In C++, a default constructor is a constructor that can be called with no arguments. If you don't provide a constructor for a class, the compiler will automatically generate a default constructor for you.

Here's an example of a class with a default constructor:

class MyClass {
  public:
    int myInt;
    MyClass() {
      myInt = 0;
    }
};

In this example, we've defined a class called MyClass with a public member variable myInt and a default constructor. The default constructor initializes the value of myInt to 0.

You can create an object of the MyClass class using the default constructor like this:

MyClass obj;

In this example, we've created an object of the MyClass class using the default constructor. The value of myInt in the obj object is initialized to 0.

If you provide a constructor for a class, the default constructor won't be generated by the compiler. Here's an example:

class MyClass {
  public:
    int myInt;
    MyClass(int value) {
      myInt = value;
    }
};

In this example, we've defined a class called MyClass with a public member variable myInt and a constructor that takes an integer argument. Because we've provided a constructor, the default constructor won't be generated by the compiler.

If you want to create an object of the MyClass class using the default constructor in this case, you need to explicitly define the default constructor:

class MyClass {
  public:
    int myInt;
    MyClass() {
      myInt = 0;
    }
    MyClass(int value) {
      myInt = value;
    }
};

In this example, we've defined both the default constructor and the constructor that takes an integer argument. Now you can create an object of the MyClass class using the default constructor:

MyClass obj;

In this example, we've created an object of the MyClass class using the default constructor. The value of myInt in the obj object is initialized to 0.