Thread: constructor purpose?

Try removing your constructor that you declared in your base class operation and see what happens.

You don't have to supply a constructor. A default constructor is called if you don't specify one. A constructor is used to initialize values when the class object is created. So if you create a class called square with members for length and width you can initialze them both to whatever values you need.

Code:

class square{
public:
         int length;
         int width;
//constructor definition
         square (int L,int W)   
              {
               length=L;
               width=W;  
              }
};

square mysquare(12,10);  creates and intializes length and width;

Now that you have created a constructor what happesn if you create an instance without initialzing

Code:

square mysquare();

it assumes that you are going to take care of everything so it looks for a default constructor that you provide. A simple default constructor is just empty brackets

Code:

//default constructor
square()
{
}

blah.h:12: error: base `operation' with only non-default constructor in class
without a constructor

I think this non-default constructor imply class plus defined without one constructor function, maybe changelike this:

Code:

class plus: public operation 
{
    public:
    int performCalculation(int a, int b);  //this statement ??
    plus();          //add this , but what type define it?
    
};

I just learn class, and cant understand deeply,
I think class & constructor is most difficultly for newbie like me.
sth wrong writing told me please.
thanks.