How to handle failures on construction?

Steve7b1

Hi,

I am concerned about resource leaks on construction; suppose that I have a widget that I am adding children to, and something throws an exception... will there be a leak of the child in the following situation?

class X: public QWidget
{
public:
    X() : QWidget(nullptr)
    {
        QPushButton * b = new QPushButton ;
        b->setParent(this) ;

         // suppose something down here throws...
    }
}

// client code:
    X *x = nullptr  ;
    try
    {
        x = new X ;
    }
    catch (...) 
    {
        // ...
    }

this is a simplified example with no layouts or anything, I guess the question boils down to whether or not the widget memory management cleanup for X will be called without a call to the destructor of X?

[Edit: Added code tags -- @Wieland]

mrjj

Hi
Nothing cleans up without the call to the dtor.
but if u do QPushButton * b = new QPushButton(this) ; (or setParent)
Then X will be deleted and all children.
If X has no parents, it might be issue.
Its a system.
http://doc.qt.io/qt-5/objecttrees.html

kshegunov

@Steve7b1 said in How to handle failures on construction?:

I am concerned about resource leaks on construction; suppose that I have a widget that I am adding children to, and something throws an exception... will there be a leak of the child in the following situation?

My advice - don't throw from the constructor. It's a bad habit in my opinion and you can easily go without it. As a side note Qt will not throw from the constructor, so any allocations you make are "safe".

Steve7b1

@mrjj Hi,

Thanks for your reply; I don't understand though why the child would be cleaned up in the example I gave, because the destructor isn't being called if we throw from within the constructor... could you expand a little?

Steve.

mrjj

Hi
QPushButton * b is owned by class X

so when X is deleted then the childs are deleted too.

Often Class X is owned by main window or other toplevel parent.
So when its deleted, Class X is freed and so is button b.

But yes, if Class X have no parent and u throw in ctor. then you got a leak :)
"It cannot die, for it never lived." Kinda situation which can be avoided by not throwing in ctor.

Steve7b1

@kshegunov Thanks for your reply. So would you suggest doing allocations with

QWidget * p = new (std::nothrow) QWhatever(this) ;

and then just checking the pointer value?

Steve.

SGaist

@Steve7b1 Hi, do you have any condition in mind that would make one of your constructors throw an error ?

Steve7b1

@SGaist I am mostly concerned about allocation failures; rare, but possible. I work on a system which is to do with visualisation of large datasets. Most of my usage is related to QGraphicsView and QGraphicsScene (not much like the example I gave, that was just hypothetical).

Our users often leave sessions going for weeks, just closing their laptops and going home, so I have to maintain certain levels of paranoia...

kshegunov

No, I suggest not throwing from the constructor (and ignoring the "out of memory" exception which you can do little to prevent). Also if you want to be throw-safe then don't use heap allocation (especially since in apps you're not concerned with binary compatibility). E.g.:

class X: public QWidget
{
    Q_OBJECT  //< Don't forget this one
public:
    X(QWidget * parent = nullptr)  //< Pass on the parent, don't work against the framework!
        : QWidget(parent), b(this)
    {
         // Suppose something down here throws ...
         // ... then the stack is automatically unrolled as per documented C++ behaviour
    }
private:
    QPushButton b; //< Can't get more throw-safe than this!
};

Steve7b1

OK, I understand your strategy; thanks for the advice.

Steve.