The Other Reference Types

This section about "other reference types" covers these types:

• program or "class" (written program in Pike)

• mapping or "dictionary" (written mapping)

• multiset or "bag" (written multiset)

Just like the container types, these types are reference types: When a data item of a basic type is stored in a variable, it is not the data item itself that is stored, but a reference to it.

The Data Type program

The data type program in Pike is used to contain class definitions. Not the source code, i. e. the program text, but the internal representation that Pike generates when it has read the source code.

As described above in the chapter about object-oriented programming, programs and classes are the same in Pike. Both an explicit class definition (that is, with the keyword class) and a source code file can be used to define a program/class.

Here are some useful things that you can do with programs:

• Check if it is a program
  programp(something)

  The function programp returns 1 if the value something is a program, otherwise 0.

• Cloning
  

  program-name() or program-name(arguments)

  This creates a new object of the type program-name. If arguments are given, they are sent to the method create in this program.

• Defining variables
  program-name variable-name;

  This creates a new variable of the type program-name.

Read more about classes and objects in the chapter about object-oriented programming.

The Data Type object

In object-oriented programming, the data items that are instances (also called clones) of a class are called objects. The data type object is used to store such objects.

An object is an instance of any class. Usually you use the more specific data type object(classname), which means an object that is an instance of the class classname. You can also write just classname instead of object(classname), and this is the recommended form.

Here are some useful things that you can do with objects:

• Check if it is an object
  objectp(something)

  The function objectp returns 1 if the value something is a object, otherwise 0.

• Create an object
  program-name()

  or

  program-name(arguments)

  This creates a new object of the type program-name. If arguments are given, they are sent to the method create in the program.

• Destroy an object
  destruct(object)

  This destroys the object object. All variables that contain references to this object will be set to 0. If there is a method called destroy in the object, that method will first be called.

  Note that usually you don't need to explicitly destroy objects. Pike has automatic garbage collection, and when an object is no longer referenced from anywhere, which means that it can never be used again, it is destroyed automatically.

• Accessing a member
  object -> member-name

  This is used to access the method or member variable called member-name in the object object. Example:

  web_page->data()

Read more about classes and objects in the chapter about object-oriented programming.

The Data Type function

Something else that may surprise you is that there is a data type for methods. Sometimes you want to refer to "any method". Take for example the built-in method map, which is used to apply an operation to all the elements in an array. You call map with (at least) two arguments: the array to go through, and the method to call for each element:

void write_one(int x)
{
  write("Number: " + x + "\n");
}

int main()
{
  array(int) all_of_them = ({ 1, 5, -1, 17, 17 });
  map(all_of_them, write_one);
  return 0;
}

The first argument to map is an array, and the second argument is of the type function. As you can see, you get a method reference of the type function by just typing the method name, without the parentheses used in method calls.

A method stored in a variable can be called just like a normal method:

function w = write;
w("Hello!\n");

Here are some useful things that you can do with method references:

• Check if it is a method reference
  functionp(something)

  The function functionp returns 1 if the value something is a method reference, otherwise 0.

• Find the name of a function
  

  function_name(function) returns a string with the name of the function function. Example:

  function_name(write) gives the result "write".