不久前,我遇到了一些代码,它们用mutable关键字标记了一个类的成员变量。据我所知,它只是允许你在const方法中修改变量:

class Foo  
{  
private:  
    mutable bool done_;  
public:  
    void doSomething() const { ...; done_ = true; }  
};

这是唯一的使用这个关键字还是有更多的它比满足眼睛?从那以后,我在一个类中使用了这种技术,将boost::mutex标记为mutable,允许const函数出于线程安全的原因锁定它,但是,说实话,这感觉有点hack。


当前回答

Mutable is used when you have a variable inside the class that is only used within that class to signal things like for example a mutex or a lock. This variable does not change the behaviour of the class, but is necessary in order to implement thread safety of the class itself. Thus if without "mutable", you would not be able to have "const" functions because this variable will need to be changed in all functions that are available to the outside world. Therefore, mutable was introduced in order to make a member variable writable even by a const function.

指定的可变变量通知编译器和读取器 成员变量可以在const中修改,这是安全和预期的吗 成员函数。

其他回答

当你重写一个const虚函数并想要修改该函数中的子类成员变量时,mutable可以很方便。在大多数情况下,您不希望更改基类的接口,因此必须使用自己的可变成员变量。

对于那些对用户来说是逻辑上无状态的东西(因此在公共类的api中应该有“const”getter),但在底层实现(你的.cpp中的代码)中不是无状态的东西,使用“mutable”。

The cases I use it most frequently are lazy initialization of state-less "plain old data" members. Namely, it is ideal in the narrow cases when such members are expensive to either build (processor) or carry around (memory) and many users of the object will never ask for them. In that situation you want lazy construction on the back end for performance, since 90% of the objects built will never need to build them at all, yet you still need to present the correct stateless API for public consumption.

它在有隐藏内部状态(如缓存)的情况下非常有用。例如:

class HashTable
{
...
public:
    string lookup(string key) const
    {
        if(key == lastKey)
            return lastValue;

        string value = lookupInternal(key);

        lastKey = key;
        lastValue = value;

        return value;
    }

private:
    mutable string lastKey, lastValue;
};

然后可以让一个const HashTable对象仍然使用它的lookup()方法,该方法修改内部缓存。

使用boost::mutex正是这个关键字的目的。另一个用途是内部结果缓存,以加快访问速度。

基本上,'mutable'适用于任何不影响对象外部可见状态的类属性。

在你的问题中的示例代码中,mutable可能是不合适的,如果done_的值影响外部状态,它取决于…;部分。

是的,这就是它的作用。我将它用于那些被方法修改的成员,而这些方法在逻辑上不会改变类的状态——例如,通过实现缓存来加速查找:

class CIniWrapper
{
public:
   CIniWrapper(LPCTSTR szIniFile);

   // non-const: logically modifies the state of the object
   void SetValue(LPCTSTR szName, LPCTSTR szValue);

   // const: does not logically change the object
   LPCTSTR GetValue(LPCTSTR szName, LPCTSTR szDefaultValue) const;

   // ...

private:
   // cache, avoids going to disk when a named value is retrieved multiple times
   // does not logically change the public interface, so declared mutable
   // so that it can be used by the const GetValue() method
   mutable std::map<string, string> m_mapNameToValue;
};

现在,你必须小心使用它——并发性问题是一个大问题,因为如果只使用const方法,调用者可能会认为它们是线程安全的。当然,修改可变数据不应该以任何重要的方式改变对象的行为,我给出的例子可能违反了这一点,例如,如果期望写入磁盘的更改将立即对应用程序可见。