Why c++ is not pure object oriented language?

I have just been asked why c++ is not pure object oriented language.

Well the answer is simple: – C++ supports object-oriented programming, but OO is not intrinsic to the language. In c++ you can write a valid, well-coded, excellently-styled C++ program without using a single object, while in Java or c# you cannot write anything without using classes and objects. For example:

class HelloWorld  {
public static void main(String[] args) {
System.out.println("Hello World!"); 
}
}

and c++

int main ()
{
cout << "Hello World!";
return 0;
}

There a good article related to OOPSLA: http://www.stroustrup.com/oopsla.pdf