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 constructors of the base classes and nonstatic data members of a class A before defining the implicitly declared 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 defined
• A has no virtual functions and no virtual base classes
• All the direct base classes of A have trivial constructors
• The classes of all the nonstatic data members of A have 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.

A copy constructor for a class A is a constructor whose first parameter is of type A&, const A&, volatile A&, or const volatile A&. Copy constructors are used to make a copy of one class object from another class object of the same class type. You cannot use a copy constructor with an argument of the same type as its class; you must use a reference. You can provide copy constructors with additional parameters as long as they all have default arguments. If a user-defined copy constructor does not exist for a class and one is needed, the compiler implicitly creates a copy constructor, with public access, for that class. A copy constructor is not created for a class if any of its members or base classes have an inaccessible copy constructor.

class X {
public:

  // default constructor, no arguments
  X();

  // constructor
  X(int, int , int = 0);

  // copy constructor
  X(const X&);

  // error, incorrect argument type
  X(X);
};

class Y {
public:

  // default constructor with one
  // default argument
  Y( int = 0);

  // default argument
  // copy constructor
  Y(const Y&, int = 0);
};