Constructor

• Declaration of Constructor

  class Human
  {
    Human(); //declaration of a constructor
  };

• Implementation of Constructor

  The first case: Implementation in class

  class Human
  {
    Human()
    {
      //constructor code here
    }
  };

  The second case: Implementation out class

  class Human
  {
    public :
    	Human(); //constructor declaration
  };
  //constructor implementation (definition)
  Human::Human() //'::': Domain Resolution Operator
  {
    //constructor code here
  }

• When and How to use Constructor?

  When creating object, which made constructor a ideal place to initialize class menmber variables to a specific value.

  • When you invoke a function without providing parameters which is called Default Constructor.
  • And the Default Constructor is optional,if you don’t provide one, compiler will creat one for you, it just create member attribute, but not initialize POD type(such int) to a non-zero value.

• Overload Constructor

  Like function, Constructor can also Overload.(overload func, the same func, which you can provide different parameter such as you can do not provide or provide a string or int)

  class Human 
  {
    Human()
    {
      //default constructor code here
    }
    Human(string humansName)
    {
      //overload constructor code here
    }
  }

  The following program will tell you

  #include <iostream>
  #include <string>
  using namespace std;
  
  class Human 
  {
    private:
    	string name;
      int age;
    public:
    Human() //You can regard this as Setter
    {
      age = 0;
      cout << "Default constructor: name and age not Set" << endl;
    }
    
    Human(string humansName, int humansAge) //You can regard this as Getter and Setter!!!
    {
      name = humansName;
      age = humansAge;
      cout << "Overload constructor creates" << endl;
      cout << name << " of" << age << " years" << endl;
    }
  };
  
  int main()
  {
    Human firstMan; //use default constructor
    Human firstWoman("Eve",20); //use overload constructor
  }

  • You can choose do not implement Default constructor, so you must provide some parameters when instantiating object.

• You do not need default constructor and just keep Overload constructor.

  And it can have default value.

  Human(string humansName,int humansAge)
  {
    //...
  }
  Human(string humansName, int humansName = 20)
  {
    name = humansName;
    age = humansAge;
  }
  
  //By the way, overload constructor can cantain initialzation list.
  Human(string humansName, int humansAge)
  :name(humansName), age(humansAge)
  {
    //cout name,age...
  }