C++ 借来的资源，如何还的潇洒？

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"正文"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所谓的资源就是，一旦用了它，将来必须还给系统。如果不是这样，糟糕的事情就会发生。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"C++ 程序内常见的资源："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"动态分配内存"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件描述符"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"互斥锁"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"图形页面中的字型和笔刷"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"数据库连接"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"网络 sockets"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"无论哪一种资源，重要的是，当你不再使用它时，必须将它还给系统，有借有还是个好习惯。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"细节 01 ： 以对象管理资源"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"把资源放在析构函数，交给析构函数释放资源"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假设某个 class 含有个工厂函数，该函数获取了对象的指针："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"A* createA(); // 返回指针，指向的是动态分配对象。\n // 调用者有责任删除它。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如上述注释所言，createA 的调用端使用了函数返回的对象后，有责任删除它。现在考虑有个"},{"type":"codeinline","content":[{"type":"text","text":"f"}]},{"type":"text","text":"函数履行了这个责任:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"void f()\n{\n A *pa = createA(); // 调用工厂函数\n ... // 其他代码\n delete pa; // 释放资源\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这看起来稳妥，但存在若干情况"},{"type":"codeinline","content":[{"type":"text","text":"f"}]},{"type":"text","text":"函数可能无法执行到"},{"type":"codeinline","content":[{"type":"text","text":"delete pa"}]},{"type":"text","text":"语句，也就会造成"},{"type":"text","marks":[{"type":"strong"}],"text":"资源泄漏"},{"type":"text","text":"，例如如下情况："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"或许因为「...」区域内的一个过早的 return 语句；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"或许因为「...」区域内的一个循环语句过早的continue 或 goto 语句退出；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"或许因为「...」区域内的语句抛出异常，无法执行到 delete。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当然可以通过谨慎地编写程序可以防止这一类错误，但你必须想想，代码可能会在时间渐渐过去后被修改，如果是一个新手没有注意这一类情况，那必然又会再次有内存泄漏的可能性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为确保 A 返回的资源都是被回收，我们需要将资源放进对象内，当对象离开作用域时，该对象的析构函数会自动释放资源。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"「智能指针」是个好帮手，交给它去管理指针对象。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"对于是由动态分配（new）于堆内存的对象，指针对象离开了作用域并不会自动调用析构函数（需手动delete）"},{"type":"text","text":"，为了让指针对象能像普通对象一样，离开作用域自动调用析构函数回收资源，我们需要借助「智能指针」的特性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"常用的「智能指针」有如下三个："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"std::auto_ptr（ C++ 98 提供、C++ 11 建议摒弃不用 ）"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"std::unique_ptr（ C++ 11 提供 ）"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"std::shared_ptr（ C++ 11 提供 ）"}]}]}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"std::auto_ptr"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面示范如何使用 std::auto_ptr 以避免 "},{"type":"codeinline","content":[{"type":"text","text":"f"}]},{"type":"text","text":" 函数潜在的资源泄漏可能性："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"void f()\n{\n std::auto_ptr pa (createA()); // 调用工厂函数\n ... // 一如既往的使用pa\n} // 离开作用域后，经由 auto_ptr 的析构函数自动删除pa;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个简单的例子示范「以对象管理资源」的两个关键想法："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"获得资源后立刻放进管理对象内"},{"type":"text","text":"。以上代码中 createA 返回的资源被当做其管理者 auto_ptr 的初值，也就立刻被放进了管理对象中。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"管理对象运用析构函数确保资源释放"},{"type":"text","text":"。不论控制流如何离开区块，一旦对象被销毁（例如当对象离开作用域）其析构函数自然会被自动调用，于是资源被释放。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么在 C++11 建议弃用 auto"},{"type":"text","marks":[{"type":"italic"}],"text":"ptr 吗？当然是 auto"},{"type":"text","text":"ptr 存在缺陷，所以后续不被建议使用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"auto_ptr 有一个不寻常的特质：若通过「复制构造函数或赋值操作符函数」 "},{"type":"text","marks":[{"type":"strong"}],"text":"copy"},{"type":"text","text":" 它们，它们会变成 "},{"type":"text","marks":[{"type":"strong"}],"text":"null"},{"type":"text","text":" ，而复制所得的指针将获取资源的唯一拥有权！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"见如下例子说明："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"std::auto_ptr pa1(createA()); // pa1 指向 createA 返回物\n\nstd::auto_ptr pa2(pa1); // 现在 pa2 指向对象，pa1将被设置为 null\n\npa1 = pa2; // 现在 pa1 指向对象，pa2 将被设置为 null"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这一诡异的复制行为，如果再次使用指向为 null 的指针，那必然会导致程序"},{"type":"text","marks":[{"type":"strong"}],"text":"奔溃"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"意味着 auto_ptr 并非管理动态分配资源的神兵利器。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"std::unique_ptr"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"unique_ptr 也采用所有权模型，但是在使用时，是直接"},{"type":"text","marks":[{"type":"strong"}],"text":"禁止"},{"type":"text","text":"通过复制构造函数或赋值操作符函数 copy 指针对象，如下例子在编译时，会出错："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"std::unique_ptr pa1(createA()); // pa1 指向 createA 返回物\n\nstd::unique_ptr pa2(pa1); // 编译出错！\n\npa1 = pa2; // 编译出错！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"std::shared_ptr"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"shared_ptr 在使用复制构造函数或赋值操作符函数后，"},{"type":"text","marks":[{"type":"strong"}],"text":"引用计会数累加并且两个指针对象指向的都是同一个块内存"},{"type":"text","text":"，这就与 unique"},{"type":"text","marks":[{"type":"italic"}],"text":"ptr、auto"},{"type":"text","text":"ptr 不同之处。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"void f()\n{\n std::shared_ptr pa1(createA()); // pa1 指向 createA 返回物\n \n std::shared_ptr pa2(pa1); // 引用计数+1，pa2和pa1指向同一个内存\n \n pa1 = pa2; // 引用计数+1，pa2和pa1指向同一个内存\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当一个对象离开作用域，shared_ptr 会把引用计数值 -1 ，"},{"type":"text","marks":[{"type":"strong"}],"text":"直到引用计数值为 0 时，才会进行删除对象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由于 shared_ptr 释放空间时会事先要判断"},{"type":"text","marks":[{"type":"strong"}],"text":"引用计数值的大小"},{"type":"text","text":"，因此不会出现多次删除一个对象的错误。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小结 - 请记住"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为防止资源泄漏，请使用 RAII（Resource Acquisition Is Initaliaztion - 资源取得时机便是初始化时机） 对象，它们在构造函数中获取资源，并在析构函数中是释放资源"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"两个建议使用的 RAII classes 分别是 std::unique"},{"type":"text","marks":[{"type":"italic"}],"text":"ptr 和 std::shared"},{"type":"text","text":"ptr。前者不允许 copy 动作，后者允许 copy 动作。但是不建议用 std::auto"},{"type":"text","marks":[{"type":"italic"}],"text":"ptr，若选 auto"},{"type":"text","text":"ptr，复制动作会使它（被复制物）指向 null 。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"细节 02：在资源管理类中小心 copying 行为"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假设，我们使用 C 语音的 API 函数处理类型为 Mutex 的互斥对象，共有 lock 和 unlock 两函数可用："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"void locak(Mutex *pm); // 锁定 pm 所指的互斥器\nvoid unlock(Mutex* pm); // 将互斥器解除锁定"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为确保绝不会忘记一个被锁住的 Mutex 解锁，我们可能会希望创立一个 class 来管理锁资源。这样的 class 要遵守 RAII 守则，也就是「资源在构造期间获得，在析构释放期间释放」："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"class Lock\n{\npublic:\n explicit Lock(Mutex *pm) // 构造函数\n : pMutex(pm)\n {\n lock(pMutex);\n }\n \n ~Lock() // 析构函数\n {\n unlock(pMutex);\n }\nprivate:\n Mutex* pMutex;\n};"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这样定义的 Lock，用法符合 RAII 方式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"Mutex m; //定义你需要的互斥锁\n... \n{ // 建立一个局部区块作用域\n Lock m1(&m); // 锁定互斥器\n ...\n} // 在离开区块作用域，自动解除互斥器锁定"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这很好，但如果 Lock 对象被复制，会发生什么事情？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"Lock m1(&m); // 锁定m\nLock m2(&m1); // 将 m1 复制到 m2身上，这会发生什么？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这是我们需要思考和面对的：「当一个 RAII 对象被复制，会发生什么事情？」大多数时候你会选择以下两种可能："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"禁止复制"},{"type":"text","text":"。如果 RAII 不允许被复制，那我们需要将 class 的复制构造函数和赋值操作符函数声明在 private。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用引用计数法"},{"type":"text","text":"。有时候我们希望保有资源，直到它直的最后一个对象被消耗。这种情况下复制 RAII 对象时，应该将资源的「被引用数」递增。std::shared_ptr 便是如此。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果前述的 Lock 打算使用"},{"type":"text","marks":[{"type":"strong"}],"text":"使用引用计数法"},{"type":"text","text":"，它可以使用 std::shared"},{"type":"text","marks":[{"type":"italic"}],"text":"ptr 来管理 pMutex 指针，然后很不幸 std::shared"},{"type":"text","text":"ptr 的默认行为是「当引用次数为 0 时删除其所指物」那不是我们想要的行为，"},{"type":"text","marks":[{"type":"strong"}],"text":"因为要对 Mutex 释放动作是解锁而非删除。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"幸运的是 std::shared_ptr 允许指定"},{"type":"text","marks":[{"type":"strong"}],"text":"自定义的删除方式"},{"type":"text","text":"，那是一个函数或函数对象。如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"class Lock\n{\npublic:\n explicit Lock(Mutex *pm) \n : pMutex(pm, unlock) // 以某个 Mutex 初始化 shared_ptr，\n // 并以 unlock 函数为删除器。\n {\n lock(pMutex.get()); // get 获取指针地址\n }\n \nprivate:\n std::shared_ptr pMutex; // 使用 shared_ptr\n};"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"请注意，本例的 Lock class 不再声明析构函数。因为编译器会自动创立默认的析构函数，来自动调用其 non-static 成员变量（本例为 pMutex ）的析构函数。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而 pMutex 的析构函数会"},{"type":"text","marks":[{"type":"strong"}],"text":"在互斥器的引用次数为 0 时，自动调用 std::shared_ptr 的删除器（本例为 unlock ）"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小结 - 请记住"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"复制 RAII 对象必须一并复制它的所管理的资源（深拷贝），所以资源的 copying 行为决定 RAII 对象的 copying 行为。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"普通而常见的 RAII class copying 行为是：禁止 copying、施行引用计数法。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"细节 03 ：在资源类中提供对原始资源的访问"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"智能指针「显式」转换，也就是通过 get 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这个调用动作绝不至于造成泄漏。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上的方式，就能避免原本由于次序导致内存泄漏发生。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小结 - 请记住"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以独立语句将 newed （已 new 过） 对象存储于智能指针内。如果不这样做，一旦异常被抛出，有可能导致难以察觉的资源泄漏。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"最后"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文部分内容参考了《Effective C++ （第3版本）》第三章节内容，前两章节的内容可看旧文"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"《"},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/3dUQpmKsA-6LHopYY9u4HQ","title":""},"content":[{"type":"text","text":"学过 C++ 的你，不得不知的这 10 条细节！"}]},{"type":"text","text":"》"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c5/c5a651ce460ea82cc1e4a612c6a374ab.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}