C++核心准则R.1: 使用资源句柄自动管理资源并RAII(资源获取即初始化)

面向对象思考

发布于 2020-03-25 17:05:17

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发布于 2020-03-25 17:05:17

R.1: Manage resources automatically using resource handles and RAII (Resource Acquisition Is Initialization) R.1: 使用资源句柄自动管理资源并RAII(资源获取即初始化)

Reason(原因)

To avoid leaks and the complexity of manual resource management. C++'s language-enforced constructor/destructor symmetry mirrors the symmetry inherent in resource acquire/release function pairs such as fopen/fclose, lock/unlock, and new/delete. Whenever you deal with a resource that needs paired acquire/release function calls, encapsulate that resource in an object that enforces pairing for you -- acquire the resource in its constructor, and release it in its destructor.

避免手动管理资源时发生泄露和复杂性。C++语言鼓励构造函数/析构函数的对称性映射资源确保/释放函数对中包含的本质的对称性。这些函数对包括fopen/fclose，lock/unlock，new/deletre等。无论什么时候，你处理一个需要成对调用申请/释放函数时，用一个强制进行成对操作的对象封装资源--在它的构造函数中申请资源，在它的析构函数中释放资源。

Example, bad(反面示例)

Consider(考虑如下代码):

void send(X* x, cstring_span destination)
{
    auto port = open_port(destination);
    my_mutex.lock();
    // ...
    send(port, x);
    // ...
    my_mutex.unlock();
    close_port(port);
    delete x;
}

In this code, you have to remember to unlock, close_port, and delete on all paths, and do each exactly once. Further, if any of the code marked ... throws an exception, then x is leaked and my_mutex remains locked.

在这段代码中，你必须记得在所有路径上unlock，close_port和delete，而且确切地只做一次。另外，如果任何一处标有...的代码抛出了异常，那么x就会泄露，my_mutex会保持锁定。

Example(示例)

Consider(考虑):

void send(unique_ptr<X> x, cstring_span destination)  // x owns the X
{
    Port port{destination};            // port owns the PortHandle
    lock_guard<mutex> guard{my_mutex}; // guard owns the lock
    // ...
    send(port, x);
    // ...
} // automatically unlocks my_mutex and deletes the pointer in x

Now all resource cleanup is automatic, performed once on all paths whether or not there is an exception. As a bonus, the function now advertises that it takes over ownership of the pointer.

现在所有的资源清除都是自动的，在每条路径上执行一次。无论是否存在异常。作为额外的奖励，这个函数对外宣称它负责资源的所有权。

What is Port? A handy wrapper that encapsulates the resource:

什么是Port？一个封装资源的便利的容器。

class Port {
    PortHandle port;
public:
    Port(cstring_span destination) : port{open_port(destination)} { }
    ~Port() { close_port(port); }
    operator PortHandle() { return port; }

    // port handles can't usually be cloned, so disable copying and assignment if necessary
    Port(const Port&) = delete;
    Port& operator=(const Port&) = delete;
};

Note(注意)

Where a resource is "ill-behaved" in that it isn't represented as a class with a destructor, wrap it in a class or use finally

当资源由于没有表现为一个带有虚构函数的类而存在"病态行为",用一个类封装它或者使用finally。