First class property

A first-class citizen ( English first-class object , German about "Object First Class") refers to the programming of computer systems an object as arguments or return value of a function or procedure occurs or a variable can be assigned.

The term was coined by Christopher Strachey in the context of functions as first-class citizens in the mid-1960s.

definition

An object is first class if it meets the following conditions:

it can be saved in program variables,
it can be passed as a parameter to functions or procedures,
it can serve as a return value of functions,
it can be created while a program is running and
it has its own identity (regardless of name).

The term object does not have to be understood in the narrower sense as an object of an object-oriented programming language . In most programming languages, the objects of the elementary data types , e.g. B. Integer and floating point objects, always first class.

Examples

In C and C ++ functions are not first class objects because it is not possible to create them at runtime - unlike e.g. B. Numbers . Also arrays are not first class objects, as they can not be passed as function parameters.
In Fortran , strings are not first class objects because they cannot be assigned to variables.
In contrast, functions in most functional programming languages are first-class objects (or first-class functions ) e.g. B. in Haskell , Scala , Smalltalk , OCaml and various Lisp dialects such as Scheme and Clojure , but not in Common Lisp .
The educational visual programming language Snap! Based on the concepts of Scratch and Scheme . provides clear and consistent access to first-class properties.

swell

^ Michael Scott: Programming Language Pragmatics . Morgan Kaufmann Publishers, San Francisco, CA 2006, p. 140.
^ Rod Burstall, "Christopher Strachey — Understanding Programming Languages," Higher-Order and Symbolic Computation 13 : 52 (2000)
↑ First Class . C2.com. January 25, 2006. Retrieved October 9, 2010.
^ First class object . Catalysoft.com. Retrieved October 9, 2010.

[1] Michael Scott: Programming Language Pragmatics . Morgan Kaufmann Publishers, San Francisco, CA 2006, p. 140.

[2] Rod Burstall, "Christopher Strachey — Understanding Programming Languages," Higher-Order and Symbolic Computation 13 : 52 (2000)

[3] First Class . C2.com. January 25, 2006. Retrieved October 9, 2010.

[4] First class object . Catalysoft.com. Retrieved October 9, 2010.