Default constructors (C++ only)
A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values.
If no user-defined constructor exists for a class A and
one is needed, the compiler implicitly declares a default parameterless
constructor A::A(). This constructor is an inline
public member of its class. The compiler will implicitly define A::A() when
the compiler uses this constructor to create an object of type A.
The constructor will have no constructor initializer and a null body.
The compiler first implicitly defines the implicitly declared 
or explicitly defaulted
constructors of the base classes and nonstatic
data members of a class A before defining the implicitly
declared or
explicitly defaulted constructor
of A. No default constructor is created for a class
that has any constant or reference type members.
A constructor of a class A is trivial if
all the following are true:
- It is implicitly declared or explicitly defaulted.
Ahas no virtual functions and no virtual base classes- All the direct base classes of
Ahave trivial constructors - The classes of all the nonstatic data members of
Ahave trivial constructors
If any of the above are false, then the constructor is nontrivial.
A union member cannot be of a class type that has a nontrivial constructor.
Like all functions, a constructor can have default arguments. They are used to initialize member objects. If default values are supplied, the trailing arguments can be omitted in the expression list of the constructor. Note that if a constructor has any arguments that do not have default values, it is not a default constructor.
class X {
public:
X(); // Default constructor with no arguments
X(int = 0); // Default constructor with one default argument
X(int, int , int = 0); // Constructor
};