int a = 1;
float b = 2.12f;
std::string c = "hello";

try {
  thebc::transaction t();
  t.add( a );
  t.add( b );
  t.add( c );

  a = fetch_a();
  b = fetch_b();
  c = fetch_c();

  t.commit();
}
catch(..) {
  ...
}