How to enable and disable qDebug() messages inside a class

[ODБOï]

@jsulm Thx, but as i said I want do disable qDebug() for a particular class.
with QT_NO_DEBUG_OUTPUT i will disable for the whole application no ?

[jsulm]

@LeLev I don't understand the use case. How do you decide whether to activate debug output for a specific class and when do you decide (compile time? run time?)?

[ODБOï]

@jsulm sorry..
My probleme is : i have lot of messages, so i can't focus on the message i'm interested.

For the moment i'm testing my application, i just want to enable/disable output messages in a particular class, just before running the app.
So lets say i have a classes A, B and C. I want to disable messages comming from B and C, and only see messages from A.
Am I more understandable ?
Thx

[jsulm]

@LeLev

#define DEBUG
class A: public QObject
{
    Q_OBJECT
public:
       A();
    ~A(){}

    Q_INVOKABLE void connecToServer(){
        conn->start();
#ifdef DEBUG
            qDebug()<<"started !"
#endif
        }
    }
private:
  bool dbg = true;  
};

[ODБOï]

@jsulm Perfect ! Thx :)

[aha_1980]

Hi @LeLev,

you might also be interested in Qt's categorized logging function: http://blog.qt.io/blog/2014/03/11/qt-weekly-1-categorized-logging/

With that, you can enable/disable logging at runtime. You just need to replace qDebug by qCDebug and add appropriate categories.

Regards.

[ODБOï]

Thx @aha_1980 ! i will check that too.

[JonB]

@LeLev
You should probably also be aware of https://doc.qt.io/qt-5.10/qtglobal.html#qInstallMessageHandler

qInstallMessageHandler allows you to control what happens at run-time (instead of compile time) to anything going via qDebug(), qWarn() etc., whether from your own code or Qt internals.

[QtTester]

Still not perfect.
for example , I write a library and distribute to someone.
1、 I donot know what to set QCDebug() for final user's categorize.
2、I can tell someone set DEFINES += MYLIB_DEBUG to turn on the debug info, if we use :

#ifdef MYLIB_DEBUG
qDebug() <<"debug info";
#endif

it will get you crazy to add more and more #ifdef /#endif.
So i try:

#ifdef MYLIB_DEBUG
#define DBG std::cout
#else
#define DBG 0 && std::cout
#endif

// use like this,
DBG <<"my string";

Now it woks!
But sadly , when using qdebug(), it doesnot have a return value, so it compiled fail ？.

// compile fail:
#define DBG 0 && qDebug()

Or we can use :

#ifdef NO_DEBUG_ONE
    #define DBG(...)
#else
    #define DBG(x,...) qDebug(x,##__VA_ARGS__)
#endif

it's like printf, but we will be not able to use operator <<.

Is there a way to define qDebug like std::cout above???

[JonB]

@QtTester said in How to enable and disable qDebug() messages inside a class:

#define DBG 0 && std::cout

Pretty dodgy, IMHO!

Is there a way to define qDebug like std::cout above???

Did you try defining QT_NO_DEBUG_OUTPUT to the compiler? E.g. in the .pro file:

CONFIG(release, debug|release):DEFINES += QT_NO_DEBUG_OUTPUT

[QtTester]

@JonB
just disable qDebug in a.cpp , not in b.cpp ,c.cpp or other.cpp.

[JonB]

@QtTester said in How to enable and disable qDebug() messages inside a class:

just disable qDebug in a.cpp

So since you know it's a #define you know you can put #define QT_NO_DEBUG_OUTPUT as the first line in any .cpp file, before you #include any Qt stuff....

[QtTester]

@JonB Not work when add QT_NO_DEBUG_OUTPUT to cpp head, you can try.

[JonB]

@QtTester
Before I offer any further suggestions:

#define QT_NO_DEBUG_OUTPUT
#include <QApplication>
#include <QDebug>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qDebug() << "Hello world";
}

suppresses the qDebug() output for me (Linux, Qt 5.12)....

[QtTester]

@JonB

[JonB]

@QtTester
Please read and act on suggestions.

you know you can put #define QT_NO_DEBUG_OUTPUT as the first line in any .cpp file, before you #include any Qt stuff....

See the italics I had put in to make sure you got it right.

#define QT_NO_DEBUG_OUTPUT
#include <QApplication>

Please follow the instructions instead of ignoring them and doing your own thing.

[QtTester]

@JonB
you are right, it seems work in cpp, how about if we provide only a head file?

lib.h

#ifndef lib_h
#define lib_h
#define QT_NO_DEBUG_OUTPUT

class MyClass{
public :
    MyClass(){
    qDebug() <<"init";
    }
};

#endif

in a.cpp include the lib.h

#include "lib.h"
void main()
{
    // so we cannot enable this line anymore?
    qDebug()<<"main will shut down qdebug all";
}

[JonB]

@QtTester
Now you are getting demanding!

The effect of #if !defined(QT_NO_DEBUG_OUTPUT) is acted on in qloggingcategory.h (https://code.woboq.org/qt5/qtbase/src/corelib/io/qloggingcategory.h.html#121).

Since that, like all Qt header files, is inside a #ifndef QLOGGINGCATEGORY_H guard, that file is only read/included the first time that file is included into any particular source file. So switching QT_NO_DEBUG_OUTPUT on & off within one file won't have the effect you seem to want.

You could presumably achieve the same effect to "scope" the enablement/disablement with something based on:

// Next line at *beginning* of your header file
#undef qCDebug
#  define qCDebug(category, ...) QT_NO_QDEBUG_MACRO()

...

// Next line at *end* of your header file
#undef qCDebug
#  define qCDebug(category, ...) \
    for (bool qt_category_enabled = category().isDebugEnabled(); qt_category_enabled; qt_category_enabled = false) \
        QMessageLogger(QT_MESSAGELOG_FILE, QT_MESSAGELOG_LINE, QT_MESSAGELOG_FUNC, category().categoryName()).debug(__VA_ARGS__)

but it's getting messy, and relies on knowing what the definition of qCDebug is in Qt, which could change.

Better would be to write something other than qDebug() for whichever things you want to enable/disable, or use a dedicated category you define for those lines.

[QtTester]

@JonB
we discuss and you give solution, others will be benefited, tripartite win. :-)