#if inside the function
<p>anybody can explain me epsilon what is this term although i did not use in my header file. 
like </p>

<pre><code>Right (const lPoint&amp; a, const lPoint&amp; b, const lPoint&amp; c, double epsilon)
{
#if _NOT_USED_EPSILON
    return (( (a.x_ * b.y_) - (a.y_ * b.x_) +
              (a.y_ * c.x_) - (a.x_ * c.y_) +
              (b.x_ * c.y_) - (c.x_ * b.y_) ) &lt; 0);
#else
/*  return (( (a.x_ * b.y_) - (a.y_ * b.x_) +
              (a.y_ * c.x_) - (a.x_ * c.y_) +
              (b.x_ * c.y_) - (c.x_ * b.y_) ) &lt; -SPATIAL_EPSILON);*/
    if( epsilon == -1 )
        return (b.x_-a.x_)*(c.y_-a.y_)-(c.x_-a.x_)*(b.y_-a.y_) &lt; -SPATIAL_AREA_EPSILON;
    else
        return (b.x_-a.x_)*(c.y_-a.y_)-(c.x_-a.x_)*(b.y_-a.y_) &lt; -epsilon;
#endif
}
</code></pre>

<p>here i did not used epsilon in my file than wy we are saying that #if _not_used_epsilon than return this .... while my epsilon by default is 0 because its mot initialized. but its use din if condition  and her const is used because it will not change the value of arguemnt. right!</p>

<p>and this #if will not read by complier inside the function i want to ask that #directory are read by coompiler or not.. i am not getting #directories.. why we use it we can sue simple if condition with variables as we use in function,, so why #directory inside the main.. who will deal with it compiler..   </p>
