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c++qtqdebug

What is the difference between qDebug() used as a stream and as a function

I have seen bits of Qt code that uses qDebug as if it were printf()

qDebug( format, ... );

Mostly i see it used like std::cout

qDebug() << "one " << var_one;

What is the difference in the usages and when is it correct/better to use one of the other? The Qt help online somehow seems to reference the function version but never explain it.

Solution

  • qDebug(pattern, object1, object2) it's basically the old fashioned fprintf(stderr, pattern, object1, object2), as such you depend on compiler support to avoid - for instance - to crash your program with wrong patterns, like int x; qDebug("%s\n", x);. Well, GCC catches this one, but the compiler cannot always know if the pattern is appropriate, I think.

    I always use qDebug() << object << ...;, as the documentation states

    If you include QtDebug, a more convenient syntax is also available:

    qDebug() << "Brush:" << myQBrush << "Other value:" << i;

    With this syntax, the function returns a QDebug object that is configured to use the QtDebugMsg message type. It automatically puts a single space between each item, and outputs a newline at the end. It supports many C++ and Qt types.

    you can pass most of Qt objects to qDebug() << ... and get them rendered in readable way

    try for instance qDebug() << QTime::currentTime();