C++ Lambda Functions

🧩 C++ Lambda Functions

Lambda functions in C++ are anonymous (unnamed) functions that you can define inline.
They are especially useful for short operations, callbacks, and STL algorithms.

Introduced in C++11.

🔹 1. Basic Syntax

[capture](parameters) -> return_type {
// function body
};

  • capture → variables from outside the lambda

  • parameters → function parameters

  • return_type → optional (often auto-deduced)

🔹 2. Simple Lambda Example

auto greet = []() {
cout << "Hello, Lambda!";
};greet();

🔹 3. Lambda with Parameters

auto add = [](int a, int b) {
return a + b;
};cout << add(10, 20); // 30

🔹 4. Lambda with Explicit Return Type

auto divide = [](int a, int b) -> double {
return (double)a / b;
};cout << divide(10, 3);

🔹 5. Capture Variables (Very Important)

Capture by Value [=]

int x = 10;

auto show = [=]() {
cout << x;
};

show();

✔ Read-only copy inside lambda

Capture by Reference [&]

int x = 10;

auto modify = [&]() {
x = 20;
};

modify();
cout << x; // 20

Capture Specific Variables

int a = 5, b = 10;

auto sum = [a, &b]() {
b = 20;
return a + b;
};

🔹 6. Lambda in STL Algorithms (Very Common)

Example: for_each

#include <vector>
#include <algorithm>vector<int> v = {1, 2, 3, 4};

for_each(v.begin(), v.end(), [](int x) {
cout << x << ” “;
});

Example: sort

sort(v.begin(), v.end(), [](int a, int b) {
return a > b; // descending order
});

🔹 7. Lambda with mutable

Allows modification of captured-by-value variables.

int x = 10;

auto func = [x]() mutable {
x += 5;
cout << x;
};

func();
cout << x; // still 10

🔹 8. Lambda as Function Argument

void operate(int a, int b, auto func) {
cout << func(a, b);
}operate(5, 3, [](int x, int y) {
return x * y;
});

🔹 9. Lambda vs Normal Function

Lambda Normal Function
Inline & short Separate definition
Can capture variables Cannot capture
Common with STL General purpose
Anonymous Named

❌ Common Mistakes

int x = 10;

auto f = []() {
cout << x; // ❌ x not captured
};

✔ Correct:

auto f = [x]() {
cout << x;
};

📌 Summary

  • Lambda = anonymous inline function

  • Syntax: [capture](params){}

  • Can capture variables by value or reference

  • Widely used with STL algorithms

  • Makes code concise and expressive

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