Untitled :: Boost.OpenMethod

Policies and Facets

Methods and classes are scoped in a policy. A method can only reference classes registered in the same policy. If a class is used as a virtual parameter in methods using different policies, it must be registered with each of them.

Class templates use_classes, method, virtual_ptr, and macros BOOST_OPENMETHOD and BOOST_OPENMETHOD_CLASSES, accept an additional argument, a policy class, which defaults to policies::debug in debug builds, and policies::release in release builds.

A policy has a collection of facets. Each facet belongs to a facet category. A policy may contain at most one facet of a given category. Facets control how type information is obtained, how vptrs are fetched, how errors are handled and printed, etc. Some are used in initialize and method dispatch; some are used by other facets in the same policy as part of their implementation. See the reference for the list of facets. Policies and facets are placed in the boost::openmethod::policies namespace. Two stock policies are provided by the library: release and debug.

The release policy contains the following facets:

facet category	facet	role
rtti	std_rtti	provides type information for classes and objects
extern_vptr	vptr_vector	stores vptrs in an indexed collection
type_hash	fast_perfect_hash	hash type id to an index in a vector
error_handler	default_error_handler	handles errors

facet category

facet

role

rtti

std_rtti

provides type information for classes and objects

extern_vptr

vptr_vector

stores vptrs in an indexed collection

type_hash

fast_perfect_hash

hash type id to an index in a vector

error_handler

default_error_handler

handles errors

The debug policy contains the same facets as release, plus a few more:

facet category facet role

facet category	facet	role
runtime_checks	(itself)	enables runtime checks
output	basic_error_output	prints error descriptions to `stderr`
trace	basic_trace_output	enables `initialize` to print information about dispatch table construction to `stderr`

runtime_checks

(itself)

enables runtime checks

output

basic_error_output

prints error descriptions to stderr

trace

basic_trace_output

enables initialize to print information about dispatch table construction to stderr

Policies, and some facets, have static variables. When it is the case, they are implemented as CRTP classes.

Policies can be created from scratch, using the basic_policy template, or constructed from existing policies by adding and removing facets. For example, policies::debug is a tweak of policies::release:

namespace boost::openmethod::policies {

struct debug : release::fork<debug>::with<
                   runtime_checks, basic_error_output<debug>,
                   basic_trace_output<debug>> {};

}

boost::openmethod::default_registry is an alias to release or debug, depending on the value of preprocessor symbols NDEBUG. The default policy can be overriden by defining the macroprocessor symbol BOOST_OPENMETHOD_DEFAULT_REGISTRY before including <boost/openmethod/core.hpp>. The value of the symbol is used as a default template parameter for use_classes, method, virtual_ptr, and others. Once the core header has been included, changing BOOST_OPENMETHOD_DEFAULT_REGISTRY has no effect.

Edit this Page