Before understanding why functions are called first-class citizens in some programming languages and not in others, we need to understand what a first-class citizen means in a programming language.

What is a First-Class Citizen?

In programming, a first-class citizen (or first-class object) is an entity that can be treated like any other value in the language.

This means it can:

1. Be assigned to a variable.
2. Be passed as an argument to a function.
3. Be returned from a function.
4. Be stored in data structures (like lists, maps, etc.).

Example — Functions as First-Class Citizens (Python)

def greet():
    return "Hello"

# 1. Assigned to a variable
say_hello = greet

# 2. Passed as an argument
def call_func(func):
    print(func())

call_func(say_hello)

# 3. Returned from a function
def outer():
    return greet

inner = outer()
print(inner())

Here, functions are first-class citizens because they can be assigned, passed, and returned.

Who Are the First-Class Citizens in Popular Languages?

C

• ✅ Primitive types (int, float, char)

• ✅ Pointers (including function pointers)

• ⚠️ Functions are not fully first-class — you can use function pointers, but no closures or nested functions.

  → First-class citizens: Variables, pointers, primitive types

C++

• ✅ Objects (instances of classes)