192 lines
3.0 KiB
Markdown
192 lines
3.0 KiB
Markdown
# Class size calculation
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First of all, let's make a summary, and then we will give an actual example
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- The size of the empty class is 1 byte
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- In a class, the virtual function itself, member functions (including static and non-static) and static data members do not occupy the storage space of class objects
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- For classes containing virtual functions, no matter how many virtual functions there are, there is only one virtual pointer, the size of VPTR
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- Ordinary inheritance, derived class inherits all the functions and members of the base class, and the size is calculated according to byte alignment
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- Virtual function inheritance, whether single inheritance or multi inheritance, inherits the VPTR of the base class(32 bit operating system 4 bytes, 64 bit operating system 8 bytes)！
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- Virtual inheritance inherits the VPTR of the base class
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## 1.Rule 1
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```c++
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/**
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* @file blackclass.cpp
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* @brief The size of the empty class is 1 byte
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* @author 光城
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* @version v1
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* @date 2019-07-21
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*/
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#include<iostream>
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using namespace std;
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class A{};
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int main()
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{
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cout<<sizeof(A)<<endl;
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return 0;
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}
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```
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## 2.Rule 2
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```c++
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/**
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* @file static.cpp
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* @brief
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*
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* @author 光城
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* @version v1
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* @date 2019-07-21
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*/
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#include<iostream>
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using namespace std;
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class A
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{
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public:
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char b;
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virtual void fun() {};
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static int c;
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static int d;
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static int f;
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};
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int main()
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{
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/**
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* @brief 16
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*/
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cout<<sizeof(A)<<endl;
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return 0;
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}
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```
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## 3.Rule 3
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```c++
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/**
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* @file morevir.cpp
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* @brief
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* @author
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* @version v1
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* @date 2019-07-21
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*/
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#include<iostream>
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using namespace std;
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class A{
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virtual void fun();
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virtual void fun1();
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virtual void fun2();
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virtual void fun3();
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};
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int main()
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{
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cout<<sizeof(A)<<endl; // 8
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return 0;
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}
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```
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## 4.Rule 4 and 5
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```c++
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/**
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* @file geninhe.cpp
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* @brief
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*
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*
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* @author 光城
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* @version v1
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* @date 2019-07-21
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*/
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#include<iostream>
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using namespace std;
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class A
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{
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public:
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char a;
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int b;
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};
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/**
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* @brief
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* char a
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* int b
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* short a
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* long b
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* 根据字节对齐4+4=8+8+8=24
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*/
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class B:A
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{
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public:
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short a;
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long b;
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};
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class C
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{
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A a;
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char c;
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};
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class A1
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{
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virtual void fun(){}
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};
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class C1:public A1
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{
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};
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int main()
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{
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cout<<sizeof(A)<<endl; // 8
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cout<<sizeof(B)<<endl; // 24
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cout<<sizeof(C)<<endl; // 12
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/**
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* @brief
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*/
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cout<<sizeof(C1)<<endl; // 8
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return 0;
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}
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```
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## 5.Rule 6
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```c++
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/**
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* @file virnhe.cpp
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* @brief
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* @author 光城
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* @version v1
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* @date 2019-07-21
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*/
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#include<iostream>
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using namespace std;
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class A
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{
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virtual void fun() {}
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};
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class B
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{
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virtual void fun2() {}
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};
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class C : virtual public A, virtual public B
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{
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public:
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virtual void fun3() {}
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};
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int main()
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{
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/**
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* @brief 8 8 16 The derived class inherits multiple virtual functions and inherits the VPTR of all virtual functions
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*/
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cout<<sizeof(A)<<" "<<sizeof(B)<<" "<<sizeof(C);
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return 0;
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}
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```
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