As per the thread in Firebase for Games, I'm updating the sample to work verbatim. I've made the following changes:

I've replaced printf with std::cout (this is more C++, and should avoid code safety warnings on some compilers).
Rather than spinning on sign_in_future.status() and calling a non-existent function Wait, I register an OnCompletion listener as a lambda that captures no scope (for performance optimists out there, may avoid a heap allocation).
I made the type of the future explicit (the code as is wouldn't compile). It would read better as auto, but I think having a full type here is better for educational purposes.
All code after the spin lock-ish pattern is moved into the lambda.
I changed user->Anonymous() (doesn't exist) to user->is_anonymous().

I did my best to match the c++ style guide in the comment, but I expect that there'll be a critique or two.

I have verified the code working in a minimalist sample application.

I've just grabbed the suggested reviewers, sorry if this is out of nowhere!

PiperOrigin-RevId: 259400353

Author: Googler

auth/src/include/firebase/auth.h

@@ -91,28 +91,33 @@ class PhoneAuthProvider;
 ///
 /// For example:
 /// @code{.cpp}
-///  // Get the Auth class for your App.
-///  firebase::auth::Auth* auth = firebase::auth::Auth::GetAuth(app);
 ///
-///  // Request anonymous sign-in and wait until asynchronous call completes.
-///  firebase::Future<firebase::auth::User*> sign_in_future =
-///      auth->SignInAnonymously();
-///  while (sign_in_future.status() == firebase::kFutureStatusPending) {
-///    Wait(100);
-///    printf("Signing in...\n");
-///  }
+/// // Get the Auth class for your App.
+/// firebase::auth::Auth* auth = firebase::auth::Auth::GetAuth(app);
 ///
-///  // Print sign in results.
-///  const firebase::auth::AuthError error =
-///      static_cast<firebase::auth::AuthError>(sign_in_future.error());
-///  if (error != firebase::auth::kAuthErrorNone) {
-///    printf("Sign in failed with error `%s`\n",
-///           sign_in_future.error_message());
-///  } else {
-///    firebase::auth::User* user = *sign_in_future.result();
-///    printf("Signed in as %s user.\n",
-///           user->Anonymous() ? "an anonymous" : "a non-anonymous");
-///  }
+/// // Request anonymous sign-in and wait until asynchronous call completes.
+/// firebase::Future<firebase::auth::User*> sign_in_future =
+///     auth->SignInAnonymously();
+/// while(sign_in_future.status() == firebase::kFutureStatusPending) {
+///     // when polling, like this, make sure you service your platform's
+///     // message loop
+///     // see https://github.com/firebase/quickstart-cpp for a sample
+///     ProcessEvents(300);
+///     std::cout << "Signing in...\n";
+/// }
+///
+/// const firebase::auth::AuthError error =
+///     static_cast<firebase::auth::AuthError>(sign_in_future.error());
+/// if (error != firebase::auth::kAuthErrorNone) {
+///     std::cout << "Sign in failed with error '"
+///         << sign_in_future.error_message() << "'\n";
+/// } else {
+///     firebase::auth::User* user = *sign_in_future.result();
+///     // is_anonymous from Anonymous
+///     std::cout << "Signed in as "
+///         << (user->is_anonymous() ? "an anonymous" : "a non-anonymous")
+///         << " user\n";
+/// }
 /// @endcode
 /// @endif
 class Auth {