What is this code doing? (size_t)-1

(size_t)-1 is in fact the equivalent of size_t(-1)

See also the following question c cast syntax styles

This code (unnecessarily) casts -1 to size_t. The most probable intent was getting the largest possible value of size_t on this system.

Although this code doesn't have Undefined Behavior, this code is ugly - in C++ you should use std::numeric_limits<size_t>::max() and in C use SIZE_MAX macro for exactly a purpose of getting the largest size_t value.