这个额外的参数传递给虚拟析构函数是什么？ [英] What's this extra parameter passed into virtual destructor?

问题描述

我有这个代码：

  class Class $ {
 public：
 virtual〜 {} 
}; 
 
 int main（）
 {
 Class * object = new Class（）; 
 delete object; 
} 
  

我使用Visual C ++ 10编译并得到这个反汇编 delete object 语句：

  
 test eax，eax 
 je wmain + 23h（401041h）
 mov edx，dword ptr [eax] 
 push 1 
 mov ecx，eax 
 call dword ptr [edx] 
  

这是实际的析构函数：

  Class ::`scalar deleting destructor'：
 test byte ptr [esp + 4]，1 
 push esi 
 mov esi ，ecx 
 mov dword ptr [esi]，offset Class ::`vftable'（402100h）
 je Class ::`标量删除析构函数'+ 18h（401018h）
 push esi 
 call dword ptr [__imp_operator delete（4020A8h）] 
 pop ecx 
 mov eax，esi 
 pop esi 
 ret 4 
  

push 1 在调用站点做什么以及为什么是 在析构函数入口点检查该值并有条件地绕过调用 operator delete（）？

解决方案

参数被析构函数用来知道是否应该在结束时调用delete。

您不想调用它：

• 析构函数由派生类析构函数调用




• 对象是另一个对象的字段，因此不是由新创建的






编辑：添加第三个案例




I have this code:

class Class {
public:
    virtual ~Class() {}
};

int main()
{
    Class* object = new Class();
    delete object;
}

which I compile with Visual C++ 10 and get this disassembly for delete object statement:

delete object;
test        eax,eax  
je          wmain+23h (401041h)  
mov         edx,dword ptr [eax]  
push        1  
mov         ecx,eax  
call        dword ptr [edx]

and this for the actual destructor:

Class::`scalar deleting destructor':
test        byte ptr [esp+4],1  
push        esi  
mov         esi,ecx  
mov         dword ptr [esi],offset Class::`vftable' (402100h)  
je          Class::`scalar deleting destructor'+18h (401018h)  
push        esi  
call        dword ptr [__imp_operator delete (4020A8h)]  
pop         ecx  
mov         eax,esi  
pop         esi  
ret         4

What is that push 1 doing at the call site and why is the test at the destructor entry point checking for that value and conditionally bypass call to operator delete()?

解决方案

The argument is used by the destructor to know if it should call delete at the end.

3 cases where you don't want to call it :

• The destructor is called by a derived class destructor
• The object is allocated on the stack, thus not created with new.
• The object is a field of another object, thus not created by new

EDIT: Add a third case

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