First-class object
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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 (however, see discussion), 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". Similarly, strings are not first class objects in Fortran as it is not possible to assign them to variables, whereas numbers can be so assigned.