Lambda syntax

Learn the [](){} structure and capture clause.

Lambda syntax

The full lambda syntax is: [capture](parameters) -> return_type { body }. Each part is optional but the order is fixed. capture captures outer variables, parameters takes parameters, return_type sets the return type, body is the body.

The -> return_type is usually omitted; the compiler deduces the type from the return statement. For example [](int a, int b) { return a + b; } automatically deduces int as the return type. If multiple return statements return different types, an explicit return type is needed: [](int x) -> double { if (x) return x; return 0.0; }.

Return type deduction

The return type of a lambda is usually deduced automatically. If it sees return x * 2; it deduces int, if it sees return x > 0; it deduces bool. This is known as auto return type deduction.

Sometimes an explicit return type is needed: especially when different return statements return different types. Then you specify a return type like -> double: [](int x) -> double { return x / 2.0; }. This guarantees the lambda result is a double. Otherwise the compiler errors on conflicting return types.

Exercise Write a two-parameter lambda

Define a two-parameter lambda with auto add = [](int a, int b) { return a + b; };. In main, print the result of add(3, 4) with std::cout.

Show example solution

#include <iostream>

int main() {
    auto add = [](int a, int b) { return a + b; };
    std::cout << add(3, 4);
    return 0;
}

Test yourself

Complete the capture clause

auto f = ____(int x) { return x; };

Show answer

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

What does [] specify in a lambda?
```
auto f = [](int x) { return x; };
```
- Parameter list
- Return type
- Capture clause
- Body
Show answer

Capture clause — [] is the capture clause; it captures outer variables.
Match each part to its role

[capture]

Outer variable capture
(parameters)

Parameter list
-> return_type

Return type
{ body }

Body

Next lesson: Capture modes

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