CODEWITHSUNDEEP
c++qt
Othman Benchekroun
Othman Benchekroun

Reputation: 2018

Qt Mouse Buttons

I'm using Qt for a while, there is some things i'm doing that are working, but I don't understand why, like this one : 

(event->buttons() & Qt::MiddleButton)

this would return true when the MiddleButton is pressed, but my problem is the syntax, Qt says that Qt::MiddleButton has a value of 4 so as a boolean is would always return true, so that means that the expression is equivalent to this : (event->buttons()) ... And that's not logical neither ... Can someone please explain ??

Upvotes: 0

Views: 2480

Answers (2)

vahancho
vahancho

Reputation: 21220

The problem is that you mix logical operator && with bitwise AND & operator. They are not the same. For example

100 && 010 = True  (both numbers are not 0)
100 &  010 = False (gives 0)

Checking for only bool(event->buttons() == 0) will give you true if there is no button pressed and false if any button is pressed. To check for a particular button, you need to use bitwise '&' operator.

Upvotes: 1

Massimo Costa
Massimo Costa

Reputation: 1860

From Qt docs

Qt::LeftButton      0x00000001 ---> 00000001b
Qt::RightButton     0x00000002 ---> 00000010b
Qt::MiddleButton    0x00000004 ---> 00000100b

The rightmost column is a binary representation! So,

Left+Right  --> buttons(): 3
Left+Middle --> buttons(): 5
.......

Then (buttons() & Qt::MiddleButton) test if the bit related to MiddleButton is set 

Left+Right  --> 00000011 --> 00000011 & 00000100 = 00000000 --> FALSE 
Left+Middle --> 00000011 --> 00000101 & 00000100 = 00000100 --> TRUE

Upvotes: 1

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