Under C/C++, reentrant programs can be categorized by their reentrancy type as follows:

Natural reentrancy

The attribute of programs that contain no modifiable external data.

Natural reentrancy is not applicable to C++.

Constructed reentrancy

The attribute of applications that contain modifiable external data and require additional processing to become reentrant. By default, all C++ programs are made reentrant via constructed reentrancy.