What are lambdas?

Learn what lambda expressions are and anonymous functions.

What are lambdas?

A lambda is an anonymous function. It is defined where it is used; no separate function declaration is needed. Example: [](int x) { return x * 2; }. This lambda takes an int and returns double its value.

Lambdas are usually assigned to a variable with auto: auto doubleIt = [](int x) { return x * 2; };. Then it is called like a normal function: doubleIt(5) gives 10. Lambdas are especially handy for passing small operations to STL algorithms; they remove the overhead of writing a separate function.

Lambda and function

A lambda works like a normal function but with differences. A normal function is named at compile time and can be called from other places. A lambda is usually defined where it is used and has no name; it can be assigned to a variable with auto.

The power of a lambda is that it can capture surrounding variables. This is something normal functions cannot do. For example in [](int x) { return x + offset; } the offset can come from the outer scope; the lambda captures it via the capture clause. We will cover this in later lessons.

Exercise Define and call a lambda

Define a lambda with auto doubleIt = [](int x) { return x * 2; };. In main, print the result of doubleIt(5) with std::cout.

Show example solution

#include <iostream>

int main() {
    auto doubleIt = [](int x) { return x * 2; };
    std::cout << doubleIt(5);
    return 0;
}

Test yourself

Which is a valid lambda expression?
```
// valid lambda syntax
```
- function(x) { return x; }
- [](int x) { return x; }
- lambda(x) => x
- (int x) -> x
Show answer

[](int x) { return x; } — [](int x) { return x; } is a valid lambda.

Complete the lambda body

auto doubleIt = [](int x) { return x ____; };

Show answer

auto doubleIt = [](int x) { return x * 2; };

Match each piece to its role

[]

Capture clause
(int x)

Parameter list
{ return x; }

Body
auto f = [](...) {...};

Assignment

Next lesson: Lambda syntax

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