boost/log/attributes/attribute_value.hpp
/* * Copyright Andrey Semashev 2007 - 2015. * Distributed under 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) */ /*! * \file attribute_value.hpp * \author Andrey Semashev * \date 21.05.2010 * * The header contains \c attribute_value class definition. */ #ifndef BOOST_LOG_ATTRIBUTE_VALUE_HPP_INCLUDED_ #define BOOST_LOG_ATTRIBUTE_VALUE_HPP_INCLUDED_ #include <boost/type_index.hpp> #include <boost/move/core.hpp> #include <boost/smart_ptr/intrusive_ptr.hpp> #include <boost/core/explicit_operator_bool.hpp> #include <boost/log/detail/config.hpp> #include <boost/log/utility/type_dispatch/type_dispatcher.hpp> #include <boost/log/attributes/attribute.hpp> #include <boost/log/attributes/value_extraction_fwd.hpp> #include <boost/log/attributes/value_visitation_fwd.hpp> #include <boost/log/detail/header.hpp> #ifdef BOOST_HAS_PRAGMA_ONCE #pragma once #endif namespace boost { BOOST_LOG_OPEN_NAMESPACE /*! * \brief An attribute value class * * An attribute value is an object that contains a piece of data that represents an attribute state * at the point of the value acquisition. All major operations with log records, such as filtering and * formatting, involve attribute values contained in a single view. Most likely an attribute value is * implemented as a simple holder of some typed value. This holder implements the * \c attribute_value::implementation interface and acts as a pimpl for the \c attribute_value * object. The \c attribute_value class provides type dispatching support in order to allow * to extract the value from the holder. * * Normally, attributes and their values shall be designed in order to exclude as much interference as * reasonable. Such approach allows to have more than one attribute value simultaneously, which improves * scalability and allows to implement generating attributes. * * However, there are cases when this approach does not help to achieve the required level of independency * of attribute values and attribute itself from each other at a reasonable performance tradeoff. * For example, an attribute or its values may use thread-specific data, which is global and shared * between all the instances of the attribute/value. Passing such an attribute value to another thread * would be a disaster. To solve this the library defines an additional method for attribute values, * namely \c detach_from_thread. The \c attribute_value class forwards the call to its pimpl, * which is supposed to ensure that it no longer refers to any thread-specific data after the call. * The pimpl can create a new holder as a result of this method and return it to the \c attribute_value * wrapper, which will keep the returned reference for any further calls. * This method is called for all attribute values that are passed to another thread. */ class attribute_value { BOOST_COPYABLE_AND_MOVABLE(attribute_value) public: /*! * \brief A base class for an attribute value implementation * * All attribute value holders should derive from this interface. */ struct BOOST_LOG_NO_VTABLE impl : public attribute::impl { public: /*! * The method dispatches the value to the given object. * * \param dispatcher The object that attempts to dispatch the stored value. * \return true if \a dispatcher was capable to consume the real attribute value type and false otherwise. */ virtual bool dispatch(type_dispatcher& dispatcher) = 0; /*! * The method is called when the attribute value is passed to another thread (e.g. * in case of asynchronous logging). The value should ensure it properly owns all thread-specific data. * * \return An actual pointer to the attribute value. It may either point to this object or another. * In the latter case the returned pointer replaces the pointer used by caller to invoke this * method and is considered to be a functional equivalent to the previous pointer. */ virtual intrusive_ptr< impl > detach_from_thread() { return this; } /*! * \return The attribute value that refers to self implementation. */ attribute_value get_value() BOOST_OVERRIDE { return attribute_value(this); } /*! * \return The attribute value type */ virtual typeindex::type_index get_type() const { return typeindex::type_index(); } }; private: //! Pointer to the value implementation intrusive_ptr< impl > m_pImpl; public: /*! * Default constructor. Creates an empty (absent) attribute value. */ BOOST_DEFAULTED_FUNCTION(attribute_value(), BOOST_NOEXCEPT {}) /*! * Copy constructor */ attribute_value(attribute_value const& that) BOOST_NOEXCEPT : m_pImpl(that.m_pImpl) {} /*! * Move constructor */ attribute_value(BOOST_RV_REF(attribute_value) that) BOOST_NOEXCEPT { m_pImpl.swap(that.m_pImpl); } /*! * Initializing constructor. Creates an attribute value that refers to the specified holder. * * \param p A pointer to the attribute value holder. */ explicit attribute_value(intrusive_ptr< impl > p) BOOST_NOEXCEPT { m_pImpl.swap(p); } /*! * Copy assignment */ attribute_value& operator= (BOOST_COPY_ASSIGN_REF(attribute_value) that) BOOST_NOEXCEPT { m_pImpl = that.m_pImpl; return *this; } /*! * Move assignment */ attribute_value& operator= (BOOST_RV_REF(attribute_value) that) BOOST_NOEXCEPT { m_pImpl.swap(that.m_pImpl); return *this; } /*! * The operator checks if the attribute value is empty */ BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT() /*! * The operator checks if the attribute value is empty */ bool operator! () const BOOST_NOEXCEPT { return !m_pImpl; } /*! * The method returns the type information of the stored value of the attribute. * The returned type info wrapper may be empty if the attribute value is empty or * the information cannot be provided. If the returned value is not empty, the type * can be used for value extraction. */ typeindex::type_index get_type() const { if (m_pImpl.get()) return m_pImpl->get_type(); else return typeindex::type_index(); } /*! * The method is called when the attribute value is passed to another thread (e.g. * in case of asynchronous logging). The value should ensure it properly owns all thread-specific data. * * \post The attribute value no longer refers to any thread-specific resources. */ void detach_from_thread() { if (m_pImpl.get()) m_pImpl->detach_from_thread().swap(m_pImpl); } /*! * The method dispatches the value to the given object. This method is a low level interface for * attribute value visitation and extraction. For typical usage these interfaces may be more convenient. * * \param dispatcher The object that attempts to dispatch the stored value. * \return \c true if the value is not empty and the \a dispatcher was capable to consume * the real attribute value type and \c false otherwise. */ bool dispatch(type_dispatcher& dispatcher) const { if (m_pImpl.get()) return m_pImpl->dispatch(dispatcher); else return false; } #if !defined(BOOST_LOG_DOXYGEN_PASS) #if !defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) #define BOOST_LOG_AUX_VOID_DEFAULT = void #else #define BOOST_LOG_AUX_VOID_DEFAULT #endif #endif // !defined(BOOST_LOG_DOXYGEN_PASS) /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns an empty value. See description of the \c result_of::extract * metafunction for information on the nature of the result value. */ template< typename T, typename TagT BOOST_LOG_AUX_VOID_DEFAULT > typename result_of::extract< T, TagT >::type extract() const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise an exception is thrown. See description of the \c result_of::extract_or_throw * metafunction for information on the nature of the result value. */ template< typename T, typename TagT BOOST_LOG_AUX_VOID_DEFAULT > typename result_of::extract_or_throw< T, TagT >::type extract_or_throw() const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. If extraction fails, the default value is returned. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \param def_value Default value. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns the default value. See description of the \c result_of::extract_or_default * metafunction for information on the nature of the result value. */ template< typename T, typename TagT BOOST_LOG_AUX_VOID_DEFAULT > typename result_of::extract_or_default< T, T, TagT >::type extract_or_default(T const& def_value) const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. If extraction fails, the default value is returned. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \param def_value Default value. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns the default value. See description of the \c result_of::extract_or_default * metafunction for information on the nature of the result value. */ template< typename T, typename TagT BOOST_LOG_AUX_VOID_DEFAULT, typename DefaultT > typename result_of::extract_or_default< T, DefaultT, TagT >::type extract_or_default(DefaultT const& def_value) const; #if defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns an empty value. See description of the \c result_of::extract * metafunction for information on the nature of the result value. */ template< typename T > typename result_of::extract< T >::type extract() const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise an exception is thrown. See description of the \c result_of::extract_or_throw * metafunction for information on the nature of the result value. */ template< typename T > typename result_of::extract_or_throw< T >::type extract_or_throw() const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. If extraction fails, the default value is returned. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \param def_value Default value. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns the default value. See description of the \c result_of::extract_or_default * metafunction for information on the nature of the result value. */ template< typename T > typename result_of::extract_or_default< T, T >::type extract_or_default(T const& def_value) const; /*! * The method attempts to extract the stored value, assuming the value has the specified type. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. If extraction fails, the default value is returned. * * \note Include <tt>value_extraction.hpp</tt> prior to using this method. * * \param def_value Default value. * * \return The extracted value, if the attribute value is not empty and the value is the same * as specified. Otherwise returns the default value. See description of the \c result_of::extract_or_default * metafunction for information on the nature of the result value. */ template< typename T, typename DefaultT > typename result_of::extract_or_default< T, DefaultT >::type extract_or_default(DefaultT const& def_value) const; #endif // defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) #undef BOOST_LOG_AUX_VOID_DEFAULT /*! * The method attempts to extract the stored value, assuming the value has the specified type, * and pass it to the \a visitor function object. * One can specify either a single type or an MPL type sequence, in which case the stored value * is checked against every type in the sequence. * * \note Include <tt>value_visitation.hpp</tt> prior to using this method. * * \param visitor A function object that will be invoked on the extracted attribute value. * The visitor should be capable to be called with a single argument of * any type of the specified types in \c T. * * \return The result of visitation. */ template< typename T, typename VisitorT > visitation_result visit(VisitorT visitor) const; /*! * The method swaps two attribute values */ void swap(attribute_value& that) BOOST_NOEXCEPT { m_pImpl.swap(that.m_pImpl); } }; /*! * The function swaps two attribute values */ inline void swap(attribute_value& left, attribute_value& right) BOOST_NOEXCEPT { left.swap(right); } BOOST_LOG_CLOSE_NAMESPACE // namespace log } // namespace boost #include <boost/log/detail/footer.hpp> #if defined(BOOST_LOG_ATTRIBUTES_ATTRIBUTE_HPP_INCLUDED_) #include <boost/log/detail/attribute_get_value_impl.hpp> #endif #endif // BOOST_LOG_ATTRIBUTE_VALUE_HPP_INCLUDED_