Compilare: user.cpp: In function ‘void sir(std::string)’: user.cpp:19:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<x.size();++i){ ^ user.cpp: In function ‘int main()’: user.cpp:31:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<s1.size();++i){ ^ user.cpp:40:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<s2.size();++i){ ^