c++ - 为什么同一个对象的析构函数被调用了两次？

Question

在核心转储 A2 的以下回溯中:~A2 被调用了两次:

#0  0x086f5371 in B1::~B1 (this=0xe6d3a030, 
    __in_chrg=<value optimized out>)
    at /fullpath/b1.cpp:400
#1  0x086ffd43 in ~B2 (this=0xe6d3a030, 
    __in_chrg=<value optimized out>)
    at /fullpath/b2.h:21
#2  B2::~B2 (this=0xe6d3a030, 
    __in_chrg=<value optimized out>)
    at /fullpath/b2.h:21
#3  0x086ea516 in A1::~A1 (this=0xe3e93958, 
    __in_chrg=<value optimized out>)
    at /fullpath/a1.cpp:716
#4  0x0889b85d in A2::~A2 (this=0xe3e93958, 
    __in_chrg=<value optimized out>)
    at /fullpath/a2.cpp:216
#5  0x0889b893 in A2::~A2 (this=0xe3e93958, 
    __in_chrg=<value optimized out>)
    at /fullpath/a2.cpp:216
#6  0x0862c0f1 in E::Identify (this=0xe8083e20, t=PT_UNKNOWN)
    at /fullpath/e.cpp:713

A2 派生自 A1，B2 派生自 B1。只有 B2 有默认析构函数，所有基类析构函数都是虚拟的。

代码看起来像这样:

e.cpp:

E::E(){
    //... some code ...
    myA1= new A2();
}

void E::Identify(){
    //...
    if(myA1){
        delete myA1; //line 713 of e.cpp
        myA1 = NULL;
    }

}

a2.cpp:

A2::~A2(){
    //...
    if (sd) //sd is not null here and also not made null after deletion
    {
        delete [] sd; //when called the second time shouldn't it crash here?
    }
    //...
} // line 216 of a2.cpp

a1.cpp

A1::A1(){
//...
   myB1 = new B2();
//...
}

A1::~A1(){
//...
    delete myB1; //line 716 of a1.cpp
//...
}

我不明白为什么 A2::~A2 被同一个对象调用两次(回溯中的 this 指针对于第 4 帧和第 5 帧具有相同的值)。

如果我转到第 4 帧并进行反汇编，它会打印出与第 5 帧反汇编代码截然不同的结果(大约 90 行汇编代码与大约 20 行汇编代码)。

Answer 1

我将示例最小化为

#include <cassert>
class A1 {
public:
    virtual ~A1() {
        assert(false);
    }
};

class A2 : public A1 {
};

int main() {
    A1* a = new A2;
    delete a;
    return 0;
}

使用断言触发核心转储。

用g++ 4.7.2编译，我们在gdb中得到双析构函数回溯

#0  0x00007f16060e92c5 in raise () from /usr/lib/libc.so.6
#1  0x00007f16060ea748 in abort () from /usr/lib/libc.so.6
#2  0x00007f16060e2312 in __assert_fail_base () from /usr/lib/libc.so.6
#3  0x00007f16060e23c2 in __assert_fail () from /usr/lib/libc.so.6
#4  0x00000000004007c8 in A1::~A1 (this=0xf60010, __in_chrg=<optimized out>) at double.cpp:6
#5  0x000000000040084d in A2::~A2 (this=0xf60010, __in_chrg=<optimized out>) at double.cpp:10
#6  0x0000000000400880 in A2::~A2 (this=0xf60010, __in_chrg=<optimized out>) at double.cpp:10
#7  0x000000000040078c in main () at double.cpp:15

虽然使用 g++ 4.3.2 编译的相同代码的回溯看起来很相似，但 A2::~A2 只有一帧。

使用相同版本的 gdb (7.5.1) 提取的两个回溯。

因此它是 g++ 4.7 生成的代码的产物，无需担心已编译二进制文件的行为。这不是对析构函数的真正双重调用。