Constructor methods v17

A constructor method is a function that creates an instance of an object type, typically by assigning values to the members of the object. An object type can define several constructors to accomplish different tasks. A constructor method is a member function invoked with a SELF parameter whose name matches the name of the type.

For example, if you define a type named address, each constructor is named address. You can overload a constructor by creating one or more different constructor functions with the same name but with different argument types.

The SPL compiler provides a default constructor for each object type. The default constructor is a member function whose name matches the name of the type and whose argument list matches the type members in order. For example, given an object type such as:

CREATE TYPE address AS OBJECT
(
  street_address VARCHAR2(40),
  postal_code    VARCHAR2(10),
  city           VARCHAR2(40),
  state          VARCHAR2(2)
)

The SPL compiler provides a default constructor with the following signature:

CONSTRUCTOR FUNCTION address
(
  street_address VARCHAR2(40),
  postal_code    VARCHAR2(10),
  city           VARCHAR2(40),
  state          VARCHAR2(2)
)

The body of the default constructor sets each member to NULL.

To create a custom constructor, using the keyword constructor, declare the constructor function in the CREATE TYPE command, and define the construction function in the CREATE TYPE BODY command. For example, you might want to create a custom constructor for the address type that computes the city and state given a street_address and postal_code:

CREATE TYPE address AS OBJECT
(
  street_address VARCHAR2(40),
  postal_code    VARCHAR2(10),
  city           VARCHAR2(40),
  state          VARCHAR2(2),

  CONSTRUCTOR FUNCTION address
   (
     street_address VARCHAR2,
     postal_code VARCHAR2
    ) RETURN self AS RESULT
)
CREATE TYPE BODY address AS
 CONSTRUCTOR FUNCTION address
  (
    street_address VARCHAR2,
    postal_code VARCHAR2
   ) RETURN self AS RESULT
 IS
   BEGIN
     self.street_address := street_address;
     self.postal_code := postal_code;
     self.city := postal_code_to_city(postal_code);
     self.state := postal_code_to_state(postal_code);
     RETURN;
   END;
END;

To create an instance of an object type, you invoke one of the constructor methods for that type. For example:

DECLARE
  cust_addr address := address('100 Main Street', 02203');
BEGIN
  DBMS_OUTPUT.PUT_LINE(cust_addr.city);  -- displays Boston
  DBMS_OUTPUT.PUT_LINE(cust_addr.state); -- displays MA
END;

Custom constructor functions are:

• Typically used to compute member values when given incomplete information. The example computes the values for city and state when given a postal code.

• Also used to enforce business rules that restrict the state of an object. For example, if you define an object type to represent a payment, you can use a custom constructor to ensure that no object of type payment can be created with an amount that is NULL, negative, or zero. The default constructor sets payment.amount to NULL, so you must create a custom constructor whose signature matches the default constructor to prohibit NULL amounts.