Fix compiler warnings. (#9055)

Author: trivialfis

src/collective/aggregator.h

@@ -32,23 +32,23 @@ namespace collective {
  * @param function The function used to calculate the results.
  * @param args Arguments to the function.
  */
-template <typename Function, typename... Args>
-void ApplyWithLabels(MetaInfo const& info, void* buffer, size_t size, Function&& function,
+template <typename Function, typename T, typename... Args>
+void ApplyWithLabels(MetaInfo const& info, T* buffer, size_t size, Function&& function,
                      Args&&... args) {
   if (info.IsVerticalFederated()) {
     // We assume labels are only available on worker 0, so the calculation is done there and result
     // broadcast to other workers.
-    std::vector<char> message(1024);
+    std::string message;
     if (collective::GetRank() == 0) {
       try {
         std::forward<Function>(function)(std::forward<Args>(args)...);
       } catch (dmlc::Error& e) {
-        strncpy(&message[0], e.what(), message.size());
-        message.back() = '\0';
+        message = e.what();
       }
     }
-    collective::Broadcast(&message[0], message.size(), 0);
-    if (strlen(&message[0]) == 0) {
+
+    collective::Broadcast(&message, 0);
+    if (message.empty()) {
       collective::Broadcast(buffer, size, 0);
     } else {
       LOG(FATAL) << &message[0];
@@ -57,6 +57,5 @@ void ApplyWithLabels(MetaInfo const& info, void* buffer, size_t size, Function&&
     std::forward<Function>(function)(std::forward<Args>(args)...);
   }
 }
-
 }  // namespace collective
 }  // namespace xgboost