C++ Lambda Expressions
What is a Lambda?
A lambda expression (C++11) is an anonymous inline function — a function without a name, defined right where it's used. Lambdas are especially useful with STL algorithms.
Syntax: [capture](parameters) -> returnType { body }
| Capture | Meaning |
|---|---|
[] | Capture nothing |
[=] | Capture all local variables by value |
[&] | Capture all local variables by reference |
[x] | Capture x by value |
[&x] | Capture x by reference |
[=, &x] | Capture all by value, x by reference |
#include <iostream>
#include <functional>
using namespace std;
int main() {
// Basic lambda — no capture, no parameters
auto hello = []() { cout << "Hello from lambda!" << endl; };
hello();
// Lambda with parameters
auto add = [](int a, int b) { return a + b; };
cout << "3 + 4 = " << add(3, 4) << endl;
// Lambda with explicit return type
auto divide = [](double a, double b) -> double {
if (b == 0) return 0;
return a / b;
};
cout << "10 / 3 = " << divide(10, 3) << endl;
// Capture by value
int x = 10;
auto addX = [x](int n) { return n + x; }; // x captured by value
x = 99; // changing x doesn't affect the lambda
cout << addX(5) << endl; // 15 (uses captured x=10)
// Capture by reference
int counter = 0;
auto increment = [&counter]() { counter++; };
increment(); increment(); increment();
cout << "Counter: " << counter << endl; // 3
// Store lambda in std::function
function<int(int, int)> multiply = [](int a, int b) { return a * b; };
cout << "5 * 6 = " << multiply(5, 6) << endl;
return 0;
}Lambdas with STL Algorithms
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
vector<int> nums = {5, 2, 8, 1, 9, 3};
// sort descending with lambda comparator
sort(nums.begin(), nums.end(), [](int a, int b) { return a > b; });
for (int n : nums) cout << n << " "; // 9 8 5 3 2 1
cout << endl;
// find_if — first element > 4
auto it = find_if(nums.begin(), nums.end(), [](int n) { return n > 4; });
if (it != nums.end()) cout << "First > 4: " << *it << endl; // 9
// for_each — print each element
for_each(nums.begin(), nums.end(), [](int n) { cout << n * 2 << " "; });
cout << endl; // 18 16 10 6 4 2
// count_if — count even numbers
int evens = count_if(nums.begin(), nums.end(), [](int n) { return n % 2 == 0; });
cout << "Even count: " << evens << endl; // 2
// remove_if — remove elements < 4
nums.erase(remove_if(nums.begin(), nums.end(), [](int n){ return n < 4; }), nums.end());
for (int n : nums) cout << n << " "; // 9 8 5
cout << endl;
return 0;
}