struct myStupidCustomString {
  myStupidCustomString(char const *what) : what (what) {}
  char const *what;
};

void throws() {
  throw myStupidCustomString("here is some really useful information");
}

// The external library can provide a function, or you can provide a wrapper, which
// extracts information from "unknown" exception types.
std::string extract_from_unknown_external_exception() {
  try { throw; }
  catch (myStupidCustomString &e) {
    return e.what;
  }
  catch (...) {
    throw;  // Rethrow original exception.
  }
}

typedef std::string Extract();
std::vector<Extract*> chain (1, &extract_from_unknown_external_exception);
// Chain would normally be initialized using whatever scheme you prefer for
// initializing global objects.
// A list or other container (including a manual linked list that doesn't
// require dynamic allocation) may be more appropriate, depending on how you
// want to register and unregister handlers.
std::string process_chain() {
  for (std::vector<Extract*>::iterator x = chain.begin(); x != chain.end(); ++x) {
    try {
      return (*x)();
    }
    catch (...) {  // That handler couldn't handle it.  Proceed to next.
    }
  }
  throw;  // None could handle it, rethrow original exception.
}

void example() {
  try { throws(); }
  catch (...) {
    try {
      std::string extracted = process_chain();
      std::cout << "extracted: " << extracted << '\n';
    }
    catch (...) {
      throw;  // Rethrow unknown exception, or otherwise handle it.
    }
  }
}

int main() {
  example();
  return 0;
}

extracted: here is some really useful information