title: Why is there no definition of the constant pi in the C++11 standard?
body: <p>I find it quiet annoying that I have to use the macro <code>_USE_MATH_DEFINES</code> in order to get the value of pi into my program. Or I need to define it myself in one of my own headers. Or I have to use boost and all that. </p>
  
  <p>It just annoys me, that there isn't a standard c++ header defining that constant in a portable way. Is there any particular reason that math constants like pi are not in the c++ standard? Could it be in one of the next versions of the standard?</p>