To the best of my knowledge (i.e. not updated since C++14), C++ is no longer a strict superset of C as of C99, and there are C99 features that are not available in C++, so I'm not sure that's the case. But plenty of overlap, for sure.

Here's a community wiki answer on SO: https://stackoverflow.com/posts/47526708/revisions

Extracted portion of relevance:

    > The following C99 features are not supported by C++:
    >
    > * restricted pointers
    > * variable length arrays
    > * flexible array members
    > * static and type qualifiers in parameter array declarators
    > * compound literals
    > * designated initializers