Error: 'string' does not name a type

Good day. I have a problem. I opened the file “…ardupilot\ArduPlane\Plane.h” and want to add a couple of my string variables. But getting an error while building.
I tried:
std::string str;
#include string (with < >)
#include string.h
#include cstring

but nothing helps. Errors appear when building the project, like:
error: ‘string’ does not name a type
error: ‘string’ in namespace ‘std’ does not name a type
or an error on a part of the code that I did not touch in any way.
Help please