C++ Lambda Functions

What is a Lambda?

A lambda (C++11) is an anonymous, inline function — defined where you need it without naming it. Syntax:

[capture](parameters) -> returnType { body }

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    // Simple lambda
    auto greet = [](string name) {
        cout << "Hello, " << name << "!" << endl;
    };
    greet("Alice");   // Hello, Alice!

    // Lambda with return type
    auto add = [](int a, int b) -> int { return a + b; };
    cout << add(3, 5) << endl;   // 8

    // Lambda with STL
    vector<int> nums = {5, 2, 8, 1, 9, 3};
    sort(nums.begin(), nums.end(), [](int a, int b) { return a > b; }); // descending
    for (int n : nums) cout << n << " ";   // 9 8 5 3 2 1
    cout << endl;
    return 0;
}

Capture Clauses

Lambdas can capture variables from the surrounding scope:

int threshold = 5;

// Capture by value [=]
auto isAbove = [=](int n) { return n > threshold; };

// Capture by reference [&]
int count = 0;
auto countAbove = [&count, threshold](int n) {
    if (n > threshold) count++;
};

vector<int> data = {1, 7, 3, 9, 2, 6};
for (int n : data) countAbove(n);
cout << "Above " << threshold << ": " << count << endl;   // 3

Lambdas with STL

vector<string> words = {"banana", "apple", "cherry", "date"};

// Sort by length
sort(words.begin(), words.end(),
     [](const string& a, const string& b) { return a.size() < b.size(); });

// Filter with count_if
int shortWords = count_if(words.begin(), words.end(),
                          [](const string& s) { return s.size() <= 5; });
cout << "Short words: " << shortWords << endl;

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> nums = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    nums.erase(remove_if(nums.begin(), nums.end(),
               [](int n) { return n % 2 != 0; }), nums.end());
    for (int n : nums) cout << n << " ";   // 2 4 6 8 10
    cout << endl;
    return 0;
}

Summary

• Lambdas are anonymous inline functions: [capture](params){ body }.
• Capture by value with [=], by reference with [&].
• Perfect with STL algorithms: sort, find_if, count_if, transform.