How does QMessageLogger magic work? - c++

Question

I am working on a logger framework for QT applications. I am not using QMessageLogger directly because of understanding and learning purposes. There is one thing about one QMessageLogger functionality that I would really like to have in my logger but I dont know how does it work. Lets take for example the qDebug macro:
#define qDebug QMessageLogger(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC).debug
One can call this function in 2 ways:
1st way:
qDebug("abc = %u", abc);
2nd way:
qDebug() << "abc = " << abc;
I am looking at the library code, but I cannot quite understand how is it implemented that one can work with QMessageLogger by using va_args as well as some stream object.
How can I achieve such effect? I would really appreciate all help, would be grateful for an example.
Here is my print method body. I need to achieve simmilar functionality with the "stream" way:
/*!
* \brief Adds the log line to the print queue.
* \param lvl: Log level of the line.
* \param text: Formatted input for va_list.
*/
void CBcLogger::print(MLL::ELogLevel lvl, const char* text, ...)
{
// check if logger initialized
if (!m_loggerStarted)
return;
// check if log level sufficient
if (lvl > m_setLogLvl)
return;
logLine_t logline;
logline.loglvl = lvl;
logline.datetime = QDateTime::currentDateTime();
va_list argptr;
va_start(argptr, text);
char* output = NULL;
if (vasprintf(&output, text, argptr))
{
logline.logstr = output;
delete output;
}
va_end(argptr);
emit addNewLogLine(logline);
}

Answer 1

First, you need to understand what is the following
QMessageLogger(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC).debug
The above line constructs a QMessageLogger instance and immediately accesses its debug member. Since it is a macro, it's also important what you write in code right after it.
If you look at what QMessageLogger::debug is, you'll see four overloads, and the first two of them are pertinent to your question:
void debug(const char *msg, ...) const Q_ATTRIBUTE_FORMAT_PRINTF(2, 3);
QDebug debug() const;
QDebug debug(const QLoggingCategory &cat) const;
QDebug debug(CategoryFunction catFunc) const;
Now the matter should be simple. If you call qDebug("abc = %u", abc), you're calling the first overload, and the expanded macro is as follows:
QMessageLogger(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC).debug("abc = %u", abc)
which is more or less equal to
QMessageLogger temp(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC);
temp.debug("abc = %u", abc);
In the second case you're calling an overload that returns a QDebug object. QDebug has overloaded operator<<. The expanded macro is as follows:
QMessageLogger(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC).debug() << "abc = " << abc;
which is more or less equal to
QMessageLogger temp(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC);
QDebug anotherTemp = temp.debug();
anotherTemp << "abc = " << abc;
Here's a simple implementation of such logger:
void addNewLogLine(char const* ptr){
cout << "addNewLogLine: " << ptr << endl;
}
struct LoggerHelper
{
std::stringstream s;
explicit LoggerHelper()=default;
LoggerHelper(LoggerHelper&&) = default;
~LoggerHelper(){
auto str = s.str();
addNewLogLine(str.c_str());
}
template<typename T>
LoggerHelper& operator<<(T const& val){
s << val;
return *this;
}
};
struct Logger
{
void operator()(char const* fmt, ...) const {
char* buf;
va_list args;
va_start(args, fmt);
vasprintf(&buf, fmt, args);
va_end(args);
addNewLogLine(buf);
free(buf);
}
LoggerHelper operator()() const {
return LoggerHelper{};
}
};
demo
Several notes:
I adhered to your interface, but personally, I'd use variadic templates instead of va_args
you're supposed to free the buffer returned by vasprintf. free is not interchangeable with delete or delete[]
I used std::stringstream, but changing it to QTextStream or any other should be simple enough
You don't need to implement helper as a separate class if you're okay with allowing log << "foo" << "bar" syntax as opposed to log() << "foo" << "bar"