Compilare: user.cpp: In function ‘void write()’: user.cpp:14:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<a.size(); i++) ^ user.cpp: At global scope: user.cpp:19:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^ user.cpp: In function ‘int main()’: user.cpp:24:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<a.size(); i++) ^