How to add simple custom exception handler?

[arcull]

Hi everyone. I would like to create a custom class for known exception (extending QtConcurrent::Exception), but unfortunately I'm missing a few puzzles... Here is what I have s far illustrating what I would like to achieve.

class myException: public QtConcurrent::Exception {
public:
    myException(const char *desc) {
        QMessageBox::critical(NULL,QObject::tr("Warning"),QObject::tr(desc));
    }
 
    ~myException() throw();
 
};

class myException: public QtConcurrent::Exception {
public:
    myException(const char *desc) {
        QMessageBox::critical(NULL,QObject::tr("Warning"),QObject::tr(desc));
    }

    ~myException() throw();
    
};

To copy to clipboard, switch view to plain text mode 

in a function I try to raise exception like this:

int b = 0;
if (b == 0)
  myException *exc = new myException("division with 0!");

int b = 0;
if (b == 0)
  myException *exc = new myException("division with 0!");

To copy to clipboard, switch view to plain text mode 

but can't make it compile, I get:
error: undefined reference to `vtable for myException'. Can you please correct this sample so that it works, much thanks.

[anda_skoa]

You declare a destructor for myException but you are not defining it (missing body).

Also using a message box in the excepion's constructor does not make sense, especially since this is concurrent exception.

Cheers,
_

[arcull]

You declare a destructor for myException but you are not defining it (missing body).

ok I probably don't need a destructor in my case

Also using a message box in the excepion's constructor does not make sense, especially since this is concurrent exception.

well it may be a bad idea, however I would like to show a message with description of what went wrong to the user, should I do it some other way? Please provide a short sample code.. I've changed my sample like this:

class myException: public QtConcurrent::Exception {
    public:
    myException(const char *desc) {
    //QMessageBox::critical(NULL,QObject::tr("Warning"),QObject::tr(desc));
    qDebug() << desc;
    }        
    };

class myException: public QtConcurrent::Exception {
    public:
    myException(const char *desc) {
    //QMessageBox::critical(NULL,QObject::tr("Warning"),QObject::tr(desc));
    qDebug() << desc;
    }        
    };

To copy to clipboard, switch view to plain text mode 

and the calling function

myException exc("division with 0");
throw exc;

myException exc("division with 0");
throw exc;

To copy to clipboard, switch view to plain text mode 

Which works so so..., if I leave the messagebox, this gets displayed but soon afterwards the whole app crashes with something like "Microsoft Visual C++ Runtime Library: This application has requested the Runtime to terminate in an unusual way." If I use qDebug instead, the message is displayed only in "application output window", but still crashes the same way. I guesss I'm doing something wrong here, please show the right direction, a snippet of code would be useful, thanks.

[anda_skoa]

however I would like to show a message with description of what went wrong to the user, should I do it some other way?

Handling an exception is what the catch block is for.

How does your's look?

Cheers,
_

[arcull]

I was trying to handle exceptions in few functions and forgot to add a try-catch block in one of them, that's why my app crashed with "Microsoft Visual C++ Runtime Library:blah blah".My working sample now looks like this:

class myException: public QtConcurrent::Exception {
public:    
    myException(const char *desc) {
        excdesc = desc;
    }
    const char *excdesc;
};

class myException: public QtConcurrent::Exception {
public:    
    myException(const char *desc) {
        excdesc = desc;
    }
    const char *excdesc;
};

To copy to clipboard, switch view to plain text mode 

and a part of one function

try {
	//some other code...
        int b = 0;
        if (b == 0) {
            myException exc("you can not divide with 0");
            throw exc;
	}
} catch (myException & e) {
       QMessageBox::critical(this,tr("Warning"),tr(e.excdesc));
}

try {
	//some other code...
        int b = 0;
        if (b == 0) {
            myException exc("you can not divide with 0");
            throw exc;
	}
} catch (myException & e) {
       QMessageBox::critical(this,tr("Warning"),tr(e.excdesc));
}

To copy to clipboard, switch view to plain text mode 

which works ok, however I see little point have an extra class myException for as little as showing a simple error description. Couldn't I do just something like this:

try {
   //some other code...
    int b = 0;
    if (b == 0) {           
      throw "you can not divide with 0";
}
} catch (char *e) {
       QMessageBox::critical(this,tr("Warning"),tr(e));
}

try {
   //some other code...
    int b = 0;
    if (b == 0) {           
      throw "you can not divide with 0";
}
} catch (char *e) {
       QMessageBox::critical(this,tr("Warning"),tr(e));
}

To copy to clipboard, switch view to plain text mode 

but my app crashes this way. Please suggest, thanks.

[anda_skoa]

QtConcurrent::Exception is for throwing expections in a threadpool worker thread but catching it in the calling thread (usually the main thread).

If all you need is a thread local exception, just use std::exception http://www.cplusplus.com/reference/exception/exception/

Cheers,
_

[arcull]

QtConcurrent::Exception is for throwing expections in a threadpool worker thread but catching it in the calling thread (usually the main thread).

If all you need is a thread local exception, just use std::exception http://www.cplusplus.com/reference/exception/exception/

Cheers,
_

Thanks I agree, but currently I just don't see how to pass a custom error message to the user this way.

QMessageBox::critical(this,tr("Warning"),tr(e.what()));

QMessageBox::critical(this,tr("Warning"),tr(e.what()));

To copy to clipboard, switch view to plain text mode 

but how do I put custom string to "what" when throwing exceptions. Thanks again.

[stampede]

how do I put custom string to "what" when throwing exceptions

how do I put custom string to "what" when throwing exceptions

To copy to clipboard, switch view to plain text mode 

via constructor

void fn(){
    throw std::runtime_error("this is my message");
}
 
or 
 
class my_exception : public std::exception{
public:
    my_exception(const QString& string){
        _data = string.toLocal8it();
    }
    const char * what() const noexcept{
        return _data.constData();
    }
private:
    QByteArray _data;
};
...
void fn(){
    throw my_exception(QObject::tr("this is my localized message!"));
}

void fn(){
    throw std::runtime_error("this is my message");
}

or 

class my_exception : public std::exception{
public:
    my_exception(const QString& string){
        _data = string.toLocal8it();
    }
    const char * what() const noexcept{
        return _data.constData();
    }
private:
    QByteArray _data;
};
...
void fn(){
    throw my_exception(QObject::tr("this is my localized message!"));
}

To copy to clipboard, switch view to plain text mode 

[Infinity]

Simply reimplement the virtual what method of your exception subclass.
You can also extend the constructor to take an error message, store it and return it in the what reimplementation (like in std::logic_error).

[arcull]

Thanks both, ok I'll have to create my custom class extending std::exception. Good, case solved