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SDL Core Guides

SDL Logger

By default, SDL Core uses the log4cxx framework for logging.

By implementing logger abstraction, SDL now provides the capability to replace the log4cxx logger with any other logging package-such as boost or syslog.


All components have access to the Logger interface and use it for logging.

Logger Interface

Logger macros use the Logger interface for sending messages to the External Logger.
The Logger interface contains only methods required by any SDL component to perform logging :

  • instance() - singleton access
  • PushLog(LogMessage)
  • IsEnabledFor(LogLevel)
  • DeInit()
  • Flush()

Logger Implementation

Logger interface is implemented by LoggerImpl.

LoggerImpl uses the message loop thread to proxy log messages to a third party (external) logger.
LoggerImpl owns ThirdPartyLoggerInterface and controls it's lifetime.
LoggerImpl provides implementation of the singleton pattern.

Message loop thread in SDLLogger

The message loop thread is needed to avoid significant performance degradation at run time as logging calls are blocking calls and might take a significant amount of time. LoggerImpl::PushLog is a non-blocking call. It will put the log message into the queue and returns immediately.

If ThirdPartyLoggerInterface supports non blocking threaded logging, minor changes in LoggerImpl can be made with use_message_loop_thread = false.

Logger singleton

Logger is the only singleton class in SDL. The singleton pattern is required to access the logger instance from any component.


Logger::instance() provides singleton by Logger interface. So SDL components do not have information about the logger implementation and the specific external logger.

Logger singleton with plugins

SDL plugins are shared libraries, so the Logger singleton could not be implemented with a Mayers singleton. A Mayers singleton would create an SDL logger instance for each plugin.

The idea is to pass a singleton pointer to each plugin during creation so that plugins can initialize the Logger::instance pointer with the instance received from SDL core.

Singleton Instance implementation

// ilogger.h
static Logger& instance(Logger* pre_init = nullptr);

// logger_impl.cc
Logger& Logger::instance(Logger* pre_init) {
  static Logger* instance_ = nullptr;
  if (pre_init) {
    assert(instance_ == nullptr);
    instance_ = pre_init;
  return *instance_;

pre_init is nullptr by default, so all components will access instance_ static pointer for logging.
The main() function will need to create a LoggerImpl object and call Logger::instance(logger implementation object);

Plugin implementation

extern "C" PluginType* Create(Logger* logger_singleton_instance) {
  return new PluginType();

SDL Core will pass a pointer to the logger singleton to the plugin so that the plugin shared library can initialize Logger::instance with the same pointer as the core portion.

Logger detailed design

Each source file creates logger_ variable via macro SDL_CREATE_LOG_VARIABLE.
This variable is actually a string with the component name of the logger.
Some logger implementations (like log4cxx) may have separate severity or destination rules for each component.

SDL implements all info required for log message :

  • LogLevel enum
  • Location info struct : location in the code
  • TimePoint

LoggerInitializer interface

LoggerInitializer specifies the interface required for main() to initialize the logger but is not required for any other SDL components.

LoggerInitializer::Init takes the third party logger implementation as an argument.
- Init(std::unique_ptr<ThirdPartyLoggerInterface>&& third_party)

ThirdPartyLogger interface

ThirdPartyLoggerInterface describes interfaces that should be implemented by the external logger adapter.

This interface should be inherited by external logger implementations.

Implementing another logger

To use another (not log4cxx) logger, you should:

  • Create a class which inherits from the ThirdPartyLoggerInterface class
AnotherOneLoggerImpl : ThirdPartyLoggerInterface {
  void Init() override;
  void DeInit() override;
  void IsEnabledFor(LogLevel) override;
  void PushLog(const LogMessage& log_message) override;

  void SomeCustomMethod(parameters);
  • Create an instance of the third party logger implementation(AnotherOneLoggerImpl) in main() and set it up for LoggerImpl.

Logger::instance does not own the logger instance. The main function is responsible for the sdl_logger_instance_ life-cycle.

// main.cpp
int main(argc, argv) {
    auto external_logger_ = std::make_unique<AnotherOneLoggerImpl>();
    auto sdl_logger_instance_ = new LoggerImpl(std::move(external_logger_));
  // Futher application code may use Logger::instance() for logging 

    delete sdl_logger_instance_;
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