Why does rand() compile without including cstdlib or using namespace std?

Question

According to the book I'm reading, rand() requires #include <cstdlib> in C++
However, I am able to compile the following code that uses rand() without #include <cstdlib> nor using namespace std; in Visual Studio 2015.
Why are these two not needed to compile? Should I include cstdlib?

C++ Code:

#include <iostream>

int main()
{
    std::cout << rand() << std::endl;
}

Answer 1

There are two issues at play:

1. Standard library header files may include other standard library header files. So iostream may include cstdlib directly or indirectly.
2. Header files with C-standard library equivalents (e.g. cstdlib) are allowed to bring C standard library names into the global namespace, that is, outside of the std namespace (e.g. rand.) This is formally allowed since C++11, and was largely tolerated before.