Add missing virtual destructors for App Check (#1099)

Author: a-maurice

app_check/src/common/app_check.cc

@@ -52,6 +52,11 @@ DEFINE_FIREBASE_VERSION_STRING(FirebaseAppCheck);
 Mutex g_app_check_lock;  // NOLINT
 static std::map<::firebase::App*, AppCheck*>* g_app_check_map = nullptr;
 
+// Define the destructors for the virtual listener/provider/factory classes.
+AppCheckListener::~AppCheckListener() {}
+AppCheckProvider::~AppCheckProvider() {}
+AppCheckProviderFactory::~AppCheckProviderFactory() {}
+
 AppCheck* AppCheck::GetInstance(::firebase::App* app) {
   MutexLock lock(g_app_check_lock);
   if (!g_app_check_map) {

app_check/src/include/firebase/app_check.h

@@ -58,7 +58,7 @@ struct AppCheckToken {
 
 /// @brief Base class used to receive messages when AppCheck token changes.
 class AppCheckListener {
-  virtual ~AppCheckListener();
+  virtual ~AppCheckListener() = 0;
   /// This method gets invoked on the UI thread on changes to the token state.
   /// Does not trigger on token expiry.
   virtual void OnAppCheckTokenChanged(const AppCheckToken& token) = 0;
@@ -71,7 +71,7 @@ class AppCheckListener {
 /// call and returning that new token if it can.
 class AppCheckProvider {
  public:
-  virtual ~AppCheckProvider();
+  virtual ~AppCheckProvider() = 0;
   /// Fetches an AppCheckToken and then calls the provided callback method with
   /// the token or with an error code and error message.
   virtual void GetToken(
@@ -82,7 +82,7 @@ class AppCheckProvider {
 /// Interface for a factory that generates {@link AppCheckProvider}s.
 class AppCheckProviderFactory {
  public:
-  virtual ~AppCheckProviderFactory();
+  virtual ~AppCheckProviderFactory() = 0;
   /// Gets the {@link AppCheckProvider} associated with the given
   /// {@link App} instance, or creates one if none
   /// already exists.