a cleaner approach than this.