Compilare: main.cpp: In function 'int oper(int, int, char)': main.cpp:21:19: warning: converting to non-pointer type 'int' from NULL [-Wconversion-null] 21 | default: return NULL; | ^~~~ main.cpp: In function 'int res(std::string)': main.cpp:46:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 46 | for (i = 0; i < s.size(); i ++) | ~~^~~~~~~~~~ main.cpp:60:13: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 60 | while (i < s.size() && isdigit (s[i])) | ~~^~~~~~~~~~