C++ Lambda Functions

A lambda is a small function without a name, written right where it is needed. Lambdas are most often passed to the algorithms of the standard library, such as sort and count_if, to say how to compare or what to count.

The syntax

[capture](parameters) { body }

#include <iostream>

int main() {
    auto square = [](int x) { return x * x; };
    auto greet = [](const char* name) { std::cout << "Hi, " << name << "\n"; };
    std::cout << square(7) << "\n";
    greet("Asha");
    return 0;
}

49
Hi, Asha

The return type is worked out from the return statement. A lambda stored in an auto variable can be called like any function.

Lambdas with algorithms

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<int> nums = {5, 3, 8, 1, 9, 2};
    std::sort(nums.begin(), nums.end(), [](int a, int b) { return a > b; });   // largest first
    for (int n : nums) std::cout << n << " ";
    std::cout << "\n";

    int evens = std::count_if(nums.begin(), nums.end(), [](int n) { return n % 2 == 0; });
    std::cout << evens << " even numbers\n";

    std::vector<std::string> words = {"pear", "fig", "banana"};
    std::sort(words.begin(), words.end(),
              [](const std::string& a, const std::string& b) { return a.size() < b.size(); });
    for (const auto& w : words) std::cout << w << " ";
    std::cout << "\n";
    return 0;
}

9 8 5 3 2 1 
2 even numbers
fig pear banana 

Captures

The square brackets list the variables from around the lambda that it may use:

• [limit] copies limit into the lambda (capture by value);
• [&total] lets the lambda use and change total itself (capture by reference);
• [=] copies everything used, [&] references everything used.

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> marks = {45, 72, 88, 39, 91};
    int passMark = 50;
    int passed = std::count_if(marks.begin(), marks.end(), [passMark](int m) { return m >= passMark; });
    std::cout << passed << " passed\n";

    int total = 0;
    std::for_each(marks.begin(), marks.end(), [&total](int m) { total += m; });
    std::cout << "total " << total << "\n";
    return 0;
}

3 passed
total 335

Prefer listing the captures explicitly: it shows exactly what the lambda depends on. Be careful with references: a lambda that outlives a variable it captured by reference refers to nothing valid.

Storing lambdas

auto holds a lambda most efficiently. When a variable or parameter must accept any function with a given shape, use std::function<int(int)> from the functional header.

When to use a lambda

Use a lambda when a small piece of behaviour is needed in one place: a sort order, a filter, a callback for a button. If the same logic is needed in several places, or it grows beyond a few lines, give it a name as an ordinary function. Code reads best when lambdas stay short enough to understand at a glance.

Practice exercises

1. Sort a vector of numbers by their last digit using a lambda.
2. Count the words longer than a length the user types, capturing it in the lambda.
3. Use std::for_each with a lambda that captures a counter by reference to count the negative numbers in a vector.