What does "written in pure C++" mean?

Question

C++ / C#

I rummage on the git and see such words and I don’t understand the meaning

Answer 1

Using only the standard language library.
No interface, no networking, no threads/processes, etc.

Answer 2

When they say a similar phrase in relation to C, they usually mean a program written in C without additional dependencies, incl. and without dependencies on the standard library, i.e. C and OS.
I assume that in the case of C ++, this is exactly what is meant.

Answer 3

"In pure C++" this is without the involvement of external libraries and frameworks (such as boost, qt, etc.).