移动构造函数和const成员变量 [英] Move constructor and const member variables

问题描述

我喜欢const成员变量的概念，尤其是当我将C函数包装到类中时。构造函数采用资源句柄（例如文件描述符），该句柄在整个对象生命周期内保持有效，而析构函数最后将其关闭。 （那是RAII背后的想法，对吗？）

但是使用C ++ 0x move构造函数时，我遇到了问题。由于析构函数也在卸载对象上调用，因此我需要防止清理资源句柄。由于成员变量是const，因此我无法分配值-1或INVALID_HANDLE（或等效值）来指示析构函数它不应执行任何操作。

如果将一个对象的状态移到另一个对象，是否可以不调用析构函数？

示例：

 类文件
 {
 public：
 //类型为命名构造函数或静态工厂方法 
静态文件open（const char * fileName，const char * modes）
 {
 FILE * handle = fopen（文件名，模式）; 
返回文件（句柄）； 
} 
 
 private：
文件* const句柄； 
 
 public：
 File（FILE * handle）：handle（handle）
 {
} 
 
〜File（）
 {
 fclose（handle）; 
} 
 
 File（File& other）：handle（other.handle）
 {
 //编译器不应调用其他 
 //对象。 
} 
 
 File（const File& other）=删除； 
 File& operator =（const File& other）=删除; 
}; 
  

解决方案

没有，没有办法。我建议，如果您真的很喜欢 handle 变量为const，则应该有一个非const标志成员变量，该变量指示破坏是否应该做任何事情。 / p>

I like the idea of const member variables especially when I wrap C functions into classes. The constructor takes a resource handle (e.g. a file descriptor) that stays valid during the whole object life time and the destructor finally closes it. (That is the idea behind RAII, right?)

But with the C++0x move constructor i run into a problem. Since the destructor is also called on the "unloaded" object i need to prevent the cleanup of the resource handle. Since the member variable is const i have no way to assign the value -1 or INVALID_HANDLE (or equivalent values) to indicate to the destructor that it should not do anything.

Is there a way that the destructor is not called if the state of an object was moved to another object?

Example:

class File
{
public:
    // Kind of "named constructor" or "static factory method"
    static File open(const char *fileName, const char *modes)
    {
        FILE *handle = fopen(fileName, modes);
        return File(handle);
    }

private:
    FILE * const handle;

public:
    File(FILE *handle) : handle(handle)
    {
    }

    ~File()
    {
        fclose(handle);
    }

    File(File &&other) : handle(other.handle)
    {
        // The compiler should not call the destructor of the "other"
        // object.
    }

    File(const File &other) = delete;
    File &operator =(const File &other) = delete;
};

解决方案

No, there is no way to do this. I would suggest that if you're really attached to the handle variable being const you should have a non-const flag member variable that indicates whether or not destruction should do anything.

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