Fixed dup sink compile warnings in older compilers with back_inserter

include/spdlog/sinks/dup_filter_sink.h

@@ -7,7 +7,7 @@
 #include <spdlog/details/null_mutex.h>
 #include <spdlog/details/log_msg.h>
 
-#include <iterator>
+#include <cstdio>
 #include <mutex>
 #include <string>
 #include <chrono>
@@ -62,11 +62,14 @@ protected:
 
         // log the "skipped.." message
         if (skip_counter_ > 0)
-        {
-            memory_buf_t buf;
-            fmt::format_to(buf, "Skipped {} duplicate messages..", skip_counter_);
-            details::log_msg skipped_msg{msg.logger_name, level::info, string_view_t{buf.data(), buf.size()}};
-            dist_sink<Mutex>::sink_it_(skipped_msg);
+        {         
+            char buf[64];
+            auto msg_size = ::snprintf(buf, sizeof(buf), "Skipped %u duplicate messages..", static_cast<unsigned>(skip_counter_));
+            if (msg_size > 0 && msg_size < sizeof(buf))
+            {
+                details::log_msg skipped_msg{msg.logger_name, level::info, string_view_t{buf, static_cast<size_t>(msg_size)}};
+                dist_sink<Mutex>::sink_it_(skipped_msg);
+            }
         }
 
         // log current message

tests/test_dup_filter.cpp

@@ -77,6 +77,7 @@ TEST_CASE("dup_filter_test5", "[dup_filter_sink]")
 
     dup_filter_sink_mt dup_sink{std::chrono::seconds{5}};
     auto test_sink = std::make_shared<test_sink_mt>();
+    test_sink->set_pattern("%v");
     dup_sink.add_sink(test_sink);
 
     dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "message1"});
@@ -85,4 +86,6 @@ TEST_CASE("dup_filter_test5", "[dup_filter_sink]")
     dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "message2"});
 
     REQUIRE(test_sink->msg_counter() == 3); // skip 2 messages but log the "skipped.." message before message2
+    REQUIRE(test_sink->lines()[1] == "Skipped 2 duplicate messages..");
+    
 }