ceph/src/boost/libs/utility/doc/base_from_member.qbk

   1 [/
   2   Copyright 2001, 2003, 2004, 2012 Daryle Walker.
   3
   4   Distributed under the Boost Software License, Version 1.0.
   5
   6   See accompanying file LICENSE_1_0.txt
   7   or copy at http://boost.org/LICENSE_1_0.txt
   8 ]
   9
  10 [article Base_From_Member
  11     [quickbook 1.5]
  12     [authors [Walker, Daryle]]
  13     [copyright 2001, 2003, 2004, 2012 Daryle Walker]
  14     [license
  15         Distributed under the Boost Software License, Version 1.0.
  16         (See accompanying file LICENSE_1_0.txt or copy at
  17         [@http://www.boost.org/LICENSE_1_0.txt])
  18     ]
  19 ]
  20
  21 [section Rationale]
  22
  23 When developing a class, sometimes a base class needs to be initialized
  24 with a member of the current class. As a na\u00EFve example:
  25
  26     #include <streambuf>  /* for std::streambuf */
  27     #include <ostream>    /* for std::ostream */
  28
  29     class fdoutbuf
  30       : public std::streambuf
  31     {
  32     public:
  33         explicit fdoutbuf( int fd );
  34         //...
  35     };
  36
  37     class fdostream
  38       : public std::ostream
  39     {
  40     protected:
  41         fdoutbuf buf;
  42     public:
  43         explicit fdostream( int fd )
  44           : buf( fd ), std::ostream( &buf ) {}
  45         //...
  46     };
  47
  48 This is undefined because C++'s initialization order mandates that the base
  49 class is initialized before the member it uses. [@http://www.moocat.org R.
  50 Samuel Klatchko] developed a way around this by using the initialization
  51 order in his favor. Base classes are intialized in order of declaration, so
  52 moving the desired member to another base class, that is initialized before
  53 the desired base class, can ensure proper initialization.
  54
  55 A custom base class can be made for this idiom:
  56
  57     #include <streambuf>  /* for std::streambuf */
  58     #include <ostream>    /* for std::ostream */
  59
  60     class fdoutbuf
  61       : public std::streambuf
  62     {
  63     public:
  64         explicit fdoutbuf( int fd );
  65         //...
  66     };
  67
  68     struct fdostream_pbase
  69     {
  70         fdoutbuf sbuffer;
  71
  72         explicit fdostream_pbase( int fd )
  73           : sbuffer( fd ) {}
  74     };
  75
  76     class fdostream
  77       : private fdostream_pbase
  78       , public std::ostream
  79     {
  80         typedef fdostream_pbase  pbase_type;
  81         typedef std::ostream     base_type;
  82
  83     public:
  84         explicit fdostream( int fd )
  85           : pbase_type( fd ), base_type( &sbuffer ) {}
  86         //...
  87     };
  88
  89 Other projects can use similar custom base classes. The technique is basic
  90 enough to make a template, with a sample template class in this library.
  91 The main template parameter is the type of the enclosed member. The
  92 template class has several (explicit) constructor member templates, which
  93 implicitly type the constructor arguments and pass them to the member. The
  94 template class uses implicit copy construction and assignment, cancelling
  95 them if the enclosed member is non-copyable.
  96
  97 Manually coding a base class may be better if the construction and/or
  98 copying needs are too complex for the supplied template class, or if the
  99 compiler is not advanced enough to use it.
 100
 101 Since base classes are unnamed, a class cannot have multiple (direct) base
 102 classes of the same type. The supplied template class has an extra template
 103 parameter, an integer, that exists solely to provide type differentiation.
 104 This parameter has a default value so a single use of a particular member
 105 type does not need to concern itself with the integer.
 106
 107 [endsect]
 108
 109 [section Synopsis]
 110
 111     #include <type_traits>  /* exposition only */
 112
 113     #ifndef BOOST_BASE_FROM_MEMBER_MAX_ARITY
 114     #define BOOST_BASE_FROM_MEMBER_MAX_ARITY  10
 115     #endif
 116
 117     template < typename MemberType, int UniqueID = 0 >
 118     class boost::base_from_member
 119     {
 120     protected:
 121         MemberType  member;
 122
 123     #if ``['C++11 is in use]``
 124         template< typename ...T >
 125         explicit constexpr   base_from_member( T&& ...x )
 126          noexcept( std::is_nothrow_constructible<MemberType, T...>::value );
 127     #else
 128         base_from_member();
 129
 130         template< typename T1 >
 131         explicit  base_from_member( T1 x1 );
 132
 133         template< typename T1, typename T2 >
 134         base_from_member( T1 x1, T2 x2 );
 135
 136         //...
 137
 138         template< typename T1, typename T2, typename T3, typename T4,
 139          typename T5, typename T6, typename T7, typename T8, typename T9,
 140          typename T10 >
 141         base_from_member( T1 x1, T2 x2, T3 x3, T4 x4, T5 x5, T6 x6, T7 x7,
 142          T8 x8, T9 x9, T10 x10 );
 143     #endif
 144     };
 145
 146     template < typename MemberType, int UniqueID >
 147     class base_from_member<MemberType&, UniqueID>
 148     {
 149     protected:
 150         MemberType& member;
 151
 152         explicit constexpr base_from_member( MemberType& x )
 153             noexcept;
 154     };
 155
 156 The class template has a first template parameter `MemberType` representing
 157 the type of the based-member. It has a last template parameter `UniqueID`,
 158 that is an `int`, to differentiate between multiple base classes that use
 159 the same based-member type. The last template parameter has a default value
 160 of zero if it is omitted. The class template has a protected data member
 161 called `member` that the derived class can use for later base classes (or
 162 itself).
 163
 164 If the appropriate features of C++11 are present, there will be a single
 165 constructor template. It implements ['perfect forwarding] to the best
 166 constructor call of `member` (if any). The constructor template is marked
 167 both `constexpr` and `explicit`. The former will be ignored if the
 168 corresponding inner constructor call (of `member`) does not have the marker.
 169 The latter binds the other way; always taking effect, even when the inner
 170 constructor call does not have the marker. The constructor template
 171 propagates the `noexcept` status of the inner constructor call. (The
 172 constructor template has a trailing parameter with a default value that
 173 disables the template when its signature is too close to the signatures of
 174 the automatically-defined non-template copy- and/or move-constructors of
 175 `base_from_member`.)
 176
 177 On earlier-standard compilers, there is a default constructor and several
 178 constructor member templates. These constructor templates can take as many
 179 arguments (currently up to ten) as possible and pass them to a constructor
 180 of the data member.
 181
 182 A specialization for member references offers a single constructor taking
 183 a `MemberType&`, which is the only way to initialize a reference.
 184
 185 Since C++ does not allow any way to explicitly state the template parameters
 186 of a templated constructor, make sure that the arguments are already close
 187 as possible to the actual type used in the data member's desired constructor.
 188 Explicit conversions may be necessary.
 189
 190 The `BOOST_BASE_FROM_MEMBER_MAX_ARITY` macro constant specifies the maximum
 191 argument length for the constructor templates. The constant may be overridden
 192 if more (or less) argument configurations are needed. The constant may be
 193 read for code that is expandable like the class template and needs to
 194 maintain the same maximum size. (Example code would be a class that uses
 195 this class template as a base class for a member with a flexible set of
 196 constructors.) This constant is ignored when C++11 features are present.
 197
 198 [endsect]
 199
 200 [section Usage]
 201
 202 With the starting example, the `fdoutbuf` sub-object needs to be
 203 encapsulated in a base class that is inheirited before `std::ostream`.
 204
 205     #include <boost/utility/base_from_member.hpp>
 206
 207     #include <streambuf>  // for std::streambuf
 208     #include <ostream>    // for std::ostream
 209
 210     class fdoutbuf
 211       : public std::streambuf
 212     {
 213     public:
 214         explicit fdoutbuf( int fd );
 215         //...
 216     };
 217
 218     class fdostream
 219       : private boost::base_from_member<fdoutbuf>
 220       , public std::ostream
 221     {
 222         // Helper typedef's
 223         typedef boost::base_from_member<fdoutbuf>  pbase_type;
 224         typedef std::ostream                        base_type;
 225
 226     public:
 227         explicit fdostream( int fd )
 228           : pbase_type( fd ), base_type( &member ){}
 229         //...
 230     };
 231
 232 The base-from-member idiom is an implementation detail, so it should not
 233 be visible to the clients (or any derived classes) of `fdostream`. Due to
 234 the initialization order, the `fdoutbuf` sub-object will get initialized
 235 before the `std::ostream` sub-object does, making the former sub-object
 236 safe to use in the latter sub-object's construction. Since the `fdoutbuf`
 237 sub-object of the final type is the only sub-object with the name `member`
 238 that name can be used unqualified within the final class.
 239
 240 [endsect]
 241
 242 [section Example]
 243
 244 The base-from-member class templates should commonly involve only one
 245 base-from-member sub-object, usually for attaching a stream-buffer to an
 246 I/O stream. The next example demonstrates how to use multiple
 247 base-from-member sub-objects and the resulting qualification issues.
 248
 249     #include <boost/utility/base_from_member.hpp>
 250
 251     #include <cstddef>  /* for NULL */
 252
 253     struct an_int
 254     {
 255         int  y;
 256
 257         an_int( float yf );
 258     };
 259
 260     class switcher
 261     {
 262     public:
 263         switcher();
 264         switcher( double, int * );
 265         //...
 266     };
 267
 268     class flow_regulator
 269     {
 270     public:
 271         flow_regulator( switcher &, switcher & );
 272         //...
 273     };
 274
 275     template < unsigned Size >
 276     class fan
 277     {
 278     public:
 279         explicit fan( switcher );
 280         //...
 281     };
 282
 283     class system
 284       : private boost::base_from_member<an_int>
 285       , private boost::base_from_member<switcher>
 286       , private boost::base_from_member<switcher, 1>
 287       , private boost::base_from_member<switcher, 2>
 288       , protected flow_regulator
 289       , public fan<6>
 290     {
 291         // Helper typedef's
 292         typedef boost::base_from_member<an_int>       pbase0_type;
 293         typedef boost::base_from_member<switcher>     pbase1_type;
 294         typedef boost::base_from_member<switcher, 1>  pbase2_type;
 295         typedef boost::base_from_member<switcher, 2>  pbase3_type;
 296
 297         typedef flow_regulator  base1_type;
 298         typedef fan<6>          base2_type;
 299
 300     public:
 301         system( double x );
 302         //...
 303     };
 304
 305     system::system( double x )
 306       : pbase0_type( 0.2 )
 307       , pbase1_type()
 308       , pbase2_type( -16, &this->pbase0_type::member.y )
 309       , pbase3_type( x, static_cast<int *>(NULL) )
 310       , base1_type( pbase3_type::member, pbase1_type::member )
 311       , base2_type( pbase2_type::member )
 312     {
 313         //...
 314     }
 315
 316 The final class has multiple sub-objects with the name `member`, so any
 317 use of that name needs qualification by a name of the appropriate base
 318 type. (Using `typedef`s ease mentioning the base types.) However, the fix
 319 introduces a new problem when a pointer is needed. Using the address
 320 operator with a sub-object qualified with its class's name results in a
 321 pointer-to-member (here, having a type of `an_int boost::base_from_member<
 322 an_int, 0> :: *`) instead of a pointer to the member (having a type of
 323 `an_int *`). The new problem is fixed by qualifying the sub-object with
 324 `this->` and is needed just for pointers, and not for references or values.
 325
 326 There are some argument conversions in the initialization. The constructor
 327 argument for `pbase0_type` is converted from `double` to `float`. The first
 328 constructor argument for `pbase2_type` is converted from `int` to `double`.
 329 The second constructor argument for `pbase3_type` is a special case of
 330 necessary conversion; all forms of the null-pointer literal in C++ (except
 331 `nullptr` from C++11) also look like compile-time integral expressions, so
 332 C++ always interprets such code as an integer when it has overloads that can
 333 take either an integer or a pointer. The last conversion is necessary for the
 334 compiler to call a constructor form with the exact pointer type used in
 335 `switcher`'s constructor. (If C++11's `nullptr` is used, it still needs a
 336 conversion if multiple pointer types can be accepted in a constructor call
 337 but `std::nullptr_t` cannot.)
 338
 339 [endsect]
 340
 341 [section Acknowledgments]
 342
 343 * [@http://www.boost.org/people/ed_brey.htm Ed Brey] suggested some interface
 344 changes.
 345
 346 * [@http://www.moocat.org R. Samuel Klatchko] ([@mailto:rsk@moocat.org
 347 rsk@moocat.org], [@mailto:rsk@brightmail.com rsk@brightmail.com]) invented
 348 the idiom of how to use a class member for initializing a base class.
 349
 350 * [@http://www.boost.org/people/dietmar_kuehl.htm Dietmar Kuehl] popularized the
 351  base-from-member idiom in his [@http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/
 352   IOStream example classes].
 353
 354 * Jonathan Turkanis supplied an implementation of generating the constructor
 355 templates that can be controlled and automated with macros. The
 356 implementation uses the [@../../../preprocessor/index.html Preprocessor library].
 357
 358 * [@http://www.boost.org/people/daryle_walker.html">Daryle Walker] started the
 359 library. Contributed the test file [@../../base_from_member_test.cpp
 360 base_from_member_test.cpp].
 361
 362 [endsect]
 363