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[C++] 深入探索 C++ 多态 ② - 继承关系

2023-08-22
c/c++

上一章 简述了虚函数的调用链路，本章主要探索 C++ 各种继承关系的类对象的多态特性。

1. 概述

封装，继承，多态是 C++ 的三大特性，其中多态与继承有密切关系。C++ 语言支持三种继承关系：单一继承，多重继承，虚拟继承：

图片来源：《多型与虚拟》

2. 继承关系

2.1. 单一继承

C++ 的单一继承是指一个类只能从一个父类继承属性和方法。

文字来源：ChatGPT

动态多态的单一继承对象类层次结构相对简单：

  1. 对象内存只有一个虚指针，并且在其首位。
  2. 虚表上的虚函数，通过层层覆盖，最终得出对象对应的虚函数表，详看下图。
  • 测试代码。
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/* g++ -O0 -std=c++11 -fdump-class-hierarchy test.cpp -o test */
#include <iostream>

class Base {
   public:
    virtual void vBaseFunc() {}
    virtual void vBaseFunc2() {}
    virtual void vBaseFunc3() {}

    long long m_base_data;
    long long m_base_data2;
};

class Base2 : public Base {
   public:
    virtual void vBaseFunc() {}
    virtual void vBase2Func() { std::cout << "Base2::vBase2Func" << std::endl; }
    virtual void vBase2Func2() {}

    long long m_base2_data;
    long long m_base2_data2;
};

class Derived : public Base2 {
   public:
    virtual void vBaseFunc2() {}
    virtual void vBase2Func() { std::cout << "Derived::vBase2Func" << std::endl; }
    virtual void vDerivedFunc() {}
    virtual void vDerivedFunc2() {}

    long long m_derived_data;
    long long m_derived_data2;
};
  • 类布局层次。
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Vtable for Base
# _ZTV4Base: vtable for Base
Base::_ZTV4Base: 5u entries
0     (int (*)(...))0
# _ZTI4Base: typeinfo for Base
8     (int (*)(...))(& _ZTI4Base)
16    (int (*)(...))Base::vBaseFunc
24    (int (*)(...))Base::vBaseFunc2
32    (int (*)(...))Base::vBaseFunc3

Vtable for Base2
Base2::_ZTV5Base2: 7u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI5Base2)
16    (int (*)(...))Base2::vBaseFunc
24    (int (*)(...))Base::vBaseFunc2
32    (int (*)(...))Base::vBaseFunc3
40    (int (*)(...))Base2::vBase2Func
48    (int (*)(...))Base2::vBase2Func2

# Derived 虚表。
Vtable for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTV7Derived: 9u entries
0     (int (*)(...))0
# _ZTI7Derived: typeinfo for Derived
8     (int (*)(...))(& _ZTI7Derived) 
16    (int (*)(...))Base2::vBaseFunc
24    (int (*)(...))Derived::vBaseFunc2
32    (int (*)(...))Base::vBaseFunc3
40    (int (*)(...))Derived::vBase2Func
48    (int (*)(...))Base2::vBase2Func2
56    (int (*)(...))Derived::vDerivedFunc
64    (int (*)(...))Derived::vDerivedFunc2

# 类的继承关系
Class Derived
   size=56 align=8
   base size=56 base align=8
Derived (0x0x7fb058fa8478) 0
    # 虚指针指向虚表的位置。
    vptr=((& Derived::_ZTV7Derived) + 16u)
  Base2 (0x0x7fb058fa8a28) 0
      primary-for Derived (0x0x7fb058fa8478)
    Base (0x0x7fb058ee87e0) 0
        primary-for Base2 (0x0x7fb058fa8a28)
  • 虚表整合。
  • 对象整体布局。

  • 虚函数调用。

    1. 对象首位保存的是虚指针 vptr，虚指针指向虚表。
    2. 虚指针指向的虚表地址向高地址偏移 0x18 个字节，这样可以获取 Derived::vBase2Func 虚函数地址，然后进行调用。
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int main() {
    auto d = new Derived;
    std::cout << d << std::endl;

    auto b = static_cast<Base2 *>(d);
    std::cout << b << std::endl;
    b->vBase2Func();
    return 0;
}

// 输出：
// 0x13a0010
// 0x13a0010
// Derived::vBase2Func

2.2. 多重继承

C++ 支持多重继承，这意味着一个类可以从多个父类继承属性和方法，在 C++ 中，可以使用逗号分隔的方式来指定多个父类。

文字来源：ChatGPT。

  • 测试代码。
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/* g++ -O0 -std=c++11 -fdump-class-hierarchy test.cpp -o test */
#include <iostream>

class Base {
   public:
    virtual void vBaseFunc() {}
    virtual void vBaseFunc2() {}

    long long m_base_data;
    long long m_base_data2;
};

class Base2 {
   public:
    virtual void vBase2Func() {}
    virtual void vBase2Func2() { std::cout << "Base2::vBase2Func2" << std::endl; }

    long long m_base2_data;
    long long m_base2_data2;
};

class Base3 {
   public:
    virtual void vBase3Func() {}
    virtual void vBase3Func2() {}

    long long m_base3_data;
    long long m_base3_data2;
};

class Derived : public Base, public Base2, public Base3 {
   public:
    virtual void vBaseFunc() {}
    virtual void vBase2Func2() { std::cout << "Derived::vBase2Func2" << std::endl; }
    virtual void vBase3Func2() {}
    virtual void vDerivedFunc() {}
    virtual void vDerivedFunc2() {}

    long long m_derived_data;
    long long m_derived_data2;
};
  • 类布局层次。
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Vtable for Base
# _ZTV4Base: vtable for Base
Base::_ZTV4Base: 4u entries
0     (int (*)(...))0
# _ZTI4Base: typeinfo for Base
8     (int (*)(...))(& _ZTI4Base)
16    (int (*)(...))Base::vBaseFunc
24    (int (*)(...))Base::vBaseFunc2

Vtable for Base2
# _ZTV5Base2: vtable for Base2
Base2::_ZTV5Base2: 4u entries
0     (int (*)(...))0
# _ZTI5Base2: typeinfo for Base2
8     (int (*)(...))(& _ZTI5Base2)
16    (int (*)(...))Base2::vBase2Func
24    (int (*)(...))Base2::vBase2Func2

Vtable for Base3
# _ZTV5Base3: vtable for Base3
Base3::_ZTV5Base3: 4u entries
0     (int (*)(...))0
# _ZTI5Base3: typeinfo for Base3
8     (int (*)(...))(& _ZTI5Base3)
16    (int (*)(...))Base3::vBase3Func
24    (int (*)(...))Base3::vBase3Func2

Vtable for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTV7Derived: 16u entries
0     (int (*)(...))0
# _ZTI7Derived: typeinfo for Derived
8     (int (*)(...))(& _ZTI7Derived)
16    (int (*)(...))Derived::vBaseFunc
24    (int (*)(...))Base::vBaseFunc2
32    (int (*)(...))Derived::vBase2Func2
40    (int (*)(...))Derived::vBase3Func2
48    (int (*)(...))Derived::vDerivedFunc
56    (int (*)(...))Derived::vDerivedFunc2
64    (int (*)(...))-24
72    (int (*)(...))(& _ZTI7Derived)
80    (int (*)(...))Base2::vBase2Func
# _ZThn24_N7Derived11vBase2Func2Ev: non-virtual thunk to Derived::vBase2Func2()
88    (int (*)(...))Derived::_ZThn24_N7Derived11vBase2Func2Ev
96    (int (*)(...))-48
104   (int (*)(...))(& _ZTI7Derived)
112   (int (*)(...))Base3::vBase3Func
# _ZThn48_N7Derived11vBase3Func2Ev: non-virtual thunk to Derived::vBase3Func2()
120   (int (*)(...))Derived::_ZThn48_N7Derived11vBase3Func2Ev

Class Derived
   size=88 align=8
   base size=88 base align=8
Derived (0x0x7f4196042348) 0
    vptr=((& Derived::_ZTV7Derived) + 16u)
  Base (0x0x7f4195f3e840) 0
      primary-for Derived (0x0x7f4196042348)
  Base2 (0x0x7f4195f3e8a0) 24
      vptr=((& Derived::_ZTV7Derived) + 80u)
  Base3 (0x0x7f4195f3e900) 48
      vptr=((& Derived::_ZTV7Derived) + 112u)

  • 虚表整合。

    1. 首先派生类的虚表与第一个基类的虚表结合成一个虚表单元，并覆盖基类的虚函数。
    2. 其它的基类，作为一个独立虚表单元。当派生类虚函数有重写基类的虚函数时，基类对应虚函数，通过 thunk 技术跳转到第一个虚表单元的对应虚函数。

  • 对象整体布局。由下图可见：

    1. 多重继承有多个虚指针，并指向对应的虚表单元。
    2. 如果派生类有 N 个多重继承单一基类，那么它的对象有 N 个虚指针和虚表单元。
  • 虚函数调用。有了上面内存布局的理解，我们应该不难理解下面这个基类指针是怎么调用派生类虚函数的：
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int main() {
    auto d = new Derived;
    std::cout << d << std::endl;

    auto b = static_cast<Base2 *>(d);
    std::cout << b << std::endl;
    b->vBase2Func2();
    return 0;
}

// 输出：
// 0x13db010
// 0x13db028
// Derived::vBase2Func2
  1. Base2 指针指向存储 vptr2 的地址：从对象内存顶部向高地址偏移 0x18 个字节，获得 vptr2 虚指针。
  2. vptr2 指针指向的虚表地址向高地址偏移 0x8 个字节，获得 non-virtual thunk to Derived::vBase2Func2() 地址。
  3. 通过 non-virtual thunk to Derived::vBase2Func2() 地址跳转到 Derived::vBase2Func2 虚函数，获取虚表上对应的虚函数地址进行调用。
  • 通过汇编理解函数 thunk to 跳转的工作原理。
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# thunk to 跳转原理（汇编）。
0000000000400aba <non-virtual thunk to Derived::vBase2Func2()>:
  # rdi 寄存器保存的是 b 指针指向的地址，该地址向低地址偏移 0x18 个字节，
  # 也就是 rdi 寄存器保存的是 Derived 内存首位地址，
  # 换句话说，将 Derived 的 this 指针作为参数传入 Derived::vBase2Func2 函数。
  400aba:    48 83 ef 18      sub    $0x18,%rdi
  # 调用 Derived::vBase2Func2() 函数。
  400abe:    eb d0            jmp    400a90 <Derived::vBase2Func2()>
  • 思考，上面多重继承的多态实例对象，下面这样释放是否正确？！（详情请参考：虚析构）。
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int main() {
    Base2* b = new Derived;
    delete b;
    return 0;
}

2.3. 虚拟继承

多重继承可以让一个类具有多个不同父类的特性，但也可能引发一些问题，比如菱形继承问题。为了解决这个问题，C++ 提供了虚继承和虚基类的概念。虚继承可以解决菱形继承问题，确保只有一个实例的共享基类。

在 C++ 中，虚拟继承（virtual inheritance）是一种特殊的继承方式。它用于解决多重继承中的菱形继承问题。当一个类通过虚拟继承从多个基类继承时，只会保留一个基类的实例，而不会重复继承。这样可以避免菱形继承带来的二义性和冗余。在虚拟继承中，派生类需要使用关键字 “virtual” 来声明基类。

文字来源：ChatGPT

因为继承关系中有共享基类，为了避免共享基类产生多个对象副本浪费内存，虚拟继承的内存布局，也会与单一继承和多重继承不一样：

  1. 公共基类的成员数据，存放于对象内存底部。
  2. 虚拟继承引入 VTT（Virtual Table Table）构造虚表。
  3. 虚表前缀引入 vbase_offset 偏移量：当前虚表与公共基类虚表的内存位置偏移量。

虚拟继承的类层次关系结构有点复杂，有兴趣的朋友可以参考：What is the VTT for a class。

2.3.1. 对象整体布局

  • 测试代码。
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/* g++ -O0 -std=c++11 -fdump-class-hierarchy test.cpp -o test */
#include <iostream>

class Base {
   public:
    virtual void vBaseFunc() {}
    virtual void vBaseFunc2() {}

    long long m_base_data = 0x11;
    long long m_base_data2 = 0x12;
};

class Base2 : virtual public Base {
   public:
    virtual void vBaseFunc() {}
    virtual void vBase2Func() {}
    virtual void vBase2Func2() {}

    long long m_base2_data = 0x21;
    long long m_base2_data2 = 0x22;
};

class Base3 : virtual public Base {
   public:
    virtual void vBaseFunc2() {}
    virtual void vBase3Func() {}
    virtual void vBase3Func2() { std::cout << "Base3::vBase3Func2" << std::endl; }
    long long m_base3_data = 0x31;
    long long m_base3_data2 = 0x32;
};

class Derived : public Base2, public Base3 {
   public:
    virtual void vBase2Func() {}
    virtual void vBase3Func2() { std::cout << "Derived::vBase3Func2" << std::endl; }
    virtual void vDerivedFunc() {}
    virtual void vDerivedFunc2() {}

    long long m_derived_data = 0x41;
    long long m_derived_data2 = 0x42;
};
  • 类对象内存布局。
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Vtable for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTV7Derived: 21u entries
0     64u
8     (int (*)(...))0
# _ZTI7Derived: typeinfo for Derived
16    (int (*)(...))(& _ZTI7Derived)
24    (int (*)(...))Base2::vBaseFunc
32    (int (*)(...))Derived::vBase2Func
40    (int (*)(...))Base2::vBase2Func2
48    (int (*)(...))Derived::vBase3Func2
56    (int (*)(...))Derived::vDerivedFunc
64    (int (*)(...))Derived::vDerivedFunc2
72    40u
80    (int (*)(...))-24
88    (int (*)(...))(& _ZTI7Derived)
96    (int (*)(...))Base3::vBaseFunc2
104   (int (*)(...))Base3::vBase3Func
# _ZThn24_N7Derived11vBase3Func2Ev: non-virtual thunk to Derived::vBase3Func2()
112   (int (*)(...))Derived::_ZThn24_N7Derived11vBase3Func2Ev
120   18446744073709551576u # -40
128   18446744073709551552u # -64
136   (int (*)(...))-64
144   (int (*)(...))(& _ZTI7Derived)
# _ZTv0_n24_N5Base29vBaseFuncEv: virtual thunk to Base2::vBaseFunc()
152   (int (*)(...))Base2::_ZTv0_n24_N5Base29vBaseFuncEv
# _ZTv0_n32_N5Base310vBaseFunc2Ev: virtual thunk to Base3::vBaseFunc2()
160   (int (*)(...))Base3::_ZTv0_n32_N5Base310vBaseFunc2Ev

Construction vtable for Base2 (0x0x7fd19d6aea90 instance) in Derived
# _ZTC7Derived0_5Base2: construction vtable for Base2-in-Derived
Derived::_ZTC7Derived0_5Base2: 12u entries
0     64u
8     (int (*)(...))0
# _ZTI5Base2: typeinfo for Base2
16    (int (*)(...))(& _ZTI5Base2)
24    (int (*)(...))Base2::vBaseFunc
32    (int (*)(...))Base2::vBase2Func
40    (int (*)(...))Base2::vBase2Func2
48    0u
56    18446744073709551552u # -64
64    (int (*)(...))-64
# _ZTI5Base2: typeinfo for Base2
72    (int (*)(...))(& _ZTI5Base2)
# _ZTv0_n24_N5Base29vBaseFuncEv: virtual thunk to Base2::vBaseFunc()
80    (int (*)(...))Base2::_ZTv0_n24_N5Base29vBaseFuncEv
88    (int (*)(...))Base::vBaseFunc2

Construction vtable for Base3 (0x0x7fd19d6aeaf8 instance) in Derived
Derived::_ZTC7Derived24_5Base3: 12u entries
0     40u
8     (int (*)(...))0
# _ZTI5Base3: typeinfo for Base3
16    (int (*)(...))(& _ZTI5Base3)
24    (int (*)(...))Base3::vBaseFunc2
32    (int (*)(...))Base3::vBase3Func
40    (int (*)(...))Base3::vBase3Func2
48    18446744073709551576u # -40
56    0u
64    (int (*)(...))-40
# _ZTI5Base3: typeinfo for Base3
72    (int (*)(...))(& _ZTI5Base3)
80    (int (*)(...))Base::vBaseFunc
# _ZTv0_n32_N5Base310vBaseFunc2Ev: virtual thunk to Base3::vBaseFunc2()
88    (int (*)(...))Base3::_ZTv0_n32_N5Base310vBaseFunc2Ev

VTT for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTT7Derived: 7u entries
0     ((& Derived::_ZTV7Derived) + 24u)
# _ZTC7Derived0_5Base2: construction vtable for Base2-in-Derived
8     ((& Derived::_ZTC7Derived0_5Base2) + 24u)
16    ((& Derived::_ZTC7Derived0_5Base2) + 80u)
# _ZTC7Derived24_5Base3: construction vtable for Base3-in-Derived
24    ((& Derived::_ZTC7Derived24_5Base3) + 24u)
32    ((& Derived::_ZTC7Derived24_5Base3) + 80u)
40    ((& Derived::_ZTV7Derived) + 152u)
48    ((& Derived::_ZTV7Derived) + 96u)

Class Derived
   size=88 align=8
   base size=64 base align=8
Derived (0x0x7fd19d7401c0) 0
    vptridx=0u vptr=((& Derived::_ZTV7Derived) + 24u)
  Base2 (0x0x7fd19d6aea90) 0
      primary-for Derived (0x0x7fd19d7401c0)
      subvttidx=8u
    Base (0x0x7fd19d5ee840) 64 virtual
        vptridx=40u vbaseoffset=-24 vptr=((& Derived::_ZTV7Derived) + 152u)
  Base3 (0x0x7fd19d6aeaf8) 24
      subvttidx=24u vptridx=48u vptr=((& Derived::_ZTV7Derived) + 96u)
    Base (0x0x7fd19d5ee840) alternative-path

2.3.2. 构造顺序

我们可以通过类的构造顺序去理解：对象内存布局如何一步一步构造出来的。在构造派生类 Derived 时，先构造基类，当基类构造完了，才构造自己。

  • 构造流程。
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|-- main
    |-- ...
    |-- Derived::Derived()
        |-- Base::Base()
        |-- Base2::Base2()
        |-- Base3::Base3()
  • 构造流程（汇编）。
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...
0x400b33:    e8 34 02 00 00    callq  0x400d6c <Derived::Derived()>
...
0x400d83:    e8 06 ff ff ff    callq  400c8e <Base::Base()>
...
0x400d97:    e8 20 ff ff ff    callq  400cbc <Base2::Base2()>
...
0x400daf:    e8 60 ff ff ff    callq  400d14 <Base3::Base3()>
  • 构造 Base。
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Vtable for Base
# _ZTV4Base: vtable for Base
Base::_ZTV4Base: 4u entries
0     (int (*)(...))0
# _ZTI4Base: typeinfo for Base
8     (int (*)(...))(& _ZTI4Base)
16    (int (*)(...))Base::vBaseFunc
24    (int (*)(...))Base::vBaseFunc2

Class Base
   size=24 align=8
   base size=24 base align=8
Base (0x0x7fd19d5ee720) 0
    vptr=((& Base::_ZTV4Base) + 16u)
  • 构造 Base2。
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Construction vtable for Base2 (0x0x7fd19d6aea90 instance) in Derived
# _ZTC7Derived0_5Base2: construction vtable for Base2-in-Derived
Derived::_ZTC7Derived0_5Base2: 12u entries
0     64u
8     (int (*)(...))0
# _ZTI5Base2: typeinfo for Base2
16    (int (*)(...))(& _ZTI5Base2)
24    (int (*)(...))Base2::vBaseFunc
32    (int (*)(...))Base2::vBase2Func
40    (int (*)(...))Base2::vBase2Func2
48    0u
56    18446744073709551552u # -64
64    (int (*)(...))-64
# _ZTI5Base2: typeinfo for Base2
72    (int (*)(...))(& _ZTI5Base2)
# _ZTv0_n24_N5Base29vBaseFuncEv: virtual thunk to Base2::vBaseFunc()
80    (int (*)(...))Base2::_ZTv0_n24_N5Base29vBaseFuncEv
88    (int (*)(...))Base::vBaseFunc2

VTT for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTT7Derived: 7u entries
0     ((& Derived::_ZTV7Derived) + 24u)
# _ZTC7Derived0_5Base2: construction vtable for Base2-in-Derived
8     ((& Derived::_ZTC7Derived0_5Base2) + 24u)
16    ((& Derived::_ZTC7Derived0_5Base2) + 80u)
# _ZTC7Derived24_5Base3: construction vtable for Base3-in-Derived
24    ((& Derived::_ZTC7Derived24_5Base3) + 24u)
32    ((& Derived::_ZTC7Derived24_5Base3) + 80u)
40    ((& Derived::_ZTV7Derived) + 152u)
48    ((& Derived::_ZTV7Derived) + 96u)
  • 构造 Base3。
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VTT for Derived
# _ZTV7Derived: vtable for Derived
Derived::_ZTT7Derived: 7u entries
0     ((& Derived::_ZTV7Derived) + 24u)
# _ZTC7Derived0_5Base2: construction vtable for Base2-in-Derived
8     ((& Derived::_ZTC7Derived0_5Base2) + 24u)
16    ((& Derived::_ZTC7Derived0_5Base2) + 80u)
# _ZTC7Derived24_5Base3: construction vtable for Base3-in-Derived
24    ((& Derived::_ZTC7Derived24_5Base3) + 24u)
32    ((& Derived::_ZTC7Derived24_5Base3) + 80u)
40    ((& Derived::_ZTV7Derived) + 152u)
48    ((& Derived::_ZTV7Derived) + 96u)

Construction vtable for Base3 (0x0x7fd19d6aeaf8 instance) in Derived
# _ZTC7Derived24_5Base3: construction vtable for Base3-in-Derived
Derived::_ZTC7Derived24_5Base3: 12u entries
0     40u
8     (int (*)(...))0
# _ZTI5Base3: typeinfo for Base3
16    (int (*)(...))(& _ZTI5Base3)
24    (int (*)(...))Base3::vBaseFunc2
32    (int (*)(...))Base3::vBase3Func
40    (int (*)(...))Base3::vBase3Func2
48    18446744073709551576u # -40
56    0u
64    (int (*)(...))-40
# _ZTI5Base3: typeinfo for Base3
72    (int (*)(...))(& _ZTI5Base3)
80    (int (*)(...))Base::vBaseFunc
# _ZTv0_n32_N5Base310vBaseFunc2Ev: virtual thunk to Base3::vBaseFunc2()
88    (int (*)(...))Base3::_ZTv0_n32_N5Base310vBaseFunc2Ev
  • 构造 Derived （参考上面 整体布局）。

2.3.3. 虚函数调用

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int main() {
    auto d = new Derived;
    std::cout << d << std::endl;

    auto b = static_cast<Base3 *>(d);
    std::cout << b << std::endl;
    b->vBase3Func2();
    return 0;
}

// 输出：
// 0x9fa010
// 0x9fa028
// Derived::vBase3Func2
  1. b 指针指向存储 vptr.base3 的地址：从 Derived 对象内存顶部向高地址偏移 0x18 个字节。
  2. vptr.base3 指针指向的虚表地址向高地址偏移 0x10 个字节，获得 non-virtual thunk to Derived::vBase3Func2() 函数地址。
  3. 通过 non-virtual thunk to Derived::vBase3Func2() 地址跳转到 Derived::vBase3Func2 虚函数，获取虚表上对应的虚函数进行调用。

3. 后记

  • 要理解多态的对象内存布局，要注意理解（多个）虚指针是如何根据不同的基类指针进行偏移的，当虚指针指向虚表后，要获得对应的虚函数，虚指针要偏移一定的位置才能定位到对应的虚表上的虚函数。

  • 如果要用一个词来形容多态，那就是 覆盖，派生类重写基类虚函数，像图层一样，（派生类）上层覆盖下层（基类），层层叠加，最后得出了被覆盖的结果；这也是我们理解 虚表 结构的核心思维方式。

  • 关于有继承关系的 C++ 多态探索，因为本人水平有限，以上只作了一些基础简单的 Demo 的分析，还有一些应用场景没有涉及（例如 虚析构）。

  • 很多技术细节没有在文章中提及，有兴趣的朋友可以动手写写 demo 用 gdb 调试一下，查看对象内存布局上的地址数据，以及反汇编查看对象构造的逻辑，是否与自己的理解一致，这样才能在不断变化的问题表象里，寻获答案本质。

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