Week 10 - task 0- execution process instance resolution of constructors and destructors

【 subject 】 Reading programs , First analyze the execution result of the program , Run the program on the computer for comparison , And then through the single step execution to track the operation of the program , To understand the base class 、 Constructors in derived classes 、 The purpose of the destructor execution process .

The procedure is as follows ：

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#include <iostream>
using namespace std;
class Part  // Component class 
{
public:
    Part();
    Part(int i);
    ~Part();
 private:
    int val;
};

class Whole: public Part
{
public:
    Whole();
    Whole(int,int,int,int);
    ~Whole();
private:
    Part one;
    Part two;
    int data;
};

Part::Part()
{
    val=0;
    cout<<"The default constructor of part was called "<<val<<endl;
}

Part::Part(int i)
{
    val=i;
    cout<<"The constructor of part was called "<<val<<endl;
}

Part::~Part()
{  
    cout<<"The destructor of part was called "<<val<<endl;
}

Whole::Whole()
{
    data=0;
    cout<<"The default constructor of whole was called "<<data<<endl;
}

Whole::Whole(int p, int i,int j,int k):Part(p),two(i),one(j),data(k)
{
   cout<<"The constructor of whole was called "<<data<<endl;
}

Whole::~Whole()
{  
    cout<<"The destructor of whole was called "<<data<<endl;
}

void f()
{
    Whole w1;
    Whole w2(1,2,3,4);
}

int main()
{
    f();
    system("pause");
    return 0;
}
      

       
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Operation results and explanation ：

Running results	Description of operation results
The default constructor of part was called 0 The default constructor of part was called 0 The default constructor of part was called 0 The default constructor of whole was called 0	The second part of the procedure is 60 Row definition object w1 when , The result of executing the constructor . For the base class part、 In a derived class part Object members execute the default constructor ; Finally, execute the derived class whole The statement in the function body of the default constructor of .
The constructor of part was called 1 The constructor of part was called 3 The constructor of part was called 2 The constructor of whole was called 4	The second part of the procedure is 61 Row definition object w2 when , The result of executing the constructor . The process of calling the constructor is also based on the base class 、 Then derive the object members in the class , Finally, execute the function body of the derived class constructor . All the constructors executed here are constructors with parameters . Notice that you are creating objects w2(1,2,3,4) when , The first 48 The arguments of the line function call are ：Part(1),two(2),one(3),data(4). Perhaps surprisingly , The number given is 1 3 2 4, instead of 1 2 3 4. This tells us ,w2 Object members in one Constructor for one(3) Execute before , Object members two Constructor for two(2) After the execution . Object members are constructed in the order they are defined in the object The order of （ See the first 20 and 21 That's ok ）, Instead of the writing order in the constructor . Why? ？ Because in one whole In the object , The data members are stored sequentially , Allocate space ,one before ,two After . C++ It's artificial language . Most of the questions are well founded , It's good to think more , But beginners often don't think . therefore , Once there is something you can't figure out , There are images left in the brain , With the further study, I will understand .
The destructor of whole was called 4 The destructor of part was called 2 The destructor of part was called 3 The destructor of part was called 1	sign out f() When a function is called , End local object w2 Life cycle of , Execute destructor . The destructors to be called are in the reverse order of the previous constructors ： First constructed, then deconstructed , The first analysis of the latter structure .
The destructor of whole was called 0 The destructor of part was called 0 The destructor of part was called 0 The destructor of part was called 0	sign out f() When a function is called , End local object w1 Life cycle of , Execute destructor . w1 Than w2 First define , The destructor is executed later . The result is automatically determined by the system , Programmers need to understand the rules of the game .