RAII教程C ++

问题描述

我想学习如何在c ++中使用RAII。我想我知道它是什么，但不知道如何在我的程序中实现它。

有没有任何一个链接可以告诉我RAII？

解决方案

没有什么（就是说，我不认为你需要一个完整的教程）。

换句话说：

指针应该被封装在智能指针类中（参见std :: auto_ptr，boost :: shared_ptr和boost :: scoped_ptr）。

需要清理的句柄应该是

同步应该依赖于在范围退出时释放mutex /同步原语（参见boost :: mutex :: scoped_lock使用示例）。

我不认为你真的可以有一个RAII教程（不再是你可以有一个设计模式）。

例如，目前我使用WinAPI进行编码，我写了以下类：

  template< typename H，BOOL _stdcall CloseFunction（H）> 
 class checked_handle 
 {
 public：
 typedef checked_handle< H，CloseFunction> MyType; 
 typedef typename H HandleType; 
 
 static const HandleType NoValue; 
 
 checked_handle（const HandleType value）
：_value（value）
 {
} 
 
〜checked_handle（）
 { 
 Close（）; 
} 
 
 HandleType * operator&（）
 {
 return& _value; 
} 
 
 operator HandleType（）
 {
 return _value; 
} 
 
 private：
 HandleType _value; 
 
 void Close（const HandleType newValue = NoValue）
 {
 CloseFunction（_value）; 
 _value = newValue; 
} 
}; 
 
 template< typename H，BOOL _stdcall CloseFunction（H）> 
 const typename checked_handle< H，CloseFunction> :: HandleType 
 checked_handle< H，CloseFunction> :: NoValue = 
 checked_handle< H，CloseFunction> :: HandleType（INVALID_HANDLE_VALUE）; 
 
 typedef checked_handle< HANDLE，:: CloseHandle> CheckedHandle; 
 typedef checked_handle< HWINSTA，:: CloseWindowStation> WinStationHandle; 
 typedef checked_handle< HDESK，:: CloseDesktop> DesktopHandle; 
 typedef checked_handle< HDEVNOTIFY，:: UnregisterDeviceNotification> DevNotifyHandle; 
 typedef checked_handle< HWND，:: DestroyWindow> WindowHandle; 
 
 BOOL __stdcall CloseKey（HKEY hKey）; 
 typedef checked_handle< HKEY，CloseKey> RegHandle; 
  

这个类不包括赋值语句和复制语义

以下是它的使用方法：

类声明：

  class东西
 {
 public：
 // ... 
 private：
 WindowHandle _window; 
}; 
  

此成员已分配，但我从不调用 :: CloseWindow（_window._handle ）显式（当 Something 的实例超出作用域时会被调用（ Something ::〜Something - > WindowHandle :: WindowHandle - > :: Close（_window._value））。 / p>


I'd like to learn how to use RAII in c++. I think I know what it is, but have no idea how to implement it in my programs. A quick google search did not show any nice tutorials.

Does any one have any nice links to teach me RAII?

解决方案

There's nothing to it (that is, I don't think you need a full tutorial).

RAII can be shortly explained as "Every resource requiring cleanup should be given to an object's constructor."

In other words:

Pointers should be encapsulated in smart pointer classes (see std::auto_ptr, boost::shared_ptr and boost::scoped_ptr for examples).

Handles requiring cleanup should be encapsulated in classes that automatically free/release the handles upon destruction.

Synchronization should rely on releasing the mutex/synchronization primitive upon scope exit (see boost::mutex::scoped_lock usage for an example).

I don't think you can really have a tutorial on RAII (not anymore than you can have one on design patterns for example). RAII is more of a way of looking at resources than anything else.

For example, at the moment I'm coding using WinAPI and I wrote the following class:

template<typename H, BOOL _stdcall CloseFunction(H)>
class checked_handle
{
public:
    typedef checked_handle<H,CloseFunction> MyType;
    typedef typename H HandleType;

    static const HandleType     NoValue;

    checked_handle(const HandleType value)
        : _value(value)
    {
    }

    ~checked_handle()
    {
        Close();
    }

    HandleType* operator &()
    {
        return &_value;
    }

    operator HandleType()
    {
        return _value;
    }

private:
    HandleType      _value;

    void Close(const HandleType newValue = NoValue)
    {
        CloseFunction(_value);
        _value = newValue;
    }
};

template<typename H,BOOL _stdcall CloseFunction(H)>
const typename checked_handle<H,CloseFunction>::HandleType 
    checked_handle<H,CloseFunction>::NoValue = 
    checked_handle<H,CloseFunction>::HandleType(INVALID_HANDLE_VALUE);

typedef checked_handle<HANDLE,::CloseHandle> CheckedHandle;
typedef checked_handle<HWINSTA,::CloseWindowStation> WinStationHandle;
typedef checked_handle<HDESK,::CloseDesktop> DesktopHandle;
typedef checked_handle<HDEVNOTIFY,::UnregisterDeviceNotification> DevNotifyHandle;
typedef checked_handle<HWND,::DestroyWindow> WindowHandle;

BOOL __stdcall CloseKey(HKEY hKey);
typedef checked_handle<HKEY,CloseKey> RegHandle;

This class doesn't include assignment and copy semantics (I removed them to provide a minimal example) so returning by value, will cause the handles to be closed twice.

Here's how it's used:

class declaration:

class Something
{
public:
    // ...
private:
    WindowHandle        _window;
};

This member is allocated but I never call ::CloseWindow(_window._handle) explicitely (it will be called when instances of Something go out of scope (as Something::~Something -> WindowHandle::WindowHandle -> ::Close(_window._value) ).

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