您的位置: 首页  >  文章  >  C++类内存分布

C++类内存分布

分类: 文章 • 2023-10-27 09:33:28

C++类内存分布

转自：https://www.cnblogs.com/jerry19880126/p/3616999.html
C++类内存分布

选择左侧的C/C+±>命令行，然后在其他选项这里写上/d1 reportAllClassLayout，它可以看到所有相关类的内存布局，如果写上/d1 reportSingleClassLayoutXXX（XXX为类名），则只会打出指定类XXX的内存布局。

多重继承

class Base
{
    int a;
    int b;
public:
    void CommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedClass1 : public Base
{
    int c;
public:
    void DerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedClass2 : public Base
{
    int d;
public:
    void DerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedDerivedClass : public DerivedClass1, public DerivedClass2
{
    int e;
public:
    void DerivedDerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};

C++类内存分布
C++类内存分布

下面我们重点看看这个类DerivedDerivedClass，由外向内看，它并列地排布着继承而来的两个父类DerivedClass1与DerivedClass2，还有自身的成员变量e。DerivedClass1包含了它的成员变量c，以及Base，Base有一个0地址偏移的虚表指针，然后是成员变量a和b；DerivedClass2的内存排布类似于DerivedClass1，注意到DerivedClass2里面竟然也有一份Base。

  • DerivedDerivedClass1内存分布
1>class DerivedClass1    size(16):
1>  +---
1> 0    | +--- (base class Base)
1> 0    | | {vfptr}
1> 4    | | a
1> 8    | | b
1>    | +---
1>12    | c
1>  +---
1>
1>DerivedClass1::[email protected]:
1>  | &DerivedClass1_meta
1>  |  0
1> 0    | &DerivedClass1::VirtualFunction
1>
1>DerivedClass1::VirtualFunction this adjustor: 0
  • DerivedDerivedClass2内存分布
1>class DerivedClass2    size(16):
1>  +---
1> 0    | +--- (base class Base)
1> 0    | | {vfptr}
1> 4    | | a
1> 8    | | b
1>  | +---
1>12    | d
1>  +---
1>
1>DerivedClass2::[email protected]:
1>  | &DerivedClass2_meta
1>  |  0
1> 0    | &DerivedClass2::VirtualFunction
1>
1>DerivedClass2::VirtualFunction this adjustor: 0
1>
  • DerivedDerivedClass内存分布
1>class DerivedDerivedClass    size(36):
1>  +---
1> 0    | +--- (base class DerivedClass1)
1> 0    | | +--- (base class Base)
1> 0    | | | {vfptr}
1> 4    | | | a
1> 8    | | | b
1>  | | +---
1>12    | | c
1>  | +---
1>16    | +--- (base class DerivedClass2)
1>16    | | +--- (base class Base)
1>16    | | | {vfptr}
1>20    | | | a
1>24    | | | b
1>  | | +---
1>28    | | d
1>  | +---
1>32    | e
1>  +---
1>
1>DerivedDerivedClass::[email protected]@:
1>  | &DerivedDerivedClass_meta
1>  |  0
1> 0    | &DerivedDerivedClass::VirtualFunction
1>
1>DerivedDerivedClass::[email protected]@:
1>  | -16
1> 0    | &thunk: this-=16; goto DerivedDerivedClass::VirtualFunction
1>
1>DerivedDerivedClass::VirtualFunction this adjustor: 0

这里有两份虚表了，分别针对DerivedClass1与DerivedClass2，在&DerivedDericedClass_meta下方的数字是首地址偏移量，靠下面的虚表的那个-16表示指向这个虚表的虚指针的内存偏移，这正是DerivedClass2中的{vfptr}在DerivedDerivedClass的内存偏移。

多重继承使用虚继承

class Base
{
    int a;
    int b;
public:
    void CommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedClass1 : public virtual  Base
{
    int c;
public:
    void DerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedClass2 : virtual public Base
{
    int d;
public:
    void DerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};

class DerivedDerivedClass : public DerivedClass1, public DerivedClass2
{
    int e;
public:
    void DerivedDerivedCommonFunction() {};
    void virtual VirtualFunction() {};
};
  • DerivedDerivedClass1内存分布
1>class DerivedClass1    size(20):
1>  +---
1> 0    | {vbptr}
1> 4    | c
1>  +---
1>    +--- (virtual base Base)
1> 8    | {vfptr}
1>12    | a
1>16    | b
1>  +---
1>
1>DerivedClass1::[email protected]:
1> 0    | 0
1> 1    | 8 (DerivedClass1d(DerivedClass1+0)Base)
1>
1>DerivedClass1::[email protected]:
1>  | -8
1> 0    | &DerivedClass1::VirtualFunction
1>
1>DerivedClass1::VirtualFunction this adjustor: 8
1>vbi:       class  offset o.vbptr  o.vbte fVtorDisp
1>            Base       8       0       4 0
1>

DerivedClass1就已经有变化了，原来是先排虚表指针与Base成员变量，vfptr位于0地址偏移处；但现在有两个虚表指针了，一个是vbptr，另一个是vfptr。vbptr是这个DerivedClass1对应的虚表指针，它指向DerivedClass1的虚表vbtable，另一个vfptr是虚基类表对应的虚指针，它指向vftable。

  • DerivedDerivedClass2内存分布
1>class DerivedClass2    size(20):
1>  +---
1> 0    | {vbptr}
1> 4    | d
1>  +---
1>  +--- (virtual base Base)
1> 8    | {vfptr}
1>12    | a
1>16    | b
1>  +---
1>
1>DerivedClass2::[email protected]:
1> 0    | 0
1> 1    | 8 (DerivedClass2d(DerivedClass2+0)Base)
1>
1>DerivedClass2::[email protected]:
1>  | -8
1> 0    | &DerivedClass2::VirtualFunction
1>
1>DerivedClass2::VirtualFunction this adjustor: 8
1>vbi:       class  offset o.vbptr  o.vbte fVtorDisp
1>            Base       8       0       4 0
1>
  • DerivedDerivedClass内存分布
1>class DerivedDerivedClass    size(32):
1>  +---
1> 0    | +--- (base class DerivedClass1)
1> 0    | | {vbptr}
1> 4    | | c
1>  | +---
1> 8    | +--- (base class DerivedClass2)
1> 8    | | {vbptr}
1>12    | | d
1>  | +---
1>16    | e
1>  +---
1>  +--- (virtual base Base)
1>20    | {vfptr}
1>24    | a
1>28    | b
1>  +---
1>
1>DerivedDerivedClass::[email protected]@:
1> 0    | 0
1> 1    | 20 (DerivedDerivedClassd(DerivedClass1+0)Base)
1>
1>DerivedDerivedClass::[email protected]@:
1> 0    | 0
1> 1    | 12 (DerivedDerivedClassd(DerivedClass2+0)Base)
1>
1>DerivedDerivedClass::[email protected]:
1>  | -20
1> 0    | &DerivedDerivedClass::VirtualFunction
1>
1>DerivedDerivedClass::VirtualFunction this adjustor: 20
1>vbi:       class  offset o.vbptr  o.vbte fVtorDisp
1>            Base      20       0       4 0

总结

虚继承的作用是减少了对基类的重复，代价是增加了虚表指针的负担（更多的虚表指针）。

下面总结一下（当基类有虚函数时）：

  1. 每个类都有虚指针和虚表；

  2. 如果不是虚继承，那么子类将父类的虚指针继承下来，并指向自身的虚表（发生在对象构造时）。有多少个虚函数，虚表里面的项就会有多少。多重继承时，可能存在多个的基类虚表与虚指针；

  3. 如果是虚继承，那么子类会有两份虚指针，一份指向自己的虚表，另一份指向虚基表，多重继承时虚基表与虚基表指针有且只有一份。

相关推荐