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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]
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