复制句柄，然后关闭原始句柄

Question

我正在开发以下类：

class Handle
{
public:
    inline Handle()
    {
        handle = INVALID_HANDLE_VALUE;
    }
    inline Handle(HANDLE handle)
    {
        this->handle = copyHandle(handle);
    }
    inline Handle(const Handle& rhs)
    {
        this->handle = copyHandle(rhs.handle);
    }
    inline bool isValid()
    {
        return handle != INVALID_HANDLE_VALUE;
    }
    inline HANDLE getNativeHandle()
    {
        return copyHandle(this->handle);
    }
    inline void close()
    {
        if(handle != INVALID_HANDLE_VALUE)
        {
            CloseHandle(handle);
            handle = INVALID_HANDLE_VALUE;
        }

    }
    inline virtual ~Handle()
    {
        if(handle != INVALID_HANDLE_VALUE)
            CloseHandle(handle);
    }
protected:
    HANDLE handle;
    HANDLE copyHandle(HANDLE copyable);
};

.cpp文件：

HANDLE Handle::copyHandle(HANDLE copyable)
{
    HANDLE ret;
    HANDLE current = GetCurrentProcess();
    if(copyable == INVALID_HANDLE_VALUE)
        ret = copyable;

    else if(DuplicateHandle(current, copyable, current, &ret, 0, TRUE , DUPLICATE_SAME_ACCESS) == 0)
        {
            if(GetLastError() == ERROR_ACCESS_DENIED)
                throw SecurityException("The handle duplication was denied!");
            else
                throw InvalidHandleException("The handle could not be duplicated!");
        }

    return ret;
}

这个类看起来工作正常，但复制句柄，然后关闭原始句柄，然后复制新句柄将抛出异常或Windows Errorcode 6，这是“无效句柄值”。

目前，我认为关闭原始句柄也会导致副本完全销毁，并使我无法在以后使用它们。

Handle test = CreateMutex(NULL, FALSE, NULL);
Handle copy = test;
test.close();
std::cout << copy.getNativeHandle() << std::endl; // throws an exception, but uses the same function as above
return 0;

是否有可能复制句柄，使其不依赖于原始句柄的存在？

Answer 1

尝试这个实现：

class Handle
{
public:
    Handle(HANDLE ahandle = INVALID_HANDLE_VALUE)
    {
        handle = ahandle; // <- take ownership of the original, not a copy
    }

    Handle(const Handle& src)     
    {
        handle = src.duplicate(); // <-- take ownership of a copy
    }

    ~Handle()
    {
        close();
    }

    void close()
    {
        if (handle != INVALID_HANDLE_VALUE)
        {
            CloseHandle(handle);
            handle = INVALID_HANDLE_VALUE;
        }
    }

    HANDLE getNativeHandle() const
    {
        return handle;
    }

    bool isValid() const
    {
        return (handle != INVALID_HANDLE_VALUE);
    }

    HANDLE duplicate()
    {
        if (handle == INVALID_HANDLE_VALUE)
            return handle;

        HANDLE ret, current = GetCurrentProcess();
        if (!DuplicateHandle(current, handle, current, &ret, 0, TRUE, DUPLICATE_SAME_ACCESS))
        {
            if (GetLastError() == ERROR_ACCESS_DENIED)
                throw SecurityException("The handle duplication was denied!");
            else
                throw InvalidHandleException("The handle could not be duplicated!");
        }

        return ret;
    }

    Handle& operator=(HANDLE &rhs)
    {
        close();
        handle = rhs; // <-- take ownership of the original, not a copy
        return *this;
    }

    Handle& operator=(const Handle &rhs)
    {
        close();
        handle = rhs.duplicate(); // <-- take ownership of a copy
        return *this;
    }

protected:
    HANDLE handle;
};

另外，一些API函数使用NULL而不是INVALID_HANDLE_VALUE，还有一些函数不使用CloseHandle()。您应该考虑考虑这些差异。我建议将Handle类更新为使用C++模板，这样您就可以在每个实例的基础上专门化行为，例如：

struct InvalidHandleTrait
{
    static const HANDLE InvalidValue = INVALID_HANDLE_VALUE;
};

struct NullHandleTrait
{
    static const HANDLE InvalidValue = NULL;
};

struct CloseHandleTrait
{
    static bool close(HANDLE handle)
    {
        return CloseHandle(handle);
    }
};

template< typename HandleTrait = InvalidHandleTrait, typename CloseTrait = CloseHandleTrait >
class Handle
{
public:
    Handle(HANDLE ahandle = HandleTrait::InvalidValue)
    {
        handle = ahandle; // <- take ownership of the original, not a copy
    }

    Handle(const Handle& src)     
    {
        handle = src.duplicate(); // <-- take ownership of a copy
    }

    ~Handle()
    {
        close();
    }

    void close()
    {
        if (handle != HandleTrait::InvalidValue)
        {
            CloseTrait::close(handle);
            handle = HandleTrait::InvalidValue;
        }
    }

    HANDLE getNativeHandle() const
    {
        return handle;
    }

    bool isValid() const
    {
        return (handle != HandleTrait::InvalidValue);
    }

    HANDLE duplicate()
    {
        if (handle == HandleTrait::InvalidValue)
            return handle;

        HANDLE ret, current = GetCurrentProcess();
        if (!DuplicateHandle(current, handle, current, &ret, 0, TRUE, DUPLICATE_SAME_ACCESS))
        {
            if (GetLastError() == ERROR_ACCESS_DENIED)
                throw SecurityException("The handle duplication was denied!");
            else
                throw InvalidHandleException("The handle could not be duplicated!");
        }

        return ret;
    }

    Handle& operator=(HANDLE &rhs)
    {
        close();
        handle = rhs; // <-- take ownership of the original, not a copy
        return *this;
    }

    Handle& operator=(const Handle &rhs)
    {
        close();
        handle = rhs.duplicate(); // <-- take ownership of a copy
        return *this;
    }

protected:
    HANDLE handle;
};

Answer 2

您还需要为Handle定义赋值运算符。我怀疑真正崩溃的代码是这样的：

Handle test = CreateMutex(NULL, FALSE, NULL);
Handle copy;
copy= test; // assigned instead of using copy constructor
test.close();
std::cout << copy.getNativeHandle() << std::endl;
return 0;

如果没有赋值运算符，则无法正确复制句柄。