It is quite interesting actually the Google and Facebook uses it for faster string manipulation purposes the performance of fbstring is better so using of normal std:: string given by c++ is replaced for better performance.we create a file string and in that we include <folly/FBString.h> so when we call std:: string the backend fbstring will work it helps to boost the performance.it simple terms it is like overriding( remember don't question me not having same name and parameters) just explaining in conceptual way for std:: string we are writing our own backend code where it helps to boost our performance but be careful to placing the folder and tell the cmake to include before system include.your aim is to compiler need to give priority to your string file