Compilare: user.cpp: In function ‘std::stack<node*> build_expr_tree(std::__cxx11::string)’: user.cpp:123:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < infix.size(); i++) ^ user.cpp: In function ‘std::__cxx11::string infix2postfix(std::__cxx11::string)’: user.cpp:183:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < data.size(); i++) ^ user.cpp: In function ‘bool eval(node*)’: user.cpp:107:1: warning: control reaches end of non-void function [-Wreturn-type] } ^