6 logger

[gwolf-011235]

Overview

1. Creates a Logger class which can be used to log messages
2. Used for logging is class LogData which has a templated << operator to receive data the same way as known from std::cout printing: << "logmessage: " << 1
3. Interface for Outputters and two implementations - console and file - which can be appended to route log messages. These Outputters also format the log message
4. Macros, init functions, namespace weblog
5. Various

1. Logger class

• Logger class is implemented as singleton. This means there should only be one instantiation and it is globally accessible. This is called an anti-pattern almost everywhere, except for very specific cases - like for example logging.
• The Logger::getInstance() has a static instance of the Logger and returns it. The first time calling this function the Logger will be constructed. Each call after will just return the reference.
• Logger has a LogLevel which can be DEBUG, INFO, WARN, ERROR.
• Logger has a vector of ILogOutputters, to register different Outputters (like console or file)
• Logger::init can be used to set LogLevel and Outputter
• The Logger receives its log via an overload of the += operator. It can receive a class LogData, which it will then pass onto all its registered Outputters

2. LogData class

• Container to hold LogData
  • timestamp : created in ctor
  • message : can be appended with <<
  • LogLevel
  • Name of function : passed via compiler macro
  • Line in the file : passed via compiler macro
  • Name of file : passed via compiler macro

3. ILogOutputter Interface

• Defines a LogOutputter
  • log() writes the message somewhere
  • getFormattedMessage() constructs the message via the LogData
• Two implementations: LogOutputterConsole and LogOutputterFile
  • Console: prints with color to std::clog or std::cerr depending on LogLevel
  • File: opens file and appends to it. Filename gets passed in ctor
• Outputters need to outlive Logger usage. Therefore their lifetime should also be static

4. Macros, Init, Namespace

• Many logging libraries provide Macros for easier usage, followed this approach
• Macros hides complicated call and makes it easier to use:
  • LOG_DEBUG, LOG_INFO, LOG_WARN, LOG_ERROR
  • Usage: LOG_DEBUG << "Hello my " << 1 << "friend"
• Two init functions to either init a file or console outputter. Both can be called to write to both, but every init function has one static Outputter -> two LogOutputterFile are not possible this way. But they can be appended via Logger::addLogOutputter
• Everything is in namespace weblog

5. Various

• Create sample implementation ofstream operator<< for ConfigFile, and its substructs. This way a ConfigFile config can be output like: LOG_DEBUG << config. Right now in LogConfigFile.cpp maybe other name? Needs to be updated if ConfigFile changes.
• Create new make target comp. This runs make clean and then bear -- make to create a new compile_commands.json

Closes #6