C ++ Primer Note 9 _ constructor _ copy constructor (deep copy and shortest copy)

1. constructor:

> Constructor is a special member function with the same name as the class. It is used to set an appropriate initial value for each member.

1. > The constructor cannot return values. The function name is the same as the class name.

2. > When the default constructor is used, the system automatically calls the default constructor to initialize the object. The default constructor initializes all data members to zero or null. The default constructor is a constructor without parameters.

3. > When an object is created, the system automatically calls the constructor.

   
   

   Features of constructor:

   1.The constructor can be overloaded. The input parameters determine whether to call constructors of different versions.

   2. const cannot be declared as const or virtual. (The Destructor can)

   3. If a class customizes a constructor, the compiler will not generate a default constructor.

   4. Only constructors can initialize member variables in the form similar to the initialization list (especially the const member variables can only be initialized in the initialization list)

   
   

   Example:

   # Include
      
       
   # Include
       
        
   Class Person {public: Person (); // default constructor Person (int n, const string & str); // overload of the constructor private: int age; string * name ;};
       
      

   
   
   

   2. Copy Structure

   
   

   First introduceDeep copyAndShortest copy:

   > Shallow copy: When copying an object, only simple values are assigned to the data members of the object. By default, the copy constructor executes the shortest copy function. In most cases, "Shallow copy" can work well, but once the object has dynamic members, there will be problems with the shallow copy.

   > Deep copy: when the class member variables have pointer types, we should re-allocate space for the pointer variables when copying objects to avoid copying only pointer values in the shortest copy, so that the two pointers point to the same memory space.
   

   
   

   Example of a copy defect:

   # Include
      
       
   # Include
       
        
   Using namespace std; class Person {public: Person (); Person (int n, const string & str); // overload the constructor ~ Person (); private: int age; string * name;}; Person: Person (): age (0), name (NULL) // list of constructor initialization, you do not need to write the definition! {Cout <"Default Person" <endl;} Person: Person (int n, const string & str): age (n), name (new string (str )) {cout <"Init Person" <endl;} Person ::~ Person () {if (name) {cout <"~ Person "<" name: "<* name <" age: "<age <endl;} delete name;} int main () {Person p1 (10, string ("SCOTT"); Person p2 = p1; return 0 ;}
       
      

   Running result:

   Init Person
   ~ Person name: SCOTT age: 10
   Segmentation fault (core dumped)
   

   Cause of program crash:

   P2 is obtained by p1 initialization. Since we do not have a custom copy constructor, we call the default copy constructor, which is a shortest copy and only copies the value of name; the name pointer in p1 and p2 points to the same new space. When the Destructor is called, two delete operations may occur and a segment error may occur.

   As shown in:

   
   

   
   

   
   

   Solution:Custom copy constructor! Achieve the effect of deep copy

   Deep copy example: <喎?http: www.bkjia.com kf ware vc " target="_blank" class="keylink"> VcD4KPHA + PC9wPgo8cHJlIGNsYXNzPQ = "brush: java;"> # include # Include Using namespace std; class Person {public: Person (); Person (int n, const string & str); Person (const Person & n );~ Person (); private: int age; string * name;}; Person: Person (): age (0), name (NULL) {cout <"Default Person" <endl;} Person: Person (int n, const string & str): age (n), name (new string (str )) {cout <"Init Person" <endl;} // custom copy constructor Person: Person (const Person & n) {if (n. name) {name = new string (* n. name); age = n. age ;}} Person ::~ Person () {if (name) {cout <"~ Person "<" name: "<* name <" age: "<age <endl;} delete name;} int main () {Person p1 (10, string ("SCOTT"); Person p2 = p1; return 0 ;}

   Running result:

   Init Person

   ~ Person name: SCOTT age: 10

   ~ Person name: SCOTT age: 10
   The preceding solution is feasible because no segment error occurs.