The second week of learning: copy constructor copy

concept

1. There is only one parameter, which is a reference to the same object
2. X::X(X&)Or X::X(const X&)often quoted more often
3. The system will generate a default copy constructor to complete the shallow copy function
4. If you define your own copy constructor, the default one will be useless

Three situations that work

1. When using an object to initialize another object,
   classname object1(object2)
classname object1 = object2this is an initialization statement, not an assignment
2. The parameter is an object of class A. When invoking, the copy constructor of class A will be called
   void func(A a1), and all class objects must be initialized. This initialization will call the copy constructor function (the overhead will be relatively large, so generally consider constant reference parameters )
3. The return value is an object of class A, and the copy constructor is called upon return.

WAITING

1. Assignment between objects does not result in a copy constructor call (if it is not initialized or assigned)

Example

#include<iostream>
#include<string>
#include<cstring>
using namespace std;
class student
{
    
    
	private:
		string name;
		char* hobby;
		int age;
	public:
		student(string n="no",char h[]=nullptr,int a=0);
		student(const student& s);
		~student();
		void displace();
 } ;
 
 student::student(string n,char h[],int a):name(n),age(a)
 {
    
    
 	hobby = new char[strlen(h)+1];
 	strcpy(hobby,h);
 }
 
 student::student(const student& s):name(s.name),age(s.age)
 {
    
    
 	hobby = new char[strlen(s.hobby)+1];
 	strcpy(hobby,s.hobby);
 }
 
 student::~student()
 {
    
    
 	delete [] hobby;
 }
 
 void student::displace()
 {
    
    
 	cout<<name<<" is "<<age<<" years old and good at "
 	<<hobby<<endl;
 }
 
 int main()
 {
    
    
 	student s2("Jeff","studying",20),s3("Alice","talking",18)
 	,s1=s2;
 	s2.displace();
 	s3.displace();
 	cout<<"S1 as followed: "<<endl;
 	s1.displace();
 }

ps. If there is a dynamic allocation of memory, it must be the destructor delete