Difference between QSharedPointer::isNull() and operator!()

Question

in Qt docs we read:

bool QSharedPointer::operator! () const

Returns true if this object is null. 
This function is suitable for use in if-constructs, like:

 if (!sharedptr) { ... }

and

bool QSharedPointer::isNull () const
Returns true if this object is holding a reference to a null pointer.

What is the difference between these two functions? This is clear what is reference to a null pointer, but what means here

"if the object is null" ?

What determines if QSharedPointer is null? How do these functions correspond to QSharedPointer::data() != null ?

I would say both are equivalent. If you check the source, it might even be that one of the functions call the other. — Some programmer dude, Nov 12 '13 at 09:47
From Qt sources of `QSharedPointer` class: `inline bool operator !() const { return isNull(); }` - confirms what @JoachimPileborg said. — vahancho, Nov 12 '13 at 09:58
@vahancho you can put it as an answer, this is what I was looking for — 4pie0, Nov 12 '13 at 10:40
Some coding standards frown upon verbosity, others enshrine it. Qt caters to both crowds, hence the two methods. — MSalters, Nov 12 '13 at 12:26

score 6 · Accepted Answer · answered Nov 12 '13 at 10:49

6

From Qt sources of the QSharedPointer class:

inline bool operator !() const { return isNull(); }

This confirms what @JoachimPileborg said in his comment - isNull() function and operator!() are equivalent.

answered Nov 12 '13 at 10:49

vahancho

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Frank Osterfeld · Answer 2 · 2016-05-09T17:57:53.317

3

A "null" QSharedPointer wraps a T* t where t equals 0/NULL/nullptr. That's what's meant with "object is null"

isNull() and operator!() are equivalent, you can use either one.

A shared pointer is null by default, or when set to 0/nullptr explicitly:

QSharedPointer<T> t; //null
QSharedPointer<T> t2(new T); //not null
QSharedPointer<T> t3(0); //null
QSharedPointer<T> t4(nullptr); //null
t2.clear(); //not null before, now null

edited May 09 '16 at 17:57

answered Nov 12 '13 at 09:51

Frank Osterfeld

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@jdi: It has several reset() overloads in Qt 5: http://doc.qt.io/qt-5/qsharedpointer.html#reset But as clear() exists in both Qt 4 and Qt 5, this is indeed more generic. – Frank Osterfeld Jun 08 '15 at 15:08
Sorry. I was looking at Qt4 compatibility. I should have checked Qt5 before editing! – jdi Jun 08 '15 at 20:04

Difference between QSharedPointer::isNull() and operator!()

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