Your information is very much out of date. C++ makes a few important changes that make most c code incompatible. #1, implicit cast between void and non-void pointers is not allowed, so something like:

    int *x = malloc(sizeof(int) * 15)

is legal c, but in c++ you would have to cast the malloc call. Also, type of character literals is int in c but char in c++. String literals are const only in c++. Functions that take no arguments must be declared as taking void (as in void func(void)) in c, but simply () in c++.

Even back then, C++ introduced new keywords, so valid C code like

  int new;

would not compile in C++.