C/C++ is the intersection of C and C++.
int new; is not C/C++, and neither is vector<int> foo;.
Similarly, C89/C99 is the intersection of these two languages, where neither enum bool { false, true }; or for(int i = 0;;) is allowed.
And C++11/C++14, etc.
It is possible to write code that compiles (and correctly runs) under C++11 and C++14, even though compiling under one doesn't imply it compiles under the other. In fact, a lot people do this.
And a lot of people write code that works in C and C++.
Obviously, the greater the overlap, the more sense it makes; I don't expect to see any questions about C/C++/Java code.
Though it does "make sense" to talk about a common subset of these languages, many questions will not have answers in this subset, e.g. What should main() return in C and C++?
But you can talk about code that works for multiple language specifications, whether those specs are differentiated by "version" or "language name" or otherwise.