#Don’t write using namespace std;.

Don’t write using namespace std;.

You can, however, in a CPP file (not H file) or inside a function put individual using std::string; etc. (See SF.7.)

You can, however, in a CPP file (not H file), or inside a function, put individual using std::string; etc. (See SF.7.)

Don’t use the C NULL macro (ES.47). But, don’t write explicit tests against nullptr’. ([ES.87](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es87-dont-add-redundant--or--to-conditions)) Use the contextual conversion to bool`, which works efficiently for raw pointers and any kind of smart pointer instance.

Don’t use the C NULL macro (ES.47). But, don’t write explicit tests against nullptr. (ES.87 ) Use the contextual conversion to bool, which works efficiently for raw pointers and any kind of smart pointer instance.