Difference between size_t and std::size_t

What are the differences between size_t and std::size_t in terms of where they are declared, when they should be used and any other differentiating features?

C's size_t and C++'s std::size_t are both same.

In C, it's defined in <stddef.h> and in C++, its defined in <cstddef> whose contents are the same as C header (see the quotation below). Its defined as unsigned integer type of the result of the sizeof operator.

C Standard says in §17.7/2,

size_t which is the unsigned integer type of the result of the sizeof operator

And C++ Standard says (about cstddef header) in §18.1/3,

The contents are the same as the Standard C library header <stddef.h>, with the following changes.

So yeah, both are same; the only difference is that C++ defines size_t in std namespace.

Please also notice that the above line also says "with the following changes" which isn't referring to size_t. Its rather referring to the new additions (mostly) made by C++ into the language (not present in C) which are also defined in the same header.

Wikipedia has very good info about range and storage size of size_t:

Range and storage size of size_t

The actual type of size_t is platform-dependent; a common mistake is to assume size_t is the same as unsigned int, which can lead to programming errors,[3][4] when moving from 32 to 64-bit architecture, for example.

According to the 1999 ISO C standard (C99), size_t is an unsigned integer type of at least 16 bits.

And the rest you can read from this page at Wikipedia.