I ran gdb with this program. The result is as follows:-
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This GDB was configured as "i386-linux"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb)
(gdb)
(gdb) start
Breakpoint 1 at 0x804b7ac: file main.cpp, line 6.
Starting program: /home/gajendra/test/testkdoublespinbox/testkdoublespinbox
[Thread debugging using libthread_db enabled]
[New Thread 1092003168 (LWP 3732)]
[Switching to Thread 1092003168 (LWP 3732)]
main (argc=1, argv=0x0) at main.cpp:6
6 QApplication a( argc, argv );
(gdb) next
5 {
(gdb) next
7 Form1 w;
(gdb)
6 QApplication a( argc, argv );
(gdb)
5 {
(gdb)
7 Form1 w;
(gdb)
6 QApplication a( argc, argv );
(gdb)


Session management error: Could not open network socket
7 Form1 w;
(gdb)

Program received signal SIGSEGV, Segmentation fault.
0x40dbf92a in KLocale::doFormatInit () from /usr/lib/libkdecore.so.4
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Couldn't get registers: No such process.
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)
Single stepping until exit from function _ZNK7KLocale12doFormatInitEv,
which has no line number information.
Cannot fetch general-purpose registers for thread 1092003168: generic error
(gdb)

I cannot make any head or tail of it. I tried replacing the QApplication definition with KApplication but I got some Message about some init function not being defined.
Can you offer some explanation from the gdb output?