Add custom json format example

examples/CMakeLists.txt

@@ -23,6 +23,7 @@ set(EXAMPLE_TARGETS
         quill_example_user_defined_sink
         quill_example_tags_logging
         quill_example_json_console_logging
+        quill_example_json_console_logging_custom_json
         quill_example_csv_writing
         quill_example_json_file_logging
         quill_example_user_defined_types_logging

examples/json_console_logging_custom_json.cpp

@@ -0,0 +1,65 @@
+#include "quill/Backend.h"
+#include "quill/Frontend.h"
+#include "quill/LogMacros.h"
+#include "quill/Logger.h"
+#include "quill/sinks/JsonSink.h"
+
+#include <string>
+
+/**
+ * Overrides generate_json_message to use a custom json format
+ */
+class MyJsonConsoleSink : public quill::JsonConsoleSink
+{
+  void generate_json_message(quill::MacroMetadata const* /** log_metadata **/, uint64_t log_timestamp,
+                             std::string_view /** thread_id **/, std::string_view /** thread_name **/,
+                             std::string const& /** process_id **/, std::string_view /** logger_name **/,
+                             quill::LogLevel /** log_level **/, std::string_view log_level_description,
+                             std::string_view /** log_level_short_code **/,
+                             std::vector<std::pair<std::string, std::string>> const* named_args,
+                             std::string_view /** log_message **/,
+                             std::string_view /** log_statement **/, char const* message_format) override
+  {
+    _json_message.append(fmtquill::format(R"({{"timestamp":"{}","log_level":"{}","message":"{}")",
+                                          std::to_string(log_timestamp), log_level_description, message_format));
+
+    // Add args as key-values
+    if (named_args)
+    {
+      for (auto const& [key, value] : *named_args)
+      {
+        _json_message.append(std::string_view{",\""});
+        _json_message.append(key);
+        _json_message.append(std::string_view{"\":\""});
+        _json_message.append(value);
+        _json_message.append(std::string_view{"\""});
+      }
+    }
+  }
+};
+
+int main()
+{
+  // Start the backend thread
+  quill::BackendOptions backend_options;
+  quill::Backend::start(backend_options);
+
+  // Frontend
+
+  // Create a json sink
+  auto json_sink = quill::Frontend::create_or_get_sink<MyJsonConsoleSink>("json_sink_1");
+
+  // When logging json, it is ideal to set the logging pattern to empty to avoid unnecessary message formatting.
+  quill::Logger* logger = quill::Frontend::create_or_get_logger(
+    "json_logger", std::move(json_sink),
+    quill::PatternFormatterOptions{"", "%H:%M:%S.%Qns", quill::Timezone::GmtTime});
+
+  int var_a = 123;
+  std::string var_b = "test";
+
+  // Log via the convenient LOGJ_ macros
+  LOGJ_INFO(logger, "A json message", var_a, var_b);
+
+  // Or manually specify the desired names of each variable
+  LOG_INFO(logger, "A json message with {var_1} and {var_2}", var_a, var_b);
+}