Boost C++ Libraries

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Setting Polices at Namespace Scope

Sometimes what you really want to do is bring all the special functions, or all the distributions into a specific namespace-scope, along with a specific policy to use with them. There are two macros defined to assist with that:

BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS(Policy)

and:

BOOST_MATH_DECLARE_DISTRIBUTIONS(Type, Policy)

You can use either of these macros after including any special function or distribution header. For example:

#include <boost/math/special_functions/gamma.hpp>
//using boost::math::tgamma;
// Need not declare using boost::math::tgamma here,
// because will define tgamma in myspace using macro below.

namespace myspace
{
  using namespace boost::math::policies;

  // Define a policy that does not throw on overflow:
  typedef policy<overflow_error<errno_on_error> > my_policy;

  // Define the special functions in this scope to use the policy:   
  BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS(my_policy)
}

// Now we can use myspace::tgamma etc.
// They will automatically use "my_policy":
//
double t = myspace::tgamma(30.0); // Will *not* throw on overflow,
// despite the large value of factorial 30 = 265252859812191058636308480000000
// unlike default policy boost::math::tgamma;

In this example, using BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS results in a set of thin inline forwarding functions being defined:

template <class T>
inline T tgamma(T a){ return ::boost::math::tgamma(a, mypolicy()); }

template <class T>
inline T lgamma(T a) ( return ::boost::math::lgamma(a, mypolicy()); }

and so on. Note that while a forwarding function is defined for all the special functions, however, unless you include the specific header for the special function you use (or boost/math/special_functions.hpp to include everything), you will get linker errors from functions that are forward declared, but not defined.

We can do the same thing with the distributions, but this time we need to specify the floating-point type to use:

#include <boost/math/distributions/cauchy.hpp>

namespace myspace
{ // using namespace boost::math::policies; // May be convenient in myspace.

  // Define a policy called my_policy to use.
  using boost::math::policies::policy;

// In this case we want all the distribution accessor functions to compile,
// even if they are mathematically undefined, so
// make the policy assert_undefined.
  using boost::math::policies::assert_undefined;

typedef policy<assert_undefined<false> > my_policy;

// Finally apply this policy to type double.
BOOST_MATH_DECLARE_DISTRIBUTIONS(double, my_policy)
} // namespace myspace

// Now we can use myspace::cauchy etc, which will use policy
// myspace::mypolicy:
//
// This compiles but throws a domain error exception at runtime.
// Caution! If you omit the try'n'catch blocks, 
// it will just silently terminate, giving no clues as to why! 
// So try'n'catch blocks are very strongly recommended.

void test_cauchy()
{
   try
   {
      double d = mean(myspace::cauchy());  // Cauchy does not have a mean!
   }
   catch(const std::domain_error& e)
   {
      cout << e.what() << endl;
   }
}

In this example the result of BOOST_MATH_DECLARE_DISTRIBUTIONS is to declare a typedef for each distribution like this:

typedef boost::math::cauchy_distribution<double, my_policy> cauchy;
tyepdef boost::math::gamma_distribution<double, my_policy> gamma;

and so on. The name given to each typedef is the name of the distribution with the "_distribution" suffix removed.


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