c++ - 有没有办法调用纯虚类的 "deleting destructor"？

Question

我在 Ubuntu Trusty 上使用 C++11 和 g++4.8。

考虑这个片段

class Parent {
public:
    virtual ~Parent() =  default;
    virtual void f() = 0;
};

class Child: public Parent {
public:
    void f(){}
};

使用调用

{
    Child o;
    o.f();
}
{
    Parent * o  = new Child;
    delete o;
}
{
    Child * o  = new Child;
    delete o;
}

我使用 gcov 生成我的代码覆盖率报告。它报告从未调用带有符号 _ZN6ParentD0Ev 的析构函数，而 _ZN6ParentD2Ev 是。

回答Dual emission of constructor symbols和 GNU GCC (g++): Why does it generate multiple dtors?报告 _ZN6ParentD0Ev 是删除构造函数。

是否有在 Parent 类上调用此“删除析构函数”的情况？

附属问题:如果没有，是否有办法让 gcov/lcov 代码覆盖工具(在 Detailed guide on using gcov with CMake/CDash? 的回答后使用)在其报告中忽略该符号？

Answer 1

我认为这是因为您有 Child 对象，而不是 Parent 对象。

{
    Child o;
    o.f();
} // 1

{
    Parent * o  = new Child;
    delete o;
} // 2

{
    Child * o  = new Child;
    delete o;
} // 3

在//1中，o被销毁，调用了Child的完整对象析构函数。由于 Child 继承了 Parent，它会调用 的 base object destructor，即 _ZN6ParentD2Ev父级。

在//2中，o是动态分配和删除的，Child的删除析构函数是叫。然后，它将调用Parent 的基础对象析构函数。在两者中，都调用了基础对象析构函数。

//3 相同。它等于 //2，除了 o 的类型。

---

我已经在 cygwin & g++ 4.8.3 & windows 7 x86 SP1 上测试过它。这是我的测试代码。

class Parent
{
public:
    virtual ~Parent() { }
    virtual void f() = 0;
};

class Child : public Parent
{
public:
    void f() { }
};

int main()
{
    {
        Child o;
        o.f();
    }
    {
        Parent * o  = new Child;
        delete o;
    }
    {
        Child * o  = new Child;
        delete o;
    }
}

和编译 & gcov 选项:

$ g++ -std=c++11 -fprofile-arcs -ftest-coverage -O0 test.cpp -o test
$ ./test
$ gcov -b -f test.cpp

这是结果。

        -:    0:Source:test.cpp
        -:    0:Graph:test.gcno
        -:    0:Data:test.gcda
        -:    0:Runs:1
        -:    0:Programs:1
function _ZN6ParentC2Ev called 2 returned 100% blocks executed 100%
        2:    1:class Parent
        -:    2:{
        -:    3:public:
function _ZN6ParentD0Ev called 0 returned 0% blocks executed 0%
function _ZN6ParentD1Ev called 0 returned 0% blocks executed 0%
function _ZN6ParentD2Ev called 3 returned 100% blocks executed 75%
        3:    4:    virtual ~Parent() = default;
call    0 never executed
call    1 never executed
branch  2 never executed
branch  3 never executed
call    4 never executed
branch  5 taken 0% (fallthrough)
branch  6 taken 100%
call    7 never executed
        -:    5:    virtual void f() = 0;
        -:    6:};
        -:    7:
function _ZN5ChildD0Ev called 2 returned 100% blocks executed 100%
function _ZN5ChildD1Ev called 3 returned 100% blocks executed 75%
function _ZN5ChildC1Ev called 2 returned 100% blocks executed 100%
        7:    8:class Child : public Parent
call    0 returned 100%
call    1 returned 100%
call    2 returned 100%
branch  3 taken 0% (fallthrough)
branch  4 taken 100%
call    5 never executed
call    6 returned 100%
        -:    9:{
        -:   10:public:
function _ZN5Child1fEv called 1 returned 100% blocks executed 100%
        1:   11:    void f() { }
        -:   12:};
        -:   13:
function main called 1 returned 100% blocks executed 100%
        1:   14:int main()
        -:   15:{
        -:   16:    {
        1:   17:        Child o;
        1:   18:        o.f();
call    0 returned 100%
call    1 returned 100%
        -:   19:    }
        -:   20:    {
        1:   21:        Parent * o  = new Child;
call    0 returned 100%
call    1 returned 100%
        1:   22:        delete o;
branch  0 taken 100% (fallthrough)
branch  1 taken 0%
call    2 returned 100%
        -:   23:    }
        -:   24:    {
        1:   25:        Child * o  = new Child;
call    0 returned 100%
call    1 returned 100%
        1:   26:        delete o;
branch  0 taken 100% (fallthrough)
branch  1 taken 0%
call    2 returned 100%
        -:   27:    }
        1:   28:}

如您所见，_ZN6ParentD2Ev，Base 的基础对象解构被调用，而 Base 的其他对象未被调用。

然而，_ZN5ChildD0Ev，删除 Child 的析构函数，被调用两次，_ZN5ChildD1Ev，Child 的完整对象析构函数>，被调用了三次，因为有 delete o; 和 Child o;。

但根据我的解释，_ZN5ChildD0Ev 应该被调用两次而 _ZN5ChildD1Ev 应该被调用一次，不是吗？为了找出原因，我这样做了:

$ objdump -d test > test.dmp

结果:

00403c88 <__ZN5ChildD0Ev>:
  403c88:   55                      push   %ebp
  403c89:   89 e5                   mov    %esp,%ebp
  403c8b:   83 ec 18                sub    $0x18,%esp
  403c8e:   a1 20 80 40 00          mov    0x408020,%eax
  403c93:   8b 15 24 80 40 00       mov    0x408024,%edx
  403c99:   83 c0 01                add    $0x1,%eax
  403c9c:   83 d2 00                adc    $0x0,%edx
  403c9f:   a3 20 80 40 00          mov    %eax,0x408020
  403ca4:   89 15 24 80 40 00       mov    %edx,0x408024
  403caa:   8b 45 08                mov    0x8(%ebp),%eax
  403cad:   89 04 24                mov    %eax,(%esp)
  403cb0:   e8 47 00 00 00          call   403cfc <__ZN5ChildD1Ev>
  403cb5:   a1 28 80 40 00          mov    0x408028,%eax
  403cba:   8b 15 2c 80 40 00       mov    0x40802c,%edx
  403cc0:   83 c0 01                add    $0x1,%eax
  403cc3:   83 d2 00                adc    $0x0,%edx
  403cc6:   a3 28 80 40 00          mov    %eax,0x408028
  403ccb:   89 15 2c 80 40 00       mov    %edx,0x40802c
  403cd1:   8b 45 08                mov    0x8(%ebp),%eax
  403cd4:   89 04 24                mov    %eax,(%esp)
  403cd7:   e8 a4 f9 ff ff          call   403680 <___wrap__ZdlPv>
  403cdc:   a1 30 80 40 00          mov    0x408030,%eax
  403ce1:   8b 15 34 80 40 00       mov    0x408034,%edx
  403ce7:   83 c0 01                add    $0x1,%eax
  403cea:   83 d2 00                adc    $0x0,%edx
  403ced:   a3 30 80 40 00          mov    %eax,0x408030
  403cf2:   89 15 34 80 40 00       mov    %edx,0x408034
  403cf8:   c9                      leave  
  403cf9:   c3                      ret    
  403cfa:   90                      nop
  403cfb:   90                      nop

是的，因为 _ZN5ChildD0Ev 调用了 _ZN5ChildD1Ev，所以 _ZN5ChildD1Ev 被调用了三次。 (1 + 2) 我想这只是 GCC 的实现 - 用于减少重复。