https://www.geeksforgeeks.org/greedy-algorithms-set-3-huffman-coding/ c++ STD library, questions about the code?

the struc defines the data types, why cant we pass into the class in the struc, then do actual logic in a function to compare? Last question, how is the queue filled up with minimum already, is there a predefined library that does this? I definitely have a better understanding, but still little confused. Thanks!