移动构造函数似乎没有执行 [英] Move constructor seemingly not executed
问题描述
这是我对C ++ 0x右值引用的第一个实验,并且有些奇怪的事情正在发生。
在下面的代码示例中, $ c> MakeWindow 按值返回一个Window对象。调用者使用它来初始化一个Window对象。如果我理解正确,这应该调用移动构造函数。为了检测这个,我抛出一个异常。除此之外,我禁用了复制构造函数:
#include< iostream>
//假的WinAPI
typedef void * HWND;
HWND CreateWindow(){return(void *)1; }
void DestroyWindow(HWND){}
//结束WinAPI
// C ++ WinAPI Wrapper库
类Window
{
public:
Window(HWND inHandle):
mHandle(inHandle)
{
std :: cout< Window constructor。Handle:< inHandle<< std :: endl;
}
窗口(窗口&& rhs):
mHandle(rhs.mHandle)
{
std :: cout< Window move constructor。Handle:< mHandle< std :: endl;
rhs.mHandle = 0;
throw 1; //这是我的断点
}
〜Window()
{
std :: cout< Window destructor。Handle:< mHandle< std :: endl;
if(mHandle)
{
DestroyWindow(mHandle);
}
}
private:
Window(const Window&);
窗口& operator =(const Window&);
HWND mHandle;
};
//工厂函数
窗口MakeWindow()
{
return Window(CreateWindow());
}
int main()
{
{
窗口窗口(MakeWindow
}
std :: cout<< 一切都好。 << std :: endl;
return 0;
}
然而,代码运行正常,没有抛出这个异常。这是控制台输出:
窗口构造函数。句柄:0x1
窗口析构函数。句柄:0x1
一切正常。
如果我注释掉移动构造函数,那么编译失败,出现以下错误:
MysteryMove.cpp:在函数'Window MakeWindow()':
MysteryMove.cpp:39:5:error:'Window :: Window (const Window&)'是私有的
MysteryMove.cpp:49:33:error:在这个上下文中
MysteryMove.cpp:在函数'int main()':
MysteryMove.cpp: 39:5:错误:'Window :: Window(const Window&)'是私人
MysteryMove.cpp:57:35:错误:在此上下文中
make:*** [all] Error 1
似乎没有意义。
感谢@Philipp我学习了move constructors也可以被省略。这在 N3126标准草案的§12.8/ 34和脚注124中有所描述。
它还提到RVO只允许用于非易失性对象。这意味着我可以这样写工厂函数:
//工厂函数
窗口MakeWindow
{
volatile窗口窗口(CreateWindow());
return const_cast< Window&&>(window);
}
确实有效:
窗口构造函数。句柄:0x1
窗口移动构造函数。句柄:0x1
抛出一个'int'实例后调用终止
中止陷阱
这不是很明显吗?您的代码会返回本地临时 c>窗口 c:
的副本:窗口MakeWindow()
{
return Window(CreateWindow());
}
编译器事实上会优化这个拷贝(通过返回值优化)这就是为什么你的移动构造函数从未实际调用 - 但为了正确性,一个副本构造函数仍然必须存在。
This is my first experiment with C++0x rvalue references and something strange seems to be going on.
In the code sample below the factory function MakeWindow
returns a Window object by value. The caller uses it to initialize a Window object. If I understood correctly, this should invoke the move constructor. In order to detect this I throw an exception there. On top of that I disabled the copy constructor:
#include <iostream>
// Fake WinAPI
typedef void* HWND;
HWND CreateWindow() { return (void*)1; }
void DestroyWindow(HWND) { }
// End WinAPI
// C++ WinAPI Wrapper Library
class Window
{
public:
Window(HWND inHandle) :
mHandle(inHandle)
{
std::cout << "Window constructor. Handle: " << inHandle << std::endl;
}
Window(Window && rhs) :
mHandle(rhs.mHandle)
{
std::cout << "Window move constructor. Handle: " << mHandle << std::endl;
rhs.mHandle = 0;
throw 1; // this is my "breakpoint"
}
~Window()
{
std::cout << "Window destructor. Handle: " << mHandle << std::endl;
if (mHandle)
{
DestroyWindow(mHandle);
}
}
private:
Window(const Window&);
Window& operator=(const Window&);
HWND mHandle;
};
// Factory function
Window MakeWindow()
{
return Window(CreateWindow());
}
int main()
{
{
Window window(MakeWindow());
}
std::cout << "Everything is OK." << std::endl;
return 0;
}
However the code runs fine without this exception being thrown. This is the console output:
Window constructor. Handle: 0x1
Window destructor. Handle: 0x1
Everything is OK.
If I comment out the move constructor then compilation fails with the following errors:
MysteryMove.cpp: In function 'Window MakeWindow()':
MysteryMove.cpp:39:5: error: 'Window::Window(const Window&)' is private
MysteryMove.cpp:49:33: error: within this context
MysteryMove.cpp: In function 'int main()':
MysteryMove.cpp:39:5: error: 'Window::Window(const Window&)' is private
MysteryMove.cpp:57:35: error: within this context
make: *** [all] Error 1
It doesn't seem to make sense. Can anyone explain what is going on?
Update
Thanks to @Philipp I learned that move constructors can also be omitted. This is described in §12.8/34 and footnote 124 of the N3126 draft standard.
It is there also mentioned that RVO is only allowed for non-volatile objects. This means I can get around it writing the factory function like this:
// Factory function
Window MakeWindow()
{
volatile Window window(CreateWindow());
return const_cast<Window&&>(window);
}
And indeed it works:
Window constructor. Handle: 0x1
Window move constructor. Handle: 0x1
terminate called after throwing an instance of 'int'
Abort trap
Isn’t it obvious? Your code returns a copy of the local temporary Window
:
Window MakeWindow()
{
return Window(CreateWindow());
}
The compiler will in fact optimize this copy away (via return value optimization) – this is why your move constructor is never actually called – but for correctness a copy constructor must still be present.
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