GTK+ is actually a C toolkit, so if you are interested, you can use
gtkmm instead, which is just a C++ wrapper. (
http://www.gtkmm.org)
However, if you are looking for a native look and feel, here's my
opinion:
wxWindows(wxWid gets) : very good, reasonable native looks (10+ years
of history)
GTK+ : Quite decent, very gnome-like, but quite similar to Win32(Tip:
no MDI however but can be feigned by using a notebook)
gtkmm : same as GTK+
FLTK : not very good, may be portable to Macs and Win32, but looks
like it's running on X
Qt : very good, with good native looks, plus...you can make your
application adopt different styles
Also, for ease-of-use, here's what I think:
wxWindows : quite ok, uses Model-View architecture, can be abit of a
slow development without a UI builder(many available, recommend wx
Dev-C++
http://wxdsgn.sourceforge.net/)
GTK+ : If you're used to doing things in very much the C way, it is
quite good
gtkmm : good use of C++ features, quite fast to develop, quite
flexible
(both come with a UI builder called Glade)
FLTK : Quite reasonable, but because of the use of function pointer
callbacks, i don't quite trust it, but there is a projetc on
sourceforge attempting to cure its "fn ptr" usage. Comes with a UI
builder Fluid, but I can't make heads or tails of Fluid.
Qt : excellent, best I've seen so far, easy to write, syntax is very
natural, UI builder is very good
So far it sounds like Qt is the best, one catch though...if you are
writing commercial applications, be prepared to fork out USD $1550 for
a single platform package (for more info, go to
http://www.trolltech.com/products/qt/pricing.html), because Qt comes
in both GPL and commercial versions. Also, there isn't a free version
for Win32, only a non-commercial, which you still pay for and cannot
be used for commercial work, and a commercial version.
If you don't have that kind of cash, use wxWindows if you have more
time, and use gtkmm if you have less.
Use FLTK only if none of the above are viable (I'm kind of biased
towards toolkits which have a native look and feel, fast development,
and a good UI builder, and FLTK only has speed in coding, so I don't
really like it)
If you are doing GPL software, use Qt, but you probably can't port it
to Win32, so it's either wxWindows or gtkmm (wx has more nativity, but
a bit slower coding)
Here's a summary of what I have said:
wxWindows : When you have no money, you need to develop commercially,
you have more time, and you go for native looks
gtkmm or GTK+ : When you have less time, no money, and you just want
it to look something like WIndows, but don't really care nitty-gritty
(commercial)
Qt : when you have $$, but, if you aren't looking towards Win32, use
its GPL versions(genera ted apps MUST be GPL)
FLTK : Use only when necessary (like can't obtain above)
P/s gtk+ and gtkmm and FLTK are supplied as LGPL, so always use shared
libraries (like DLLs or .so files)
P/ps Hope I was useful :)
P/pps Feel free to bludgeon me for my postscripts, but sorry if I am a
bit long winded :) Have a good day