How to use the PI constant in C++

On some (especially older) platforms (see the comments below) you might need to

#define _USE_MATH_DEFINES

and then include the necessary header file:

#include <math.h>

and the value of pi can be accessed via:

M_PI

In my math.h (2014) it is defined as:

# define M_PI           3.14159265358979323846  /* pi */

but check your math.h for more. An extract from the "old" math.h (in 2009):

/* Define _USE_MATH_DEFINES before including math.h to expose these macro
 * definitions for common math constants.  These are placed under an #ifdef
 * since these commonly-defined names are not part of the C/C++ standards.
 */

However:

1. on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the _USE_MATH_DEFINES

2. On (recent) Linux platforms there are long double values too provided as a GNU Extension:

   # define M_PIl          3.141592653589793238462643383279502884L /* pi */
   

Pi can be calculated as atan(1)*4. You could calculate the value this way and cache it.

You could also use boost, which defines important math constants with maximum accuracy for the requested type (i.e. float vs double).

const double pi = boost::math::constants::pi<double>();

Check out the boost documentation for more examples.