Base-from-Member IdiomThe class template There is test/example code in base_from_member_test.cpp. ContentsRationaleWhen developing a class, sometimes a base class needs to be initialized with a member of the current class. As a naïve example: #include <streambuf> // for std::streambuf #include <ostream> // for std::ostream class fdoutbuf : public std::streambuf { public: explicit fdoutbuf( int fd ); //... }; class fdostream : public std::ostream { protected: fdoutbuf buf; public: explicit fdostream( int fd ) : buf( fd ), std::ostream( &buf ) {} //... }; This is undefined because C++'s initialization order mandates that the base class is initialized before the member it uses. R. Samuel Klatchko developed a way around this by using the initialization order in his favor. Base classes are intialized in order of declaration, so moving the desired member to another base class, that is initialized before the desired base class, can ensure proper initialization. A custom base class can be made for this idiom: #include <streambuf> // for std::streambuf #include <ostream> // for std::ostream class fdoutbuf : public std::streambuf { public: explicit fdoutbuf( int fd ); //... }; struct fdostream_pbase { fdoutbuf sbuffer; explicit fdostream_pbase( int fd ) : sbuffer( fd ) {} }; class fdostream : private fdostream_pbase , public std::ostream { typedef fdostream_pbase pbase_type; typedef std::ostream base_type; public: explicit fdostream( int fd ) : pbase_type( fd ), base_type( &sbuffer ) {} //... }; Other projects can use similar custom base classes. The technique is basic enough to make a template, with a sample template class in this library. The main template parameter is the type of the enclosed member. The template class has several (explicit) constructor member templates, which implicitly type the constructor arguments and pass them to the member. The template class uses implicit copy construction and assignment, cancelling them if the enclosed member is non-copyable. Manually coding a base class may be better if the construction and/or copying needs are too complex for the supplied template class, or if the compiler is not advanced enough to use it. Since base classes are unnamed, a class cannot have multiple (direct) base classes of the same type. The supplied template class has an extra template parameter, an integer, that exists solely to provide type differentiation. This parameter has a default value so a single use of a particular member type does not need to concern itself with the integer. Synopsis#ifndef BOOST_BASE_FROM_MEMBER_MAX_ARITY #define BOOST_BASE_FROM_MEMBER_MAX_ARITY 10 #endif template < typename MemberType, int UniqueID = 0 > class boost::base_from_member { protected: MemberType member; base_from_member(); template< typename T1 > explicit base_from_member( T1 x1 ); template< typename T1, typename T2 > base_from_member( T1 x1, T2 x2 ); //... template< typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7, typename T8, typename T9, typename T10 > base_from_member( T1 x1, T2 x2, T3 x3, T4 x4, T5 x5, T6 x6, T7 x7, T8 x8, T9 x9, T10 x10 ); }; The class template has a first template parameter
MemberType representing the type of the based-member.
It has a last template parameter UniqueID, that is an
There is a default constructor and several constructor member templates. These constructor templates can take as many arguments (currently up to ten) as possible and pass them to a constructor of the data member. Since C++ does not allow any way to explicitly state the template parameters of a templated constructor, make sure that the arguments are already close as possible to the actual type used in the data member's desired constructor. The BOOST_BASE_FROM_MEMBER_MAX_ARITY macro constant specifies the maximum argument length for the constructor templates. The constant may be overridden if more (or less) argument configurations are needed. The constant may be read for code that is expandable like the class template and needs to maintain the same maximum size. (Example code would be a class that uses this class template as a base class for a member with a flexible set of constructors.) UsageWith the starting example, the #include <boost/utility/base_from_member.hpp> #include <streambuf> // for std::streambuf #include <ostream> // for std::ostream class fdoutbuf : public std::streambuf { public: explicit fdoutbuf( int fd ); //... }; class fdostream : private boost::base_from_member<fdoutbuf> , public std::ostream { // Helper typedef's typedef boost::base_from_member<fdoutbuf> pbase_type; typedef std::ostream base_type; public: explicit fdostream( int fd ) : pbase_type( fd ), base_type( &member ) {} //... }; The base-from-member idiom is an implementation detail, so it
should not be visible to the clients (or any derived classes) of
ExampleThe base-from-member class templates should commonly involve only one base-from-member sub-object, usually for attaching a stream-buffer to an I/O stream. The next example demonstrates how to use multiple base-from-member sub-objects and the resulting qualification issues. #include <boost/utility/base_from_member.hpp> #include <cstddef> // for NULL struct an_int { int y; an_int( float yf ); }; class switcher { public: switcher(); switcher( double, int * ); //... }; class flow_regulator { public: flow_regulator( switcher &, switcher & ); //... }; template < unsigned Size > class fan { public: explicit fan( switcher ); //... }; class system : private boost::base_from_member<an_int> , private boost::base_from_member<switcher> , private boost::base_from_member<switcher, 1> , private boost::base_from_member<switcher, 2> , protected flow_regulator , public fan<6> { // Helper typedef's typedef boost::base_from_member<an_int> pbase0_type; typedef boost::base_from_member<switcher> pbase1_type; typedef boost::base_from_member<switcher, 1> pbase2_type; typedef boost::base_from_member<switcher, 2> pbase3_type; typedef flow_regulator base1_type; typedef fan<6> base2_type; public: system( double x ); //... }; system::system( double x ) : pbase0_type( 0.2 ) , pbase1_type() , pbase2_type( -16, &this->pbase0_type::member ) , pbase3_type( x, static_cast<int *>(NULL) ) , base1_type( pbase3_type::member, pbase1_type::member ) , base2_type( pbase2_type::member ) { //... } The final class has multiple sub-objects with the name
"member," so any use of that name needs qualification by
a name of the appropriate base type. (Using There are some argument conversions in the initialization. The
constructor argument for CreditsContributors
Revised: 28 August 2004 Copyright 2001, 2003, 2004 Daryle Walker. Use, modification, and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or a copy at <http://www.boost.org/LICENSE_1_0.txt>.) |