What library is the error() function in?
<p>I try to use the error() function in my program, but it shows "Error: identifier "error" is undefined". Is there a library that I need to include? 
This is found from a book called "Programming Principles and Practice Using C++". The error() function will terminate the program with a system error message plus the string that is passed to error() as an argument. </p>

<pre><code>if (x&lt;=0) error("non-positive x"); 
if (y&lt;=0) error("non-positive y");
int area1 = area(x,y); 
</code></pre>

<blockquote>
  <p>Really, the only question is what to do if we
  find an error. Here, we have called a function error() which we assume
  will do something sensible. In fact, in std_lib_facilities.h we supply
  an error() function that by default terminates the program with a
  system error message plus the string we passed as an argument to
  error(). If you prefer to write out your own error message or take
  other actions, you catch runtime_error (§5.6.2, §7.3, §7.8, §B.2.1).
  This approach suffices for most student programs and is an example of
  a style that can be used for more sophisticated error handling.</p>
</blockquote>