The guarantees the standard gives you go like this:
1 == sizeof(char) <= sizeof(short) <= sizeof (int) <= sizeof(long) <= sizeof(long long)
So it's perfectly valid for sizeof (int) and sizeof (long) to be equal, and many platforms choose to go with this approach. You will find some platforms where int is 32 bits, long is 64 bits, and long long is 128 bits, but it seems very common for sizeof (long) to be 4.
(Note that long long is recognized in C from C99 onwards, but was normally implemented as an extension in C++ prior to C++11.)
You're on a 32-bit machine or a 64-bit Windows machine. On my 64-bit machine (running a Unix-derivative O/S, not Windows), sizeof(int) == 4, but sizeof(long) == 8.
They're different types — sometimes the same size as each other, sometimes not.
(In the really old days, sizeof(int) == 2 and sizeof(long) == 4 — though that might have been the days before C++ existed, come to think of it. Still, technically, it is a legitimate configuration, albeit unusual outside of the embedded space, and quite possibly unusual even in the embedded space.)
A typical best practice is not using long/int/short directly. Instead, according to specification of compilers and OS, wrap them into a header file to ensure they hold exactly the amount of bits that you want. Then use int8/int16/int32 instead of long/int/short. For example, on 32bit Linux, you could define a header like this
typedef char int8;
typedef short int16;
typedef int int32;
typedef unsigned int uint32;
On platforms where they both have the same size the answer is nothing. They both represent signed 4 byte values.
However you cannot depend on this being true. The size of long and int are not definitively defined by the standard. It's possible for compilers to give the types different sizes and hence break this assumption.
The long must be at least the same size as an int, and possibly, but not necessarily, longer.
On common 32-bit systems, both int and long are 4-bytes/32-bits, and this is valid according to the C++ spec.
On other systems, both int and long long may be a different size. I used to work on a platform where int was 2-bytes, and long was 4-bytes.
Source: Stackoverflow.com