C++对象模型探索--04数据语义-创新互联

数据语义学数据成员绑定时机

编译器对成员函数的解析是整个类定义完毕后才开始的。因为只有整个类定义完毕后编译器才能够知道类的成员变量，才能根据时机在需
要出现类成员变量的场合做出适当的解析（成员函数中解析成类中的成员变量，全局函数中解析成全局的变量）
对于成员函数参数是在编译器第一次遇到这个类型的时候决定的

进程内存空间布局

不同的数据在内存中有不同的保存时机、保存位置。在执行程序时，总内存会被分为多个段，称为文本，未初始化的全局、初始化的全局、栈和堆段。将整个程序加载到文本段中，并在堆栈存储器中选择存储器到变量。

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High Addresses --->.----------------------.
                    |      Environment     |
                    |----------------------|
                    |                      |   Functions and variable are declared
                    |         STACK        |   on the stack.
	base pointer ->| - - - - - - - - - - -|
                    |           |          |
                    |           v          |
                    :                      :
                    .                      .   The stack grows down into unused space
                    .         Empty        .   while the heap grows up. 
                    .                      .
                    .                      .   (other memory maps do occur here, such 
                    .                      .    as dynamic libraries, and different memory
                    :                      :    allocate)
                    |           ^          |
                    |           |          |
	   brk point ->| - - - - - - - - - - -|   Dynamic memory is declared on the heap
                    |          HEAP        |
                    |                      |
                    |----------------------|
                    |          BSS         |   Uninitialized data (BSS)
                    |----------------------|   
                    |          Data        |   Initialized data (DS)
                    |----------------------|
                    |          Text        |   Binary code
Low Addresses ---->'----------------------'

栈(STACK)

它位于较高的地址，与堆段的增长和收缩方向正好相反。
函数的局部变量存在于栈上
每个函数都有一个栈帧,调用函数时，将在栈中创建一个栈帧。栈帧包含函数的局部变量参数和返回值。
栈包含一个LIFO结构。函数变量在调用时被压入栈，返回时将函数变量从栈弹出。
SP（栈指针）寄存器跟踪栈的顶部。

堆(HEAP)

堆区域由进程中的所有共享库和动态加载的模块共享。它在堆栈的相反方向上增长和收缩。
用于在运行时分配内存。由内存管理函数（如malloc、calloc、free等）管理的堆区域，这些函数可以在内部使用brk和sbrk系统调用来调整其大小。

未初始化的数据块(BSS)

此段包含所有未初始化的全局和静态变量，所有变量都由零或者空指针初始化。程序加载器在加载程序时为BSS节分配内存。

初始化的数据块(DS)

此段包含显式初始化的全局变量和静态变量,大小由程序源代码中值的大小决定，在运行时不会更改。它具有读写权限，因此可以在运行时更改此段的变量值。

代码段(TEXT)

该段是一个只读段，包含已编译程序的二进制文件。该段是可共享的，因此对于文本编辑器等频繁执行的程序，内存中只需要一个副本

C++单继承下内存布局分析

#includeclass A
{public:
    int a;
    A() : a(0x1) {}
    virtual void foo() {std::cout<< "A::foo()"<< std::endl; }
    void bar() {std::cout<< "A::bar()"<< std::endl; }
};

class B : public A
{public:
    int b;
    B() : A(), b(0x2) {}
    void foo() {std::cout<< "B::foo()"<< std::endl; }
};

class C : public B
{public:
    int c;
    C() : B(), c(0x3) {}
    void foo() {std::cout<< "C::foo()"<< std::endl; }
};

int main(int argc, char **argv)
{A a;
    B b;
    C c;
    B *p = &c;
    p->foo();

    std::cout<< sizeof(int)<< " "<< sizeof(int *)<< std::endl;
    return 0;
}

g++ main.cpp -o main -std=c++14 -g
gdb查看

(gdb) b 29
Breakpoint 1 at 0x120b: file main.cpp, line 29.
(gdb) r
Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:29
29          A a;
(gdb) n
30          B b;
(gdb) set print pretty on
(gdb) set print vtbl on
(gdb) p a
$1 = {_vptr.A = 0x555555557d38,
  a = 1
}
(gdb) p/a &a
$2 = 0x7fffffffdbf0
(gdb) p/a &a.a
$3 = 0x7fffffffdbf8
(gdb) p sizeof(a)
$4 = 16
(gdb) x/2xg &a 
0x7fffffffdbf0: 0x0000555555557d38      0x00007fff00000001
(gdb) info vtbl a
vtable for 'A' @ 0x555555557d38 (subobject @ 0x7fffffffdbf0):
[0]: 0x555555555344

_vptr.A:代表a对象所含有的虚函数表指针，0x555555557d38为第一个虚函数也即foo()的地址，真正虚函数表的起始地址为0x555555557d38 - 16，还会有一些虚函数表头信息，vptr 总是指向虚函数表的第一个函数入口
对象a所在的地址为0x7fffffffdbf0，整个对象占16个字节，其中8个字节为vptr虚函数表指针，4个字节为数据int a
vtable for ‘A’ @ 0x555555557d38

gdb查看

(gdb) n
31          C c;
(gdb) p b
$6 = {= {_vptr.A = 0x555555557d20,
    a = 1
  }, 
  members of B:
  b = 2
}
(gdb) p sizeof(b)
$7 = 16
(gdb) n
32          B *p = &c;
(gdb) p c
$8 = {= {= {  _vptr.A = 0x555555557d08,
      a = 1
    }, 
    members of B:
    b = 2
  }, 
  members of C:
  c = 3
}
(gdb) p sizeof(c)
$9 = 24
(gdb) info vtbl b
vtable for 'B' @ 0x555555557d20 (subobject @ 0x7fffffffdc00):
[0]: 0x5555555553ba(gdb) info vtbl c
vtable for 'C' @ 0x555555557d08 (subobject @ 0x7fffffffdc10):
[0]: 0x555555555430

如果class B中申明了新的虚函数（比如foo2），class B中依然只有一个虚函数表，只不过会把foo2加入到该表中。此时class A的虚函数表不会包含foo2。

C++多重继承下内存布局分析

#includeclass A
{int a;
    virtual void foo() {std::cout<< "A::foo()"<< std::endl; }
};

class B
{int b;
    virtual void bar() {std::cout<< "B::bar()"<< std::endl; }
};

class C : public A, public B
{int c;
    void foo() {std::cout<< "C::foo()"<< std::endl; }
    void bar() {std::cout<< "C::bar()"<< std::endl; }
};

int main(int argc, char **argv)
{A a;
    B b;
    C c;

    std::cout<< sizeof(int)<< " "<< sizeof(int *)<< std::endl;
    
    return 0;
}

gdb查看

gdb main 
(gdb) b 28
Breakpoint 1 at 0x122f: file main.cpp, line 28.
(gdb) set print pretty on
(gdb) set print vtbl on
(gdb) set print object on
(gdb) p a
No symbol "a" in current context.
(gdb) r
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:28
28          std::cout<< sizeof(int)<< " "<< sizeof(int *)<< std::endl;
(gdb) p a
$1 = (A) {_vptr.A = 0x555555557d20,
  a = -134528544
}
(gdb) p b
$2 = (B) {_vptr.B = 0x555555557d08,
  b = -135408993
}
(gdb) p/a c
$3 = (C) {= {_vptr.A = 0x555555557cd0,
    a = 0xfffffffff7eab60a
  }, 
   = {_vptr.B = 0x555555557cf0,
    b = 0xfffffffff7fb3e88
  }, 
  members of C:
  c = 0x7fff
--Typefor more, q to quit, c to continue without paging--
}
(gdb) p sizeof(c)
$4 = 32
(gdb) x/5ag &c
0x7fffffffdc00: 0x555555557cd0<_ZTV1C+16>  0x7ffff7eab60a<_ZNSt9basic_iosIwSt11char_traitsIwEE15_M_cache_localeERKSt6locale+90>0x7fffffffdc10: 0x555555557cf0<_ZTV1C+48>  0x7ffff7fb3e88<_ZSt5wclog+8>0x7fffffffdc20: 0x7ffff7fb3940

数据成员int a, int b, int c都未初始化，此时是UB未定义行为
对象c含有两个虚函数表指针_vptr.A和_vptr.B，占用32个字节内存，3个int数据成员，两个虚函数表指针
对象c的内存布局为 c: vptr.A | a | vptr.B | b | c

C++虚继承下内存布局分析

#includeclass A
{
    int a;
    virtual void foo() { std::cout<< "A::foo()"<< std::endl; }
};

class B : virtual public A
{
    int b;
    virtual void foo() { std::cout<< "B::foo()"<< std::endl; }
};

class C : virtual public A
{
    int c;
    void foo() { std::cout<< "C::foo()"<< std::endl; }
};

class D : public B, public C
{
    int d;
    virtual void foo() { std::cout<< "D::foo()"<< std::endl; }
};

int main(int argc, char **argv)
{
    A a;
    B b;
    C c;
    D d;

    A *pa = &d;
    B *pb = &d;
    C *p c = &d;
    std::cout<< sizeof(int)<< " "<< sizeof(int *)<< std::endl;

    return 0;
}

gdb查看

gdb main
(gdb) b 37
Breakpoint 1 at 0x1247: file main.cpp, line 37.
(gdb) r
37          std::cout<< sizeof(int)<< " "<< sizeof(int *)<< std::endl;
(gdb) set print pretty on
(gdb) set print object on
(gdb) set print vtbl on
(gdb) p a
$1 = (A) {
  _vptr.A = 0x555555557ce0,
  a = 0
}
(gdb) p b
$2 = (B) {
   = {
    _vptr.A = 0x555555557cb8,
    a = -238370304
  }, 
  members of B:
  _vptr.B = 0x555555557c98,
  b = 0
--Typefor more, q to quit, c to continue without paging--
}
(gdb) p c
$3 = (C) {
   = {
    _vptr.A = 0x555555557c68,
    a = -134528544
  }, 
  members of C:
  _vptr.C = 0x555555557c48,
  c = -134529192
}
(gdb) p d
$4 = (D) {
   = {
     = {
      _vptr.A = 0x555555557b70,
      a = -134529400
    }, 
    members of B:
    _vptr.B = 0x555555557b30,
    b = -135408993
  },= {
    members of C:
    _vptr.C = 0x555555557b50,
    c = -135612918
  }, 
  members of D:
  d = 32767
}
(gdb) p &d
$5 = (D *) 0x7fffffffdbf0

对象内存布局
- a: _vptr.A | a
- b: _vptr.A | a | _vptr.B | b
- c: _vptr.A | a | _vptr.C | c
- d: _vptr.A | a | _vptr.B | b | _vptr.C | c | d
A *pa = &d;Bpb = &d;Cp c= &d;都指向d的起始地址&d = 0x7fffffffdbf0。假如d类里实现的虚函数都放在A的虚函数表里，没有实现的放在被继承的基类里面

数据成员布局

普通成员变量的存储顺序是按照在类中定义的顺序从上到下来的
类定义中public、private、protected的数量不影响类对象的sizeof
边界调整，字节对齐

数据成员存取

静态成员变量的存取
类的静态成员变量可以当作一个全局变量，它只是在类的空间内可见，引用时用类名::静态成员变量名，静态成员变量只有一个实体，保存在可执行文件的数据段。

class A 
{public:
    int m_a;
    int m_b;
    static int m_c; // 声明
}

int A::m_c = 0; // 定义

非静态的成员变量的存取
对于普通成员变量的访问，编译器是把类对象的首地址加上成员变量的偏移值

单一继承下的数据成员布局

一个子类对象所包含的内容是它自己的成员+父类的成员的总和
从偏移值看，父类成员先出现，然后才是子类成员,即子类对象中包含父类子对象

#include#includeclass Base
{public:
    int m_a;
    int m_b;
};

class Derived : public Base
{public:
    int m_i;
    int m_j;
};

int main(int argc, char **argv)
{// 打印成员变量偏移值
    printf("Base::m_a = %d\n", &Base::m_a);
    printf("Base::m_b = %d\n", &Base::m_b);

    printf("Derived::m_a = %d\n", &Derived::m_a);
    printf("Derived::m_b = %d\n", &Derived::m_b);

    printf("Derived::m_i = %d\n", &Derived::m_i);
    printf("Derived::m_j = %d\n", &Derived::m_j);

    Base b;    // m_a | m_b
    Derived d; // m_a | m_b | m_i | m_j

    return 0;
}

#include#includeclass Base1
{
public:
    int m_a;
    char m_c1;
};

class Base2 : public Base1
{
public:
    char m_c2;
};

class Base3 : public Base2
{
public:
    char m_c3;
};

int main(int argc, char **argv)
{
    Base1 b1; //  8byte: 4 | 1 | 3padding
    Base2 b2; // 12byte: 4 | 1 | 3padding | 1 | 3padding
    Base3 b3; // 12byte: 4 | 1 | 3padding | 1 | 1 | 2padding
	
    printf("sizeof(Base1) = %d\n", sizeof(Base1));
    printf("sizeof(Base2) = %d\n", sizeof(Base2));
    printf("sizeof(Base3) = %d\n", sizeof(Base3));
	// 打印成员变量偏移值
    printf("Base3::m_a = %d\n", &Base3::m_a);
    printf("Base3::m_c1 = %d\n", &Base3::m_c1);
    printf("Base3::m_c2 = %d\n", &Base3::m_c2);
    printf("Base3::m_c3 = %d\n", &Base3::m_c3);

    return 0;
}

输出结果

sizeof(Base1) = 8
sizeof(Base2) = 12
sizeof(Base3) = 12
Base3::m_a = 0
Base3::m_c1 = 4
Base3::m_c2 = 8
Base3::m_c3 = 9

单类单继承虚函数下的数据成员布局单个类带虚函数的数据成员布局

类中引入虚函数时会有额外的成本付出：

编译时编译器会产生虚函数表
对象中会产生虚函数表指针vptr，用以指向虚函数表
增加或扩展构造函数，增加给虚函数表指针vptr，让vptr指向虚函数表
如果多重继承，比如继承2个父类，每个父类都有虚函数的话，每个父类都有vptr，那么继承时，子类会把这2个vptr都继承过来，如果子类还有自己额外的虚函数，子类与第一个基类共用一个vptr

class Base
{public:
    int m_i;
    int m_j;

    Base() {}
    ~Base(){}
    virtual void func() {}
};
//内存布局为
// vptr | m_i | m_j

单一继承下父类带虚函数的数据成员布局

#include#includeclass Base
{
public:
    int m_i;

    Base() {}
    ~Base() {}
    virtual void b_func() {}
};

class Derived : public Base
{
public:
    int m_a;
    int m_b;

    Derived() {}
    ~Derived() {}

    virtual void d_func() {}
};

int main(int argc, char **argv)
{
    printf("sizeof(Base) = %d\n", sizeof(Base));
    printf("sizeof(Derived) = %d\n", sizeof(Derived));
    Base b;
    Derived d;

    printf("Base::m_i = %d\n", &Base::m_i);
    printf("Derived::m_i = %d\n", &Derived::m_i);
    printf("Derived::m_a = %d\n", &Derived::m_a);
    printf("Derived::m_b = %d\n", &Derived::m_b);

    return 0;
}

输出结果为

sizeof(Base) = 16
sizeof(Derived) = 24
Base::m_i = 8
Derived::m_i = 8
Derived::m_a = 12
Derived::m_b = 16

//内存布局为
//Base： vptr | m_i | padding4
//Derived： vptr | m_i | m_a | m_b | padding4

单一继承下父类不带虚函数的数据成员布局

#include#includeclass Base
{public:
    int m_i;

    Base() {}
    ~Base() {}
};

class Derived : public Base
{public:
    int m_a;
    int m_b;

    Derived() {}
    ~Derived() {}

    virtual void d_func() {}
};

int main(int argc, char **argv)
{printf("sizeof(Base) = %d\n", sizeof(Base));
    printf("sizeof(Derived) = %d\n", sizeof(Derived));
    Derived d;

    d.m_i = 1;
    d.m_a = 2;
    d.m_b = 3;
    
    printf("Base::m_i = %d\n", &Base::m_i);
    printf("Derived::m_i = %d\n", &Derived::m_i);
    printf("Derived::m_a = %d\n", &Derived::m_a);
    printf("Derived::m_b = %d\n", &Derived::m_b);

    return 0;
}

gdb查看

gdb main
(gdb) set print pretty on
(gdb) set print object on
(gdb) set print vtbl on
(gdb) b 35
Breakpoint 1 at 0x123f: file main.cpp, line 35.
(gdb) r
sizeof(Base) = 4
sizeof(Derived) = 24

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:35
35          printf("Base::m_i = %d\n", &Base::m_i);
(gdb) p/a &d
$1 = 0x7fffffffdc00
(gdb) x/10xw &d
0x7fffffffdc00: 0x55557d40      0x00005555      0x00000001      0x00000002
0x7fffffffdc10: 0x00000003      0x00007fff      0x342a4e00      0xa0066d24
0x7fffffffdc20: 0xf7fb3940      0x00007fff
(gdb) info vtbl d
vtable for 'Derived' @ 0x555555557d40 (subobject @ 0x7fffffffdc00):
[0]: 0x5555555553e4

从gdb调试信息可以看出
d的vptr为0x555555557d40 即从虚拟地址0x7fffffffdc00~0x7fffffffdc07
地址0x7fffffffdc08~0x7fffffffdc0b的值为0x00000001即d.m_i的值
地址0x7fffffffdc0c~0x7fffffffdc0f的值为0x00000002即d.m_a的值
地址0x7fffffffdc10~0x7fffffffdc13的值为0x00000003即d.m_b的值
由此可得d的内存布局为：vptr | m_i | m_a | m_b | padding4

多重继承且父类都带虚函数的数据成员布局

#include#includeclass Base1
{public:
    int m_b1;

    Base1() {printf("    Base1::Base1() 的this指针是:%p\n", this); }
    ~Base1() {}
    virtual void func_b1() {}
};

class Base2
{public:
    int m_b2;

    Base2() {printf("    Base2::Base2() 的this指针是:%p\n", this); }
    ~Base2() {}
    virtual void func_b2() {}
};

class Derived : public Base1, public Base2
{public:
    int m_a;
    int m_b;

    Derived()
    {printf("Derived::Derived() 的this指针是:%p\n", this);
    }
    ~Derived() {}

    virtual void d_func() {}
};

int main(int argc, char **argv)
{printf("sizeof(Base1) = %d\n", sizeof(Base1));
    printf("sizeof(Base2) = %d\n", sizeof(Base2));
    printf("sizeof(Derived) = %d\n", sizeof(Derived));

    Derived d;
    d.m_b1 = 1;
    d.m_b2 = 2;
    d.m_a = 3;
    d.m_b = 4;

    printf("Base1::m_b1 = %d\n", &Base1::m_b1);
    printf("Base2::m_b2 = %d\n", &Base2::m_b2);

    printf("Derived::m_b1 = %d\n", &Derived::m_b1);
    printf("Derived::m_b2 = %d\n", &Derived::m_b2);
    printf("Derived::m_a = %d\n", &Derived::m_a);
    printf("Derived::m_b = %d\n", &Derived::m_b);

    return 0;
}

gdb查看

gdb main

(gdb) set print pretty on
(gdb) set print object on
(gdb) set print vtbl on
(gdb) b 51
Breakpoint 1 at 0x125f: file main.cpp, line 51.
(gdb) r
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
sizeof(Base1) = 16
sizeof(Base2) = 16
sizeof(Derived) = 40
    Base1::Base1() 的this指针是:0x7fffffffdbf0
    Base2::Base2() 的this指针是:0x7fffffffdc00
Derived::Derived() 的this指针是:0x7fffffffdbf0

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:51
51          printf("Base1::m_b1 = %d\n", &Base1::m_b1);
(gdb) p d
$1 = (Derived) {= {_vptr.Base1 = 0x555555557cd0,
    m_b1 = 1
  },= {_vptr.Base2 = 0x555555557cf0,
    m_b2 = 2
  }, 
  members of Derived:
  m_a = 3,
  m_b = 4
}
(gdb) x/10xw &d
0x7fffffffdbf0: 0x55557cd0      0x00005555      0x00000001      0x00007fff
0x7fffffffdc00: 0x55557cf0      0x00005555      0x00000002      0x00000003
0x7fffffffdc10: 0x00000004      0x00007fff
(gdb) info vtbl d
vtable for 'Derived' @ 0x555555557cd0 (subobject @ 0x7fffffffdbf0):
[0]: 0x55555555540c[1]: 0x555555555576vtable for 'Base2' @ 0x555555557cf0 (subobject @ 0x7fffffffdc00):
[0]: 0x555555555476

从gdb调试信息可以看出
Base1的虚函数指针vptr1和继承Derived的虚函数指针vptr同在0x7fffffffdbf0处，即继承类和多个基类的第一个类共用一个vptr
地址0x7fffffffdbf0~0x7fffffffdbf7的值为0x0000555555557cd0即_vptr.Base1
地址0x7fffffffdbf8~0x7fffffffdbfb的值为0x00000001即m_b1的值
地址0x7fffffffdbfc~0x7fffffffdbff的值为4字节的填充
地址0x7fffffffdc00~0x7fffffffdc07的值为0x0000555555557cf0即_vptr.Base2
地址0x7fffffffdc08~0x7fffffffdc0b的值为0x00000002即m_b2的值
地址0x7fffffffdc0c~0x7fffffffdc0f的值为0x00000003即m_a的值
地址0x7fffffffdc10~0x7fffffffdc13的值为0x00000004即m_b的值
由此可得d的内存布局为：_vptr.Base1 | m_b1 | 4padding |_vptr.Base2 | m_b2 | m_a | m_b | padding4

虚基类问题剖析虚基类（虚继承/虚派生）问题的提出

传统多重继承的问题：空间问题、效率问题、二义性问题

|-----------|   |-----------|
	|    Base   |   |    Base   |
	|-----------|   |-----------|
		  |               |
		 \|/             \|/ 
	|----------|     |----------|
	| Derived1 |     | Derived2 |
	|----------|     |----------|
		 |               |	
		 |---------------|           
				|
			   \|/
		   |----------|
		   |     C    |
		   |----------|

#include#includeclass Base
{public:
    int m_b1;
};

class Derived1 : public Base
{public:
};

class Derived2 : public Base
{public:
};

class C : public Derived1, public Derived2
{};

int main(int argc, char **argv)
{printf("sizeof(Base) = %ld\n", sizeof(Base));
    printf("sizeof(Derived1) = %ld\n", sizeof(Derived1));
    printf("sizeof(Derived2) = %ld\n", sizeof(Derived2));
    printf("sizeof(C) = %ld\n", sizeof(C));

    C c; // c的内存布局为：m_b1 | m_b1
    c.Derived1::m_b1 = 1;
    c.Derived2::m_b1 = 1;
    return 0;
}

虚基类初探

虚基类表vbtable（virtual base table）
虚基类表指针vbptr（virtual base table pointer）

2层结构时虚基类表内容分析

示例1

	|-----------|
			|    Base   |
			|-----------|
		          | 
		   ---------------
virtual   |               |  virtual
		 \|/             \|/ 
	|----------|     |----------|
	| Derived1 |     | Derived2 |
	|----------|     |----------|

#include#includeclass Base
{public:
    int m_b;
};

class Derived1 : virtual public Base
{public:
    int m_d1;
};

class Derived2 : virtual public Base
{public:
    int m_d2;
};

int main(int argc, char **argv)
{printf("sizeof(Base) = %ld\n", sizeof(Base));
    printf("sizeof(Derived1) = %ld\n", sizeof(Derived1));

    Derived1 d1;
    d1.m_b = 0x1111;
    d1.m_d1 = 0x2222;

    return 0;
}

gdb查看

gdb main
(gdb) b 31
Breakpoint 1 at 0x1230: file main.cpp, line 32.
(gdb) set print pretty on
(gdb) r
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
sizeof(Base) = 4
sizeof(Derived1) = 16

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:32
32          return 0;
(gdb) p &d1
$1 = (Derived1 *) 0x7fffffffdc10
(gdb) p d1
$2 = {= {m_b = 4369
  }, 
  members of Derived1:
  _vptr.Derived1 = 0x555555557d58,
  m_d1 = 8738
}
(gdb) x/2xg 0x7fffffffdc10
0x7fffffffdc10: 0x0000555555557d58      0x0000111100002222

其中：
sizeof(Derived1) = 16, &d=0x7fffffffdc10
0x7fffffffdc10~0x7fffffffdc17内容为0x0000555555557d58即_vptr.Derived1
0x7fffffffdc18~0x7fffffffdc1b内容为00002222即d1.m_d1 = 0x2222;
0x7fffffffdc1c~0x7fffffffdc1f内容为00001111即d1.m_b = 0x1111;
由此可得d1的内存布局为： vptr | m_d1 | m_b(虚基类对象大小)
virtual虚继承之后，Derived1、Derived2会被编译器插入一个虚基类表指针

示例2

|-----------|   |-----------|
	|   Base1   |   |    Base2  |
	|-----------|   |-----------|
 virtual |               | public
		 |---------------|           
				|
			   \|/
		   |----------|
		   | Derived1 |
		   |----------|

#include#includeclass Base1
{public:
    int m_b1;
};

class Base2
{public:
    int m_b2;
};

class Derived1 : virtual public Base1, public Base2
{public:
    int m_d1;
};

int main(int argc, char **argv)
{printf("sizeof(Base1) = %ld\n", sizeof(Base1));
    printf("sizeof(Base2) = %ld\n", sizeof(Base2));
    printf("sizeof(Derived1) = %ld\n", sizeof(Derived1));

    Derived1 d1;
    d1.m_b1 = 0xb1;
    d1.m_b2 = 0xb2;
    d1.m_d1 = 0x2222;

    return 0;
}

gdb查看

gdb main 
(gdb) b 33
Breakpoint 1 at 0x1237: file main.cpp, line 33.
(gdb) set print pretty on
(gdb) r
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
sizeof(Base1) = 4
sizeof(Base2) = 4
sizeof(Derived1) = 24

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:33
warning: Source file is more recent than executable.
33          return 0;
(gdb) p d1
$1 = {= {m_b1 = 177
  },= {m_b2 = 178
  }, 
  members of Derived1:
  _vptr.Derived1 = 0x555555557d38,
  m_d1 = 8738
}
(gdb) p d1
$2 = {= {m_b1 = 177
  },= {m_b2 = 178
  }, 
  members of Derived1:
  _vptr.Derived1 = 0x555555557d38,
  m_d1 = 8738
}
(gdb) p &d1
$3 = (Derived1 *) 0x7fffffffdc10
(gdb) x/6xw 0x7fffffffdc10
0x7fffffffdc10: 0x55557d38      0x00005555      0x000000b2      0x00002222
0x7fffffffdc20: 0x000000b1      0x00007fff

其中：
sizeof(Derived1) = 24, &d1=0x7fffffffdc10
0x7fffffffdc10~0x7fffffffdc17内容为0x0000555555557d38即_vptr.Derived1
0x7fffffffdc18~0x7fffffffdc1b内容为0x000000b2即d1.m_b2 = 0x000000b2;
0x7fffffffdc1c~0x7fffffffdc1f内容为0x00002222即d1.m_d1 = 0x2222;
0x7fffffffdc20~0x7fffffffdc23内容为0x000000b1即d1.m_b1 = 0x000000b1;
由此可得d1的内存布局为： vptr | m_b2 | m_d1 | m_b1(虚基类对象大小) | padding4 |

3层结构时虚基类表内容分析

	|-----------|
			|    Base   |
			|-----------|
		          | 
		   ---------------
virtual   |               |  virtual
		 \|/             \|/ 
	|----------|     |----------|
	| Derived1 |     | Derived2 |
	|----------|     |----------|
		 |               |	
		 |---------------|           
				|
			   \|/
		   |----------|
		   |     C    |
		   |----------|

#include#includeclass Base
{public:
    int m_b1;
};

class Derived1 : virtual public Base
{public:
    int m_d1;
};

class Derived2 : virtual public Base
{public:
    int m_d2;
};

class C : public Derived1, public Derived2
{public:
    int m_c;
};

int main(int argc, char **argv)
{printf("sizeof(Base) = %ld\n", sizeof(Base));
    printf("sizeof(Derived1) = %ld\n", sizeof(Derived1));
    printf("sizeof(Derived2) = %ld\n", sizeof(Derived2));
    printf("sizeof(C) = %ld\n", sizeof(C));

    C c;// c的内存布局为：vbptr1(from Derived1) | m_d1 | vbptr2(from Derived2) | m_d2 | m_c | m_b1(虚基类对象)
    c.m_b1 = 0xb1;
    c.m_d1 = 0xd1;
    c.m_d2 = 0xd2;
    c.m_c = 0xc;

    return 0;
}

gdb查看

gdb main
(gdb) b 41
Breakpoint 1 at 0x1257: file main.cpp, line 41.
(gdb) r
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
sizeof(Base) = 4
sizeof(Derived1) = 16
sizeof(Derived2) = 16
sizeof(C) = 40

Breakpoint 1, main (argc=1, argv=0x7fffffffdd48) at main.cpp:41
41          return 0;
(gdb) p &c
$1 = (C *) 0x7fffffffdc00
(gdb) set print pretty on
(gdb) p c
$3 = {= {= {  m_b1 = 177
    }, 
    members of Derived1:
    _vptr.Derived1 = 0x555555557c98,
    m_d1 = 209
  },= {members of Derived2:
    _vptr.Derived2 = 0x555555557cb0,
    m_d2 = 210
  }, 
  members of C:
  m_c = 12
}
(gdb) x/10xw 0x7fffffffdc00
0x7fffffffdc00: 0x55557c98      0x00005555      0x000000d1        0x00007fff
0x7fffffffdc10: 0x55557cb0      0x00005555      0x000000d2        0x0000000c
0x7fffffffdc20: 0x000000b1      0x00007fff

其中：
sizeof© = 40, &c=0x7fffffffdc00
0x7fffffffdc00~0x7fffffffdc07内容为0x0000555555557c98即_vptr.Derived1 = 0x555555557c98,由于多继承派生类与第一个基类共用一个vptr,即c的vptr=0x555555557c98
0x7fffffffdc08~0x7fffffffdc0b内容为0x000000d1即c.m_d1 = 0x000000d1;
0x7fffffffdc10~0x7fffffffdc17内容为0x0000555555557cb0即_vptr.Derived2 = 0x555555557cb0;
0x7fffffffdc18~0x7fffffffdc1b内容为0x000000d2即c.m_d2 = 0xd2;
0x7fffffffdc1c~0x7fffffffdc1f内容为0x0000000c即c.m_c = 0xc;
0x7fffffffdc20~0x7fffffffdc23内容为0x000000b1即c.m_b1 = 0xb1;
由此可得c的内存布局为： vptr1 | m_d1 | padding4 | vptr2 | m_d2 | m_c | m_b1(虚基类对象大小) | padding4 |

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