Qt Object instantiation query

Question

QObject *obj; ... if ( /* obj is already instantiated */ ) { ; } else { obj = new QObject(); }

My query is the condition of the if

Chris · Accepted Answer · 2012-04-03 14:42:38Z

5

1) Initialize your object pointer to NULL

2) Check for NULL in your if statement

QObject *obj = NULL; ... if ( obj != NULL ) { ; } else { obj = new QObject(); }

answered Apr 3, 2012 at 14:42

Chris

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3 Comments

Timo Geusch Over a year ago

Actually, NULL is a C-ism. For a modern compiler, please use nullptr instead.

Dewsworld Over a year ago

I guess Qt discarded NULL. And ask to use 0 in stead. But ISO forbids initialization of members like error I get when using QObject *obj = 0;

Chris Over a year ago

If obj is a member variable, then it should be initialized to 0/NULL/nullptr (take your pick) inside of your class's constructor, preferably in the initializer list.

Samuel Harmer · Accepted Answer · 2012-04-03 15:15:00Z

0

QObject *obj = 0; // ... if (!obj) obj = new QObject();

Note: Makes no guarantee obj is not a dangling pointer.

edited Apr 3, 2012 at 15:15

answered Apr 3, 2012 at 14:48

Samuel Harmer

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Comments

Rafael Oliveira · Accepted Answer · 2014-01-14 21:38:23Z

Use QPointer class. http://qt-project.org/doc/qt-4.8/qpointer.html#data

Whenever you call deleteLater(); it will set it to 0, than you can check it.

QPointer<QWidget> someWidget = 0;

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