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libs/math/example/policy_eg_4.cpp

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+//  Copyright John Maddock 2007.
+//  Copyright Paul A. Bristow 2010
+
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Note that this file contains quickbook mark-up as well as code
+// and comments, don't change any of the special comment mark-ups!
+
+#include <iostream>
+using std::cout;  using std::endl;
+#include <cerrno> // for ::errno
+
+//[policy_eg_4
+
+/*`
+Suppose we want `C::foo()` to behave in a C-compatible way and set
+`::errno` on error rather than throwing any exceptions.
+
+We'll begin by including the needed header for our function:
+*/
+
+#include <boost/math/special_functions.hpp>
+//using boost::math::tgamma; // Not needed because using C::tgamma.
+
+/*`
+Open up the "C" namespace that we'll use for our functions, and
+define the policy type we want: in this case a C-style one that sets
+::errno and returns a standard value, rather than throwing exceptions.
+
+Any policies we don't specify here will inherit the defaults.
+*/
+
+namespace C
+{ // To hold our C-style policy.
+  //using namespace boost::math::policies; or explicitly:
+  using boost::math::policies::policy;
+
+  using boost::math::policies::domain_error;
+  using boost::math::policies::pole_error;
+  using boost::math::policies::overflow_error;
+  using boost::math::policies::evaluation_error;
+  using boost::math::policies::errno_on_error;
+
+  typedef policy<
+     domain_error<errno_on_error>,
+     pole_error<errno_on_error>,
+     overflow_error<errno_on_error>,
+     evaluation_error<errno_on_error>
+  > c_policy;
+
+/*`
+All we need do now is invoke the BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS
+macro passing our policy type c_policy as the single argument:
+*/
+
+BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS(c_policy)
+
+} // close namespace C
+
+/*`
+We now have a set of forwarding functions defined in namespace C
+that all look something like this:
+
+``
+template <class RealType>
+inline typename boost::math::tools::promote_args<RT>::type
+   tgamma(RT z)
+{
+   return boost::math::tgamma(z, c_policy());
+}
+``
+
+So that when we call `C::tgamma(z)`, we really end up calling
+`boost::math::tgamma(z, C::c_policy())`:
+*/
+
+int main()
+{
+   errno = 0;
+   cout << "Result of tgamma(30000) is: "
+      << C::tgamma(30000) << endl; // Note using C::tgamma
+   cout << "errno = " << errno << endl; // errno = 34
+   cout << "Result of tgamma(-10) is: "
+      << C::tgamma(-10) << endl;
+   cout << "errno = " << errno << endl; // errno = 33, overwriting previous value of 34.
+}
+
+/*`
+
+Which outputs:
+
+[pre
+Result of C::tgamma(30000) is: 1.#INF
+errno = 34
+Result of C::tgamma(-10) is: 1.#QNAN
+errno = 33
+]
+
+This mechanism is particularly useful when we want to define a project-wide policy,
+and don't want to modify the Boost source,
+or to set project wide build macros (possibly fragile and easy to forget).
+
+*/
+//] //[/policy_eg_4]
+