When the program run to "if (size > (capacity - 1))" the size = 10,it seems that the word_list array isnot enough, word_list array expand as twice, it use for put the 10 pieces of data into the new_word_list, then delete word_list ,
it should use datele[] to delete all the data of word_list,because word_list is a array
next the word_list point to the address of new array new_word_list,
use this : word_list = new_word_list;

at this place we change the word_list's value, but the function build_wordlist (string * word_list, int capacity, string filename) use value delivery, when the main() use word_list
its value still the initial value
so we should use & to quote the word_list and capacity:
build_wordlist (string * &word_list, int &capacity, string filename)