First class object

An object is called a “first class object” if it ^{[3] [4]} :

• can be stored in a variable or data structures ;
• may be passed to the function as an argument ;
• can be returned from a function as a result;
• can be created at runtime ;
• internally self-identifying (independent of naming).

The term “object” is used here in a general sense, and is not limited to objects of a programming language . So the values ​​of the simplest data types , for example integer and float , in many languages ​​are “first class objects”.

In C and C ++, you cannot create functions at run time, so functions are not first-class objects in these languages. At the same time, pointers to a function can be passed as an argument and returned from another function, which is why functions in C ++ are sometimes called second-class objects . Nevertheless, in C ++ there is a concept of a functional object ( English function object ), which is an object of the first class and implements semantics equivalent to functions ^[3] .

In Smalltalk ^[5] , Scala , and JavaScript ^[6], functions (methods) and classes are first-class objects. Since the operators ( + , - ) in Smalltalk are essentially methods, they are also first-class objects.

Sample Nim code.

  # assign the procedure to a variable
 var value = proc () =
   echo "value"

 value () # procedure call
 var value2 = value
 value2 () # procedure call

 # procedure will be transferred to another
 proc two (): string =
   return "two"

 # procedure will receive another procedure
 proc wrap ( x : proc ) =
   echo "one"
   echo x ()
   echo "three"

 # calling a procedure that receives another procedure as an input
 wrap ( two )

 # procedure that returns a procedure
 proc closure ( x : int ): proc =
   proc res ( y : int ): int =
     return y * y + x
   return res

 var result = closure ( 2 ) # call the procedure that will return another procedure
 echo result ( 3 ) # call internal procedure