Compilare: user.cpp: In function ‘void add(std::__cxx11::string)’: user.cpp:26:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ~~^~~~~~~~~~ user.cpp: In function ‘void del(std::__cxx11::string)’: user.cpp:43:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ~~^~~~~~~~~~ user.cpp: In function ‘void print_apps(std::__cxx11::string)’: user.cpp:55:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ~~^~~~~~~~~~ user.cpp: In function ‘void print_prefix(std::__cxx11::string)’: user.cpp:71:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ~~^~~~~~~~~~ user.cpp: At global scope: user.cpp:84:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^