I created an application for Windows in C++ using QT. If I want to port it to Linux or Mac OS, will sizeof(int) or sizeof(long) change? In other words, do types in QT applications for different platforms have similar size?
In addition to stefaanv's answer, if you're worried about their size, use quint32 and friends. Qt guarantees them to be the same size on all supported platforms.
Check out QtGlobal:
The header file declares several type definitions that guarantee a specified bit-size on all platforms supported by Qt for various basic types, for example qint8 which is a signed char guaranteed to be 8-bit on all platforms supported by Qt. The header file also declares the qlonglong type definition for long long int (__int64 on Windows).
Several convenience type definitions are declared: qreal for double, uchar for unsigned char, uint for unsigned int, ulong for unsigned long and ushort for unsigned short.
Or, if you want a more standard solution, there's stdint.h. I like using Qt all the way, because then the porting is literally just running qmake on a different platform.