mouse buttons - my view from X11 upwards into Qt

David Faure faure at
Mon Feb 21 20:03:21 GMT 2011

On Sunday 20 February 2011, Rick Stockton wrote:
> Qt::NoButton    0x00000000      The button state does not refer to any
> button  (see QMouseEvent::button
> <>()).
> Qt::LeftButton  0x00000001      The left button is pressed, or an event 
> refers to the left button. (The left button may be the right button on 
> left-handed mice.)
> Qt::RightButton         0x00000002      The right button.
> Qt::MidButton   0x00000004      The middle button.
> Qt::MiddleButton        MidButton       The middle button.
> Qt::XButton1    0x00000008      The first X button.
> Qt::XButton2    0x00000010      The second X button.

Let's look at the header file rather than the API docs:

    enum MouseButton {
        NoButton         = 0x00000000,
        LeftButton       = 0x00000001,
        RightButton      = 0x00000002,
        MidButton        = 0x00000004, // ### Qt 5: remove me
        MiddleButton     = MidButton,
        XButton1         = 0x00000008,
        XButton2         = 0x00000010,
        MouseButtonMask  = 0x000000ff

"Qt::MouseButton defines all values as 32-bit hex values 
(with leading zeroes)" is true, but does this really matter to the compiler?
0x00000001 is really just 1.

> I think that an enumerated item with an explicit value will NEVER be 
> compressed, even though a Compiler which wanted to squeeze an enum of 7 
> items (defined without specific values) into a SINGLE BYTE would be free 
> to do so. If my understanding is correct on all platforms, then we can 
> safely use the entire Uint32.

The standard ( Enumeration specifiers) (not that I know this by heart, 
google helped) says:

Each enumerated type shall be compatible with char, a signed integer type, or 
an unsigned integer type. The choice of type is implementation-defined, but 
shall be capable of representing the values of all the members of the 

So this enum might very well be a single byte on some compilers, since 0xff 
fits. AFAIK gcc and msvc don't do this, though. I'll let Thiago answer about 
the more unusual compilers out there :-)

> This portion of "more mouse buttons", the part within Qt, isn't big 
> enough for GSOC. Perhaps the KDE follow-ups are, and I will touch on a 
> few in separate messages. I intend to write and fully document the Qt 
> Updates myself, after I receive your comments and recommendation -- and, 
> I hope, a similar input from a Qt person working in the "input devices" 
> area. (I know that they have other, big things to think about, but I'd 
> hate to do a bunch of work and be told "this isn't the right design" 
> AFTERWARDS.) So, if you can link me up with such a Qt person, please do 
> so. This is a REAL email address and a good contact point for me.

You might want to try IRC,, channel #qt-labs.

David Faure, faure at,
Sponsored by Nokia to work on KDE, incl. Konqueror (

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