...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

When using the math constants at your chosen fixed precision in non-template
code, you can simply add a `using namespace`

declaration, for example, ```
using namespace
boost::math::double_constants
```

, to make the constants
of the correct precision for your code visible in the current scope, and
then use each constant *as a simple variable - sans brackets*:

#include <boost/math/constants/constants.hpp> double area(double r) { using namespace boost::math::double_constants; return pi * r * r; }

Had our function been written as taking a `float`

rather than a `double`

, we could
have written instead:

#include <boost/math/constants/constants.hpp> float area(float r) { using namespace boost::math::float_constants; return pi * r * r; }

Likewise, constants that are suitable for use at ```
long
double
```

precision are available in
the namespace `boost::math::long_double_constants`

.

You can see the full list of available constants at math_toolkit.constants.

Some examples of using constants are at constants_eg1.