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These three data types will be described in more detail later, but here is a short explanation of what they are good for.

An object is an object, in the object-oriented sense: the values of a bunch of member variables as defined by a program (or class, as a C++ or Java or Smalltalk programmer would say). We say that the object is an instance of the program.

An object is an instance of a program, and just as we want to store objects in variables, and send them back and forth to functions, we sometimes want to do the same with the programs. For example, we may need a function that creates an instance (that is, object) of any program, and we must therefore be able to send the program as an argument to the function. This is what we use the data type program for.

It may come as a surprise that there is a special datatype for Pike programs, but it is actually very useful, and adds a lot of flexibility to Pike.

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;
}

Running this code snippet would output:

Number: 1
Number: 5
Number: -1
Number: 17
Number: 17