What does this mean "Virtually any legal C program is a legal C++ program"?

Question

The statement in the question inside double quotes bugs me. what does that mean? please help me guys.

Answer 1

C++ is meant to be the C language with augmented features and possibilities. So, any C program could be considered as a C++ program.
Even though it is meant to be this way, there are in facts some differences that sometimes make a legal C program an illegal C++ program, hence the "virtually" in your sentence. Some C features were removed from C++ with C++11, C++14 and C++17, and C got some features in C99 and C11 C++ didn’t receive.

Answer 2

I think it means C++ is an upgraded version of C. And almost all programmes written in C are compatible and can run well when ported to C++

Answer 3

Well, C++11 is C99 compliant. In other words: C++11 is a true superset (of the old standard) of C99. SoloLearn C++ still uses C++03.