id in C++ class works from Objective-C

README.md

Build and test like so:

$ clang++ -std=c++11 test_class.cc main.mm -framework Foundation
$ ./a.out
2019-09-03 12:18:13.640 a.out[85486:6585983] The answer is 42

main.mm

#import <Foundation/Foundation.h>

#include "test_class.h"

int main() {
  TestClass tc;

  NSLog(@"The answer is %d", tc.id());
  return 0;
}

test_class.cc

#include "test_class.h"

int TestClass::id() const {
  return 42;
}

test_class.h

class TestClass {
 public:
  int id() const;
};