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| 11 | <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
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| 13 | <title>Serialization - Tutorial</title> |
| 14 | </head> |
| 15 | <body link="#0000ff" vlink="#800080"> |
| 16 | <table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header"> |
| 17 | <tr> |
| 18 | <td valign="top" width="300"> |
| 19 | <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3> |
| 20 | </td> |
| 21 | <td valign="top"> |
| 22 | <h1 align="center">Serialization</h1> |
| 23 | <h2 align="center">Tutorial</h2> |
| 24 | </td> |
| 25 | </tr> |
| 26 | </table> |
| 27 | <hr> |
| 28 | <dl class="page-index"> |
| 29 | <dt><a href="#simplecase">A Very Simple Case</a> |
| 30 | <dt><a href="#nonintrusiveversion">Non Intrusive Version</a> |
| 31 | <dt><a href="#serializablemembers">Serializable Members</a> |
| 32 | <dt><a href="#derivedclasses">Derived Classes</a> |
| 33 | <dt><a href="#pointers">Pointers</a> |
| 34 | <dt><a href="#arrays">Arrays</a> |
| 35 | <dt><a href="#stl">STL Collections</a> |
| 36 | <dt><a href="#versioning">Class Versioning</a> |
| 37 | <dt><a href="#splitting">Splitting <code style="white-space: normal">serialize</code> into <code style="white-space: normal">save/load</code></a> |
| 38 | <dt><a href="#archives">Archives</a> |
| 39 | <dt><a href="#examples">List of examples</a> |
| 40 | </dl> |
| 41 | An output archive is similar to an output data stream. Data can be saved to the archive |
| 42 | with either the << or the & operator: |
| 43 | <pre><code> |
| 44 | ar << data; |
| 45 | ar & data; |
| 46 | </code></pre> |
| 47 | An input archive is similar to an input datastream. Data can be loaded from the archive |
| 48 | with either the >> or the & operator. |
| 49 | <pre><code> |
| 50 | ar >> data; |
| 51 | ar & data; |
| 52 | </code></pre> |
| 53 | <p> |
| 54 | When these operators are invoked for primitive data types, the data is simply saved/loaded |
| 55 | to/from the archive. When invoked for class data types, the class |
| 56 | <code style="white-space: normal">serialize</code> function is invoked. Each |
| 57 | <code style="white-space: normal">serialize</code> function is uses the above operators |
| 58 | to save/load its data members. This process will continue in a recursive manner until |
| 59 | all the data contained in the class is saved/loaded. |
| 60 | |
| 61 | <h3><a name="simplecase">A Very Simple Case</a></h3> |
| 62 | These operators are used inside the <code style="white-space: normal">serialize</code> |
| 63 | function to save and load class data members. |
| 64 | <p> |
| 65 | Included in this library is a program called |
| 66 | <a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a> which illustrates how |
| 67 | to use this system. Below we excerpt code from this program to |
| 68 | illustrate with the simplest possible case how this library is |
| 69 | intended to be used. |
| 70 | <pre> |
| 71 | <code> |
| 72 | #include <fstream> |
| 73 | |
| 74 | // include headers that implement a archive in simple text format |
| 75 | #include <boost/archive/text_oarchive.hpp> |
| 76 | #include <boost/archive/text_iarchive.hpp> |
| 77 | |
| 78 | ///////////////////////////////////////////////////////////// |
| 79 | // gps coordinate |
| 80 | // |
| 81 | // illustrates serialization for a simple type |
| 82 | // |
| 83 | class gps_position |
| 84 | { |
| 85 | private: |
| 86 | friend class boost::serialization::access; |
| 87 | // When the class Archive corresponds to an output archive, the |
| 88 | // & operator is defined similar to <<. Likewise, when the class Archive |
| 89 | // is a type of input archive the & operator is defined similar to >>. |
| 90 | template<class Archive> |
| 91 | void serialize(Archive & ar, const unsigned int version) |
| 92 | { |
| 93 | ar & degrees; |
| 94 | ar & minutes; |
| 95 | ar & seconds; |
| 96 | } |
| 97 | int degrees; |
| 98 | int minutes; |
| 99 | float seconds; |
| 100 | public: |
| 101 | gps_position(){}; |
| 102 | gps_position(int d, int m, float s) : |
| 103 | degrees(d), minutes(m), seconds(s) |
| 104 | {} |
| 105 | }; |
| 106 | |
| 107 | int main() { |
| 108 | // create and open a character archive for output |
| 109 | std::ofstream ofs("filename"); |
| 110 | |
| 111 | // create class instance |
| 112 | const gps_position g(35, 59, 24.567f); |
| 113 | |
| 114 | // save data to archive |
| 115 | { |
| 116 | boost::archive::text_oarchive oa(ofs); |
| 117 | // write class instance to archive |
| 118 | oa << g; |
| 119 | // archive and stream closed when destructors are called |
| 120 | } |
| 121 | |
| 122 | // ... some time later restore the class instance to its orginal state |
| 123 | gps_position newg; |
| 124 | { |
| 125 | // create and open an archive for input |
| 126 | std::ifstream ifs("filename"); |
| 127 | boost::archive::text_iarchive ia(ifs); |
| 128 | // read class state from archive |
| 129 | ia >> newg; |
| 130 | // archive and stream closed when destructors are called |
| 131 | } |
| 132 | return 0; |
| 133 | } |
| 134 | </code> |
| 135 | </pre> |
| 136 | <p>For each class to be saved via serialization, there must exist a function to |
| 137 | save all the class members which define the state of the class. |
| 138 | For each class to be loaded via serialization, there must exist a function to |
| 139 | load theese class members in the same sequence as they were saved. |
| 140 | In the above example, these functions are generated by the |
| 141 | template member function <code style="white-space: normal">serialize</code>. |
| 142 | |
| 143 | <h3><a name="nonintrusiveversion">Non Intrusive Version</a></h3> |
| 144 | <p>The above formulation is intrusive. That is, it requires |
| 145 | that classes whose instances are to be serialized be |
| 146 | altered. This can be inconvenient in some cases. |
| 147 | An equivalent alternative formulation permitted by the |
| 148 | system would be: |
| 149 | <pre><code> |
| 150 | #include <boost/archive/text_oarchive.hpp> |
| 151 | #include <boost/archive/text_iarchive.hpp> |
| 152 | |
| 153 | class gps_position |
| 154 | { |
| 155 | public: |
| 156 | int degrees; |
| 157 | int minutes; |
| 158 | float seconds; |
| 159 | gps_position(){}; |
| 160 | gps_position(int d, int m, float s) : |
| 161 | degrees(d), minutes(m), seconds(s) |
| 162 | {} |
| 163 | }; |
| 164 | |
| 165 | namespace boost { |
| 166 | namespace serialization { |
| 167 | |
| 168 | template<class Archive> |
| 169 | void serialize(Archive & ar, gps_position & g, const unsigned int version) |
| 170 | { |
| 171 | ar & g.degrees; |
| 172 | ar & g.minutes; |
| 173 | ar & g.seconds; |
| 174 | } |
| 175 | |
| 176 | } // namespace serialization |
| 177 | } // namespace boost |
| 178 | </code></pre> |
| 179 | <p> |
| 180 | In this case the generated serialize functions are not members of the |
| 181 | <code style="white-space: normal">gps_position</code> class. The two formulations function |
| 182 | in exactly the same way. |
| 183 | <p> |
| 184 | The main application of non-intrusive serialization is to permit serialization |
| 185 | to be implemented for classes without changing the class definition. |
| 186 | In order for this to be possible, the class must expose enough information |
| 187 | to reconstruct the class state. In this example, we presumed that the |
| 188 | class had <code style="white-space: normal">public</code> members - not a common occurence. Only |
| 189 | classes which expose enough information to save and restore the class |
| 190 | state will be serializable without changing the class definition. |
| 191 | <h3><a name="serializablemembers">Serializable Members</a></h3> |
| 192 | <p> |
| 193 | A serializable class with serializable members would look like this: |
| 194 | <pre><code> |
| 195 | class bus_stop |
| 196 | { |
| 197 | friend class boost::serialization::access; |
| 198 | template<class Archive> |
| 199 | void serialize(Archive & ar, const unsigned int version) |
| 200 | { |
| 201 | ar & latitude; |
| 202 | ar & longitude; |
| 203 | } |
| 204 | gps_position latitude; |
| 205 | gps_position longitude; |
| 206 | protected: |
| 207 | bus_stop(const gps_position & lat_, const gps_position & long_) : |
| 208 | latitude(lat_), longitude(long_) |
| 209 | {} |
| 210 | public: |
| 211 | bus_stop(){} |
| 212 | // See item # 14 in Effective C++ by Scott Meyers. |
| 213 | // re non-virtual destructors in base classes. |
| 214 | virtual ~bus_stop(){} |
| 215 | }; |
| 216 | </code></pre> |
| 217 | <p>That is, members of class type are serialized just as |
| 218 | members of primitive types are. |
| 219 | <p> |
| 220 | Note that saving an instance of the class <code style="white-space: normal">bus_stop</code> |
| 221 | with one of the archive operators will invoke the |
| 222 | <code style="white-space: normal">serialize</code> function which saves |
| 223 | <code style="white-space: normal">latitude</code> and |
| 224 | <code style="white-space: normal">longitude</code>. Each of these in turn will be saved by invoking |
| 225 | <code style="white-space: normal">serialize</code> in the definition of |
| 226 | <code style="white-space: normal">gps_position</code>. In this manner the whole |
| 227 | data structure is saved by the application of an archive operator to |
| 228 | just its root item. |
| 229 | |
| 230 | |
| 231 | <h3><a name="derivedclasses">Derived Classes</a></h3> |
| 232 | <p>Derived classes should include serializations of their base classes. |
| 233 | <pre><code> |
| 234 | #include <boost/serialization/base_object.hpp> |
| 235 | |
| 236 | class bus_stop_corner : public bus_stop |
| 237 | { |
| 238 | friend class boost::serialization::access; |
| 239 | template<class Archive> |
| 240 | void serialize(Archive & ar, const unsigned int version) |
| 241 | { |
| 242 | // serialize base class information |
| 243 | ar & boost::serialization::base_object<bus_stop>(*this); |
| 244 | ar & street1; |
| 245 | ar & street2; |
| 246 | } |
| 247 | std::string street1; |
| 248 | std::string street2; |
| 249 | virtual std::string description() const |
| 250 | { |
| 251 | return street1 + " and " + street2; |
| 252 | } |
| 253 | public: |
| 254 | bus_stop_corner(){} |
| 255 | bus_stop_corner(const gps_position & lat_, const gps_position & long_, |
| 256 | const std::string & s1_, const std::string & s2_ |
| 257 | ) : |
| 258 | bus_stop(lat_, long_), street1(s1_), street2(s2_) |
| 259 | {} |
| 260 | }; |
| 261 | </code> |
| 262 | </pre> |
| 263 | <p> |
| 264 | Note the serialization of the base classes from the derived |
| 265 | class. Do <b>NOT</b> directly call the base class serialize |
| 266 | functions. Doing so might seem to work but will bypass the code |
| 267 | that tracks instances written to storage to eliminate redundancies. |
| 268 | It will also bypass the writing of class version information into |
| 269 | the archive. For this reason, it is advisable to always make member |
| 270 | <code style="white-space: normal">serialize</code> functions private. The declaration |
| 271 | <code style="white-space: normal">friend boost::serialization::access</code> will grant to the |
| 272 | serialization library access to private member variables and functions. |
| 273 | <p> |
| 274 | <h3><a name="pointers">Pointers</a></h3> |
| 275 | Suppose we define a bus route as an array of bus stops. Given that |
| 276 | <ol> |
| 277 | <li>we might have several types of bus stops (remember bus_stop is |
| 278 | a base class) |
| 279 | <li>a given bus_stop might appear in more than one route. |
| 280 | </ol> |
| 281 | it's convenient to represent a bus route with an array of pointers |
| 282 | to <code style="white-space: normal">bus_stop</code>. |
| 283 | <pre> |
| 284 | <code> |
| 285 | class bus_route |
| 286 | { |
| 287 | friend class boost::serialization::access; |
| 288 | bus_stop * stops[10]; |
| 289 | template<class Archive> |
| 290 | void serialize(Archive & ar, const unsigned int version) |
| 291 | { |
| 292 | int i; |
| 293 | for(i = 0; i < 10; ++i) |
| 294 | ar & stops[i]; |
| 295 | } |
| 296 | public: |
| 297 | bus_route(){} |
| 298 | }; |
| 299 | </code> |
| 300 | </pre> |
| 301 | Each member of the array <code style="white-space: normal">stops</code> will be serialized. |
| 302 | But remember each member is a pointer - so what can this really |
| 303 | mean? The whole object of this serialization is to permit |
| 304 | reconstruction of the original data structures at another place |
| 305 | and time. In order to accomplish this with a pointer, it is |
| 306 | not sufficient to save the value of the pointer, rather the |
| 307 | object it points to must be saved. When the member is later |
| 308 | loaded, a new object has to be created and a new pointer has |
| 309 | to be loaded into the class member. |
| 310 | <p> |
| 311 | If the same pointer is serialized more than once, only one instance |
| 312 | is be added to the archive. When read back, no data is read back in. |
| 313 | The only operation that occurs is for the second pointer is set equal to the first |
| 314 | <p> |
| 315 | Note that, in this example, the array consists of polymorphic pointers. |
| 316 | That is, each array element point to one of several possible |
| 317 | kinds of bus stops. So when the pointer is saved, some sort of class |
| 318 | identifier must be saved. When the pointer is loaded, the class |
| 319 | identifier must be read and and instance of the corresponding class |
| 320 | must be constructed. Finally the data can be loaded to newly created |
| 321 | instance of the correct type. |
| 322 | |
| 323 | As can be seen in |
| 324 | <a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a>, |
| 325 | serialization of pointers to derived classes through a base |
| 326 | clas pointer may require explicit enumeration of the derived |
| 327 | classes to be serialized. This is referred to as "registration" or "export" |
| 328 | of derived classes. This requirement and the methods of |
| 329 | satisfying it are explained in detail |
| 330 | <a href="serialization.html#derivedpointers">here</a>. |
| 331 | <p> |
| 332 | All this is accomplished automatically by the serialization |
| 333 | library. The above code is all that is necessary to accomplish |
| 334 | the saving and loading of objects accessed through pointers. |
| 335 | <p> |
| 336 | <h3><a name="arrays">Arrays</a></h3> |
| 337 | The above formulation is in fact more complex than necessary. |
| 338 | The serialization library detects when the object being |
| 339 | serialized is an array and emits code equivalent to the above. |
| 340 | So the above can be shortened to: |
| 341 | <pre> |
| 342 | <code> |
| 343 | class bus_route |
| 344 | { |
| 345 | friend class boost::serialization::access; |
| 346 | bus_stop * stops[10]; |
| 347 | template<class Archive> |
| 348 | void serialize(Archive & ar, const unsigned int version) |
| 349 | { |
| 350 | ar & stops; |
| 351 | } |
| 352 | public: |
| 353 | bus_route(){} |
| 354 | }; |
| 355 | </code> |
| 356 | </pre> |
| 357 | <h3><a name="stl">STL Collections</a></h3> |
| 358 | The above example uses an array of members. More likely such |
| 359 | an application would use an STL collection for such a purpose. |
| 360 | The serialization library contains code for serialization |
| 361 | of all STL classes. Hence, the reformulation below will |
| 362 | also work as one would expect. |
| 363 | <pre> |
| 364 | <code> |
| 365 | #include <boost/serialization/list.hpp> |
| 366 | |
| 367 | class bus_route |
| 368 | { |
| 369 | friend class boost::serialization::access; |
| 370 | std::list<bus_stop *> stops; |
| 371 | template<class Archive> |
| 372 | void serialize(Archive & ar, const unsigned int version) |
| 373 | { |
| 374 | ar & stops; |
| 375 | } |
| 376 | public: |
| 377 | bus_route(){} |
| 378 | }; |
| 379 | </code> |
| 380 | </pre> |
| 381 | <h3><a name="versioning">Class Versioning</a></h3> |
| 382 | <p> |
| 383 | Suppose we're satisfied with our <code style="white-space: normal">bus_route</code> class, build a program |
| 384 | that uses it and ship the product. Some time later, it's decided |
| 385 | that the program needs enhancement and the <code style="white-space: normal">bus_route</code> class is |
| 386 | altered to include the name of the driver of the route. So the |
| 387 | new version looks like: |
| 388 | <pre> |
| 389 | <code> |
| 390 | #include <boost/serialization/list.hpp> |
| 391 | #include <boost/serialization/string.hpp> |
| 392 | |
| 393 | class bus_route |
| 394 | { |
| 395 | friend class boost::serialization::access; |
| 396 | std::list<bus_stop *> stops; |
| 397 | std::string driver_name; |
| 398 | template<class Archive> |
| 399 | void serialize(Archive & ar, const unsigned int version) |
| 400 | { |
| 401 | ar & driver_name; |
| 402 | ar & stops; |
| 403 | } |
| 404 | public: |
| 405 | bus_route(){} |
| 406 | }; |
| 407 | </code> |
| 408 | </pre> |
| 409 | Great, we're all done. Except... what about people using our application |
| 410 | who now have a bunch of files created under the previous program. |
| 411 | How can these be used with our new program version? |
| 412 | <p> |
| 413 | In general, the serialization library stores a version number in the |
| 414 | archive for each class serialized. By default this version number is 0. |
| 415 | When the archive is loaded, the version number under which it was saved |
| 416 | is read. The above code can be altered to exploit this |
| 417 | <pre> |
| 418 | <code> |
| 419 | #include <boost/serialization/list.hpp> |
| 420 | #include <boost/serialization/string.hpp> |
| 421 | #include <boost/serialization/version.hpp> |
| 422 | |
| 423 | class bus_route |
| 424 | { |
| 425 | friend class boost::serialization::access; |
| 426 | std::list<bus_stop *> stops; |
| 427 | std::string driver_name; |
| 428 | template<class Archive> |
| 429 | void serialize(Archive & ar, const unsigned int version) |
| 430 | { |
| 431 | // only save/load driver_name for newer archives |
| 432 | if(version > 0) |
| 433 | ar & driver_name; |
| 434 | ar & stops; |
| 435 | } |
| 436 | public: |
| 437 | bus_route(){} |
| 438 | }; |
| 439 | |
| 440 | BOOST_CLASS_VERSION(bus_route, 1) |
| 441 | </code> |
| 442 | </pre> |
| 443 | By application of versioning to each class, there is no need to |
| 444 | try to maintain a versioning of files. That is, a file version |
| 445 | is the combination of the versions of all its constituent classes. |
| 446 | |
| 447 | This system permits programs to be always compatible with archives |
| 448 | created by all previous versions of a program with no more |
| 449 | effort than required by this example. |
| 450 | |
| 451 | <h3><a name="splitting">Splitting <code style="white-space: normal">serialize</code> |
| 452 | into <code style="white-space: normal">save/load</code></a></h3> |
| 453 | The <code style="white-space: normal">serialize</code> function is simple, concise, and guarantees |
| 454 | that class members are saved and loaded in the same sequence |
| 455 | - the key to the serialization system. However, there are cases |
| 456 | where the load and save operations are not as similar as the examples |
| 457 | used here. For example, this could occur with a class that has evolved through |
| 458 | multiple versions. The above class can be reformulated as: |
| 459 | <pre> |
| 460 | <code> |
| 461 | #include <boost/serialization/list.hpp> |
| 462 | #include <boost/serialization/string.hpp> |
| 463 | #include <boost/serialization/version.hpp> |
| 464 | #include <boost/serialization/split_member.hpp> |
| 465 | |
| 466 | class bus_route |
| 467 | { |
| 468 | friend class boost::serialization::access; |
| 469 | std::list<bus_stop *> stops; |
| 470 | std::string driver_name; |
| 471 | template<class Archive> |
| 472 | void save(Archive & ar, const unsigned int version) const |
| 473 | { |
| 474 | // note, version is always the latest when saving |
| 475 | ar & driver_name; |
| 476 | ar & stops; |
| 477 | } |
| 478 | template<class Archive> |
| 479 | void load(Archive & ar, const unsigned int version) |
| 480 | { |
| 481 | if(version > 0) |
| 482 | ar & driver_name; |
| 483 | ar & stops; |
| 484 | } |
| 485 | BOOST_SERIALIZATION_SPLIT_MEMBER() |
| 486 | public: |
| 487 | bus_route(){} |
| 488 | }; |
| 489 | |
| 490 | BOOST_CLASS_VERSION(bus_route, 1) |
| 491 | </code> |
| 492 | </pre> |
| 493 | The macro <code style="white-space: normal">BOOST_SERIALIZATION_SPLIT_MEMBER()</code> generates |
| 494 | code which invokes the <code style="white-space: normal">save</code> |
| 495 | or <code style="white-space: normal">load</code> |
| 496 | depending on whether the archive is used for saving or loading. |
| 497 | <h3><a name="archives">Archives</a></h3> |
| 498 | Our discussion here has focused on adding serialization |
| 499 | capability to classes. The actual rendering of the data to be serialized |
| 500 | is implemented in the archive class. Thus the stream of serialized |
| 501 | data is a product of the serialization of the class and the |
| 502 | archive selected. It is a key design decision that these two |
| 503 | components be independent. This permits any serialization specification |
| 504 | to be usable with any archive. |
| 505 | <p> |
| 506 | In this tutorial, we have used a particular |
| 507 | archive class - <code style="white-space: normal">text_oarchive</code> for saving and |
| 508 | <code style="white-space: normal">text_iarchive</code> for loading. |
| 509 | text archives render data as text and are portable across platforms. In addition |
| 510 | to text archives, the library includes archive class for native binary data |
| 511 | and xml formatted data. Interfaces to all archive classes are all identical. |
| 512 | Once serialization has been defined for a class, that class can be serialized to |
| 513 | any type of archive. |
| 514 | <p> |
| 515 | If the current set of archive classes doesn't provide the |
| 516 | attributes, format, or behavior needed for a particular application, |
| 517 | one can either make a new archive class or derive from an existing one. |
| 518 | This is described later in the manual. |
| 519 | |
| 520 | <h3><a name="examples">List of Examples</h3> |
| 521 | <dl> |
| 522 | <dt><a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a> |
| 523 | <dd>This is the completed example used in this tutorial. |
| 524 | It does the following: |
| 525 | <ol> |
| 526 | <li>Creates a structure of differing kinds of stops, routes and schedules |
| 527 | <li>Displays it |
| 528 | <li>Serializes it to a file named "testfile.txt" with one |
| 529 | statement |
| 530 | <li>Restores to another structure |
| 531 | <li>Displays the restored structure |
| 532 | </ol> |
| 533 | <a href="../example/demo_output.txt" target="demo_output">Output of |
| 534 | this program</a> is sufficient to verify that all the |
| 535 | originally stated requirements for a serialization system |
| 536 | are met with this system. The <a href="../example/demofile.txt" |
| 537 | target="test_file">contents of the archive file</a> can |
| 538 | also be displayed as serialization files are ASCII text. |
| 539 | |
| 540 | <dt><a href="../example/demo_xml.cpp" target="demo_xml_cpp">demo_xml.cpp</a> |
| 541 | <dd>This is a variation the original demo which supports xml archives in addition |
| 542 | to the others. The extra wrapping macro, BOOST_SERIALIZATION_NVP(name), is |
| 543 | needed to associate a data item name with the corresponding xml |
| 544 | tag. It is importanted that 'name' be a valid xml tag, else it |
| 545 | will be impossible to restore the archive. |
| 546 | For more information see |
| 547 | <a target="detail" href="wrappers.html#nvp">Name-Value Pairs</a>. |
| 548 | <a href="../example/demo_save.xml" target="demo_save_xml">Here</a> |
| 549 | is what an xml archive looks like. |
| 550 | |
| 551 | <dt><a href="../example/demo_xml_save.cpp" target="demo_xml_save_cpp">demo_xml_save.cpp</a> |
| 552 | and <a href="../example/demo_xml_load.cpp" target="demo_xml_load_cpp">demo_xml_load.cpp</a> |
| 553 | <dd>Note also that though our examples save and load the program data |
| 554 | to an archive within the same program, this merely a convenience |
| 555 | for purposes of illustration. In general, the archive may or may |
| 556 | not be loaded by the same program that created it. |
| 557 | </dl> |
| 558 | <p> |
| 559 | The astute reader might notice that these examples contain a subtle but important flaw. |
| 560 | They leak memory. The bus stops are created in the <code style="white-space: normal"> |
| 561 | main</code> function. The bus schedules may refer to these bus stops |
| 562 | any number of times. At the end of the main function after the bus schedules are destroyed, |
| 563 | the bus stops are destroyed. This seems fine. But what about the structure |
| 564 | <code style="white-space: normal">new_schedule</code> data item created by the |
| 565 | process of loading from an archive? This contains its own separate set of bus stops |
| 566 | that are not referenced outside of the bus schedule. These won't be destroyed |
| 567 | anywhere in the program - a memory leak. |
| 568 | <p> |
| 569 | There are couple of ways of fixing this. One way is to explicitly manage the bus stops. |
| 570 | However, a more robust and transparent is to use |
| 571 | <code style="white-space: normal">shared_ptr</code> rather than raw pointers. Along |
| 572 | with serialization implementations for the Standard Library, the serialization library |
| 573 | includes implementation of serialization for |
| 574 | <code style="white-space: normal">boost::shared ptr</code>. Given this, it should be |
| 575 | easy to alter any of these examples to eliminate the memory leak. This is left |
| 576 | as an excercise for the reader. |
| 577 | |
| 578 | <hr> |
| 579 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
| 580 | Distributed under the Boost Software License, Version 1.0. (See |
| 581 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
| 582 | </i></p> |
| 583 | </body> |
| 584 | </html> |