Question

I saw this in a textbook, but the book doesn’t explain what it actually does, and why I should do this. Here is something similar to the example in the book:

 class MyClass
 {
      public:
           MyClass(int initial_capacity = 20);
      private:
           int capacity;
 }

I can’t use initial_capacity in the implementation, I can’t even implement anything, so I am confused as to what this is for? Does it set capacity to 20 somehow? How is this a default constructor?

Answer 1

Probably, there is missed implementation of constructor. For example, if constructor looks like this one :

MyClass(int initial_capacity = 20) {
     capacity = initial_capacity;
}

If you create object this way:

MyClass a(10);

capacity will be set to 10.
On the other hand, if you will create object like this:

MyClass a;

capacity will be set to 20.