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:
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//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
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#include<iostream>
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#include stdio>
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using namespace std;
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class Base
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{
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public:
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~Base() {printf("\nBase::destructor.");}
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};
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class Derived: public Base
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{
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virtual void show()
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{
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cout<<"show"<
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}
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public:
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~Derived(){printf("\nDerived::destructor.");}
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};
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int main()
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{
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Base* pB=NULL;
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Derived *pD= new Derived;
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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)。
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/*unsigned char *pC=(unsigned char*)pB;
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pC=pC-4;
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delete pC;//这样就可以释放new Derived分配的内存,而不会发生崩溃。
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*/
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delete pB;
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}
其main函数对应的汇编代码如下(VC6.0):
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int main()
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23: {
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00401060 push ebp
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00401061 mov ebp,esp
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00401063 push 0FFh
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00401065 push offset __ehhandler$_main (00416a0b)
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0040106A mov eax,fs:[00000000]
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00401070 push eax
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00401071 mov dword ptr fs:[0],esp
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00401078 sub esp,64h
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0040107B push ebx
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0040107C push esi
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0040107D push edi
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0040107E lea edi,[ebp-70h]
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00401081 mov ecx,19h
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00401086 mov eax,0CCCCCCCCh
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0040108B rep stos dword ptr [edi]
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24:Base* pB=NULL;
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0040108D mov dword ptr [ebp-10h],0
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25: Derived *pD= new Derived;
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00401094 push 4
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00401096 call operator new (00403780)
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0040109B add esp,4
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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。
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004010A1 mov dword ptr [ebp-4],0
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004010A8 cmp dword ptr [ebp-1Ch],0 //判断operator new(...)函数分配内存是否成功。
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004010AC je main+5Bh (004010bb)
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004010AE mov ecx,dword ptr [ebp-1Ch] //调用Derived::Derived()函数,ecx保存的是内存指针。
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004010B1 call @ILT+35(Derived::Derived) (00401028)
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004010B6 mov dword ptr [ebp-28h],eax
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004010B9 jmp main+62h (004010c2)
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004010BB mov dword ptr [ebp-28h],0
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004010C2 mov eax,dword ptr [ebp-28h]
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004010C5 mov dword ptr [ebp-18h],eax
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004010C8 mov dword ptr [ebp-4],0FFFFFFFFh
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004010CF mov ecx,dword ptr [ebp-18h]
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004010D2 mov dword ptr [ebp-14h],ecx
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26: pB=pD;
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004010D5 cmp dword ptr [ebp-14h],0
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004010D9 je main+86h (004010e6)
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004010DB mov edx,dword ptr [ebp-14h]
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004010DE add edx,4
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004010E1 mov dword ptr [ebp-2Ch],edx
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004010E4 jmp main+8Dh (004010ed)
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004010E6 mov dword ptr [ebp-2Ch],0
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004010ED mov eax,dword ptr [ebp-2Ch]
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004010F0 mov dword ptr [ebp-10h],eax //pB=(unsinged char*)(pD)+4
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27:
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28: delete pB;
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004010F3 mov ecx,dword ptr [ebp-10h]
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004010F6 mov dword ptr [ebp-24h],ecx
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004010F9 mov edx,dword ptr [ebp-24h]
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004010FC mov dword ptr [ebp-20h],edx
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004010FF cmp dword ptr [ebp-20h],0
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00401103 je main+0B4h (00401114)
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00401105 push 1
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00401107 mov ecx,dword ptr [ebp-20h]
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0040110A call @ILT+20(Base::`scalar deleting destructor
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