First-class object

First-class object
In computing, a first-class object (also -value, -entity, -citizen), in the context of a particular programming language, is an entity which can be used in programs without restriction (when compared to other kinds of objects in the same language). Depending on the language, this can imply:

being expressible as an anonymous literal value
being storable in variables
being storable in data structures
having an intrinsic identity (independent of any given name)
being comparable for equality with other entities
being passable as a parameter to a procedure/function
being returnable as the result of a procedure/function
being constructable at runtime
For example, in C, it is not possible to create new functions at runtime, whereas other kinds of object can be created at runtime. So functions in C are not first-class objects; sometimes they are called second-class objects because they can still be manipulated in most of the above fashions (via function pointers). Similarly, strings are not first class objects in FORTRAN 66 as it is not possible to assign them to variables, whereas numbers can be so assigned.

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First-class function
In computer science, a programming language is said to support first-class functions if it treats functions as first-class objects. Specifically, this means that functions can be created during the execution of a program, stored in data structures, passed as arguments to other functions, and returned as the values of other functions.