Difference between size_t and unsigned int?

Question

I am so confused about size_t. I have searched on the internet and everywhere mentioned that size_t is an unsigned type so, it can represent only n

Answer 1

size_t has a specific restriction.

Quoting from http://www.cplusplus.com/reference/cstring/size_t/ :

Alias of one of the fundamental unsigned integer types.

It is a type able to represent the size of any object in bytes: size_t is the type returned by the sizeof operator and is widely used in the standard library to represent sizes and counts.

It is not interchangeable with unsigned int because the size of int is specified by the data model. For example LLP64 uses a 32-bit int and ILP64 uses a 64-bit int.