unsigned int vs. size_t

Question

I notice that modern C and C++ code seems to use size_t instead of int/unsigned int pretty much everywhere - from parameters for C str

Answer 1

The size_t type is the type returned by the sizeof operator. It is an unsigned integer capable of expressing the size in bytes of any memory range supported on the host machine. It is (typically) related to ptrdiff_t in that ptrdiff_t is a signed integer value such that sizeof(ptrdiff_t) and sizeof(size_t) are equal.

When writing C code you should always use size_t whenever dealing with memory ranges.

The int type on the other hand is basically defined as the size of the (signed) integer value that the host machine can use to most efficiently perform integer arithmetic. For example, on many older PC type computers the value sizeof(size_t) would be 4 (bytes) but sizeof(int) would be 2 (byte). 16 bit arithmetic was faster than 32 bit arithmetic, though the CPU could handle a (logical) memory space of up to 4 GiB.

Use the int type only when you care about efficiency as its actual precision depends strongly on both compiler options and machine architecture. In particular the C standard specifies the following invariants: sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) placing no other limitations on the actual representation of the precision available to the programmer for each of these primitive types.

Note: This is NOT the same as in Java (which actually specifies the bit precision for each of the types 'char', 'byte', 'short', 'int' and 'long').