VC:__declspec(novtable)

C++ vtable 优化

最新推荐文章于 2024-12-18 16:06:29 发布

转载最新推荐文章于 2024-12-18 16:06:29 发布 · 577 阅读

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标题	VC:__declspec(novtable) 选择自 toby 的 Blog
关键字	VC:__declspec(novtable)
出处

C++里virtual的缺陷就是vtable会增大代码的尺寸，看vcl时，object pascal里virtual也有vtable的问题，于是又了dynamic，两种方法各有利弊。但是在C++里却没有这样的机制，原来也没深想过，今天看MFC代码时，在CObject的定义时：
class AFX_NOVTABLE CObject
{
...
}
AFX_NOVTABLE是什么东东？是个宏，在Afxver_.h中：
#if _MSC_VER >= 1100 && !defined(_DEBUG)
#define AFX_NOVTABLE __declspec(novtable)
#else
#define AFX_NOVTABLE
#endif
也就是说在你编译Release版本时，在CObject前是__declspec(novtable)，在debug版本没有这个限制。MSDN里的解释是：
-----------------------------------------------------------------

Microsoft Specific

This is a __declspec extended attribute.

This form of __declspec can be applied to any class declaration, but should only be applied to pure interface classes, that is, classes that will never be instantiated on their own. The __declspec stops the compiler from generating code to initialize the vfptr in the constructor(s) and destructor of the class. In many cases, this removes the only references to the vtable that are associated with the class and, thus, the linker will remove it. Using this form of __declspec can result in a significant reduction in code size.

If you attempt to instantiate a class marked with novtable and then access a class member, you will receive an access violation (AV).

Example

// novtable.cpp
#include <stdio.h>
class __declspec(novtable) X
{
public:
   virtual void mf();
};

class Y : public X
{
public:
   void mf()
   {
      printf("In Y/n");
   }
};

int main()
{
   // X *pX = new X();
   // pX->mf();   // AV at runtime
   Y *pY = new Y();
   pY->mf();
}

Output

In Y

END Microsoft Specific
-----------------------------------------------------------------
依照AFX_NOVTABLE的声明，对CObject在debug模式，是不起作用的，而在release模式时将移除CObject的vtable，这是release比debug版本的尺寸小的原因之一吧。