C++中delete表达式执行的操作是:1,调用析构函数;2,释放对象内存(operator delete(...))。
如果父类的析构函数没有声明为virtual函数,且子类中至少存在一个virtual函数,此时将子类的对象地址赋值给父类指针。当对父类的指针执行delete操作时,会调用父类析构函数,然后在释放内存时(即delete表达式执行的操作的2,释放对象内存)出现崩溃。
然而如果子类中不存在一个virtual函数时,执行上面同样的操作就不会出现崩溃。
原因分析如下:
-
//已知本示例 父类的析构函数应声明为virtual函数。但是由于本程序不需要析构函数执行特殊的操作,所以delete父类指针pB同样可以释放内存,然而引起出乎意料的内//存释放异常。因此对本程序进行如下分析
-
#include<iostream>
-
#include stdio>
-
using namespace std;
-
-
class Base
-
{
-
public:
-
~Base() {printf("\nBase::destructor.");}
-
};
-
-
class Derived: public Base
-
{
-
virtual void show()
-
{
-
cout<<"show"<
-
}
-
public:
-
~Derived(){printf("\nDerived::destructor.");}
-
};
-
-
int main()
-
{
-
Base* pB=NULL;
-
Derived *pD= new Derived;
-
pB=pD;//此时pD得到的是(unsigned char*)pD+4的地址,所以执行operator delete时会发生崩溃(因为此时把vptr的内存当成了待释放的内存块的大小)。
-
/*unsigned char *pC=(unsigned char*)pB;
-
pC=pC-4;
-
delete pC;//这样就可以释放new Derived分配的内存,而不会发生崩溃。
-
*/
-
delete pB;
-
}
其main函数对应的汇编代码如下(VC6.0):
-
int main()
-
23: {
-
00401060 push ebp
-
00401061 mov ebp,esp
-
00401063 push 0FFh
-
00401065 push offset __ehhandler$_main (00416a0b)
-
0040106A mov eax,fs:[00000000]
-
00401070 push eax
-
00401071 mov dword ptr fs:[0],esp
-
00401078 sub esp,64h
-
0040107B push ebx
-
0040107C push esi
-
0040107D push edi
-
0040107E lea edi,[ebp-70h]
-
00401081 mov ecx,19h
-
00401086 mov eax,0CCCCCCCCh
-
0040108B rep stos dword ptr [edi]
-
-
24:Base* pB=NULL;
-
0040108D mov dword ptr [ebp-10h],0
-
25: Derived *pD= new Derived;
-
00401094 push 4
-
00401096 call operator new (00403780)
-
0040109B add esp,4
-
0040109E mov dword ptr [ebp-1Ch],eax //eax保存的是operator new(...)函数分配的内存的首地址。
-
004010A1 mov dword ptr [ebp-4],0
-
004010A8 cmp dword ptr [ebp-1Ch],0 //判断operator new(...)函数分配内存是否成功。
-
004010AC je main+5Bh (004010bb)
-
004010AE mov ecx,dword ptr [ebp-1Ch] //调用Derived::Derived()函数,ecx保存的是内存指针。
-
004010B1 call @ILT+35(Derived::Derived) (00401028)
-
004010B6 mov dword ptr [ebp-28h],eax
-
004010B9 jmp main+62h (004010c2)
-
004010BB mov dword ptr [ebp-28h],0
-
004010C2 mov eax,dword ptr [ebp-28h]
-
004010C5 mov dword ptr [ebp-18h],eax
-
004010C8 mov dword ptr [ebp-4],0FFFFFFFFh
-
004010CF mov ecx,dword ptr [ebp-18h]
-
004010D2 mov dword ptr [ebp-14h],ecx
-
26: pB=pD;
-
004010D5 cmp dword ptr [ebp-14h],0
-
004010D9 je main+86h (004010e6)
-
004010DB mov edx,dword ptr [ebp-14h]
-
004010DE add edx,4
-
004010E1 mov dword ptr [ebp-2Ch],edx
-
004010E4 jmp main+8Dh (004010ed)
-
004010E6 mov dword ptr [ebp-2Ch],0
-
004010ED mov eax,dword ptr [ebp-2Ch]
-
004010F0 mov dword ptr [ebp-10h],eax //pB=(unsinged char*)(pD)+4
-
27:
-
28: delete pB;
-
004010F3 mov ecx,dword ptr [ebp-10h]
-
004010F6 mov dword ptr [ebp-24h],ecx
-
004010F9 mov edx,dword ptr [ebp-24h]
-
004010FC mov dword ptr [ebp-20h],edx
-
004010FF cmp dword ptr [ebp-20h],0
-
00401103 je main+0B4h (00401114)
-
00401105 push 1
-
00401107 mov ecx,dword ptr [ebp-20h]
-
0040110A call @ILT+20(Base::`scalar deleting destructor
阅读(6641) | 评论(0) | 转发(0) |