Closure (computer programming) Facts for Kids

In programming languages, a closure is a special kind of function that "remembers" the environment where it was created. Think of it like a backpack that a function carries around. This backpack holds all the variables that the function needs, even if those variables were defined outside the function itself. This allows the function to use those remembered variables later, even when it's called from a different part of your program.

What is a Closure?
Closure-Like Ideas in Other Languages
See Also

What is a Closure?

A closure is a function that keeps track of the variables from its "birthplace." Imagine you have a function that creates another function. The inner function might need to use some variables from the outer function. A closure makes sure that the inner function can always access those variables, even after the outer function has finished running.

How Closures Remember Things

When a closure is made, it doesn't just store the code of the function. It also stores a snapshot of the variables it needs from its surrounding environment. This snapshot is like a special record. So, when you call the closure later, it looks into its record to find the values of those variables. This is different from a regular function, which usually forgets about variables from its creation place once that part of the program is done.

Why Are Closures Useful?

Closures are super helpful in programming, especially in languages where functions can be treated like any other piece of data, such as numbers or text. This means you can pass functions around, return them from other functions, and store them in variables.

Functions as First-Class Citizens

Many modern programming languages, like Python, JavaScript, and Julia, treat functions as "first-class objects." This means functions are just like any other value. You can:

Give them to other functions as inputs.
Get them back as results from other functions.
Store them in variables.

When you return a function that uses variables from its original home, you're creating a closure.

Here's a simple example in JavaScript:

// This function creates a "greeter" function
function makeGreeter(greeting) {
  // The inner function "remembers" the 'greeting' variable
  return function(name) {
    return greeting + ", " + name + "!";
  };
}

// Create a greeter that says "Hello"
var sayHello = makeGreeter("Hello");
// Create a greeter that says "Hi"
var sayHi = makeGreeter("Hi");

// Now use them!
alert(sayHello("Alice")); // Shows "Hello, Alice!"
alert(sayHi("Bob"));     // Shows "Hi, Bob!"

In this example, `sayHello` and `sayHi` are closures. They both remember their own `greeting` (either "Hello" or "Hi") even though `makeGreeter` has already finished running.

Keeping Secrets: Private Variables

Closures can also help you create "private" variables. These are variables that only a specific function can see and change. It's like having a secret compartment that only your function can open. This is similar to how "private" parts of an object work in object-oriented programming.

Imagine you want a counter that only a specific function can increase:

function createCounter() {
  var count = 0; // This is the "private" variable

  return function() {
    count = count + 1; // Only this function can change 'count'
    return count;
  };
}

var myCounter = createCounter();
alert(myCounter()); // Shows 1
alert(myCounter()); // Shows 2
alert(myCounter()); // Shows 3

var anotherCounter = createCounter(); // A brand new counter
alert(anotherCounter()); // Shows 1

Each `createCounter` call makes a new `count` variable that only its returned function can access.

Other Cool Uses

Making Control Structures: In some languages, closures can be used to build things like `if/then/else` statements or `while` loops.
Private Communication: Multiple functions can share and change the same "remembered" variables, allowing them to communicate with each other secretly.

How Computers Handle Closures

When a computer runs a program with closures, it needs a special way to store them.

Memory and Closures

Normally, when a function finishes, its local variables are removed from the computer's memory (usually from a place called the "stack"). But for closures, the variables they "remember" need to stick around longer. So, these variables are often stored in a different part of memory called the "heap."

To make sure these variables are only kept as long as they are needed, many languages that use closures also use something called "garbage collection." This is like a cleanup crew that automatically finds and removes data from memory that is no longer being used. This way, programmers don't have to worry about manually managing this memory.

Closure-Like Ideas in Other Languages

While the term "closure" is specific, many languages have features that work in a similar way, allowing functions to remember their environment.

Callbacks (C)

In C, you can use "callbacks" to achieve similar results. You give a function a pointer to another function and some data. When the first function calls the callback, it also passes along that data. This lets the callback function use the data it was given, much like a closure uses its remembered variables.

Local Classes and Lambda Functions (Java)

Java has "local classes" and "anonymous classes" that can access variables from the method they are defined in. Before Java 8, these variables had to be marked as `final` (meaning they couldn't change).

Since Java 8, Java also has "lambda expressions," which are a more direct way to create closure-like functions. They make the code much shorter and easier to read.

// Java 8 Lambda Example
class Example {
    private int baseValue = 10; // A variable from the enclosing class

    public void calculateAndPrint(int input) {
        // This is a lambda expression, acting like a closure
        // It "remembers" 'baseValue' and 'input'
        Runnable task = () -> {
            int result = baseValue + input;
            System.out.println("Result: " + result);
        };
        new Thread(task).start(); // Run the task in a new thread
    }

    public static void main(String[] args) {
        new Example().calculateAndPrint(5); // Will print "Result: 15"
    }
}

Blocks (C, C++, Objective-C 2.0)

Apple added "blocks" to C, C++, and Objective-C 2.0. Blocks are a form of closure. They let you write small pieces of code that can capture variables from their surroundings. You can choose if these variables are captured by value (a copy) or by reference (the original variable).

Delegates (C#, VB.NET, D)

Languages like C# and Visual Basic .NET use "anonymous methods" and "lambda expressions" that support closures. They allow you to write short functions that capture variables from their environment.

// C# Example
var multiplier = 2; // This variable is "captured"
var numbers = new[] { 1, 2, 3 };

// This lambda expression is a closure
// It uses the 'multiplier' variable
var doubledNumbers = numbers.Select(x => x * multiplier);

// doubledNumbers will contain {2, 4, 6}

In D, "delegates" are used to implement closures. A delegate is like a function pointer that also carries a context (the remembered variables).

Function Objects (C++)

In C++, you can create "function objects" by making a class that acts like a function. Since C++11, the language also has "lambda expressions" that automatically create these function objects. These lambdas can capture variables from their surrounding code, either by making copies of them or by referring directly to them.

Closure (computer programming) facts for kids

Contents