When the following occurs 

1.A C++ class 'class1' contains a member of class 'class2,' 

and 

2.You want to construct an object of type 'class1' (but not from another object of type 'class1'). There must be a constructor class2::class2() so that the member can be constructed. 

This constructor without parameters is called the default constructor. 

The compiler will supply a default constructor automatically unless you have defined any constructor for class 'class2'. 

In that case, the compiler will not supply the default constructor automatically ( you must supply one.