Compilare: user.cpp: In function ‘std::__cxx11::string getNext(std::__cxx11::string, int&)’: user.cpp:34:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i<expr.length() && !is_operator(expr[i])) ~^~~~~~~~~~~~~~ user.cpp: In function ‘std::__cxx11::string infix_to_postfix(std::__cxx11::string)’: user.cpp:48:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i= 0; i<expr.length(); i++) ~^~~~~~~~~~~~~~ user.cpp: In function ‘std::__cxx11::string postfix_getNext(std::__cxx11::string, int&)’: user.cpp:98:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i<expr.length() && (expr[i]!=' ')) ~^~~~~~~~~~~~~~ user.cpp: In function ‘int eval_expr(std::__cxx11::string)’: user.cpp:120:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i =0;i<expr.length();i++) ~^~~~~~~~~~~~~~ user.cpp: In function ‘int interpret(int, int, char)’: user.cpp:113:1: warning: control reaches end of non-void function [-Wreturn-type] } ^