Lambdas & Functional Programming

Master C++ lambdas — capture lists, mutable lambdas, generic lambdas, and their use as algorithm predicates. Learn std::function, std::invoke, and the IIFE pattern.

Lambda Expressions

A lambda expression creates an anonymous function object (closure) inline. Lambdas are the primary way to pass behavior to STL algorithms, callbacks, and event handlers.

The full syntax is:

``cpp [captures](parameters) mutable -> return_type { body }``

- Captures — variables from the enclosing scope to use inside the lambda
- Parameters — like regular function parameters
- mutable — allows modifying captured-by-value variables
- Return type — usually deduced automatically
- Body — the function body

Lambdas compile down to compiler-generated function objects (structs with operator()) — they have zero overhead compared to hand-written functors.

Basic Lambda Syntax and Captures

The capture list is the defining feature of lambdas. It determines which enclosing variables the lambda can access and how.

lambda_captures.cpp

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

int main() {
    std::vector<int> nums = {1, 5, 3, 8, 2, 9, 4, 7, 6};

    // Simplest lambda — no captures
    std::sort(nums.begin(), nums.end(),
              [](int a, int b) { return a > b; });  // descending

    // Capture by value [=] — copies variables
    int threshold = 5;
    auto above = std::count_if(nums.begin(), nums.end(),
                               [threshold](int x) { return x > threshold; });
    std::cout << "Above " << threshold << ": " << above << '\n';

    // Capture by reference [&] — references variables
    int sum = 0;
    std::for_each(nums.begin(), nums.end(),
                  [&sum](int x) { sum += x; });
    std::cout << "Sum: " << sum << '\n';

    // Mixed captures
    int min_val = 3, max_val = 7;
    auto in_range = std::count_if(nums.begin(), nums.end(),
        [min_val, &max_val](int x) { return x >= min_val && x <= max_val; });
    // min_val captured by value, max_val captured by reference

    // Capture all by value [=] or all by reference [&]
    auto print_all = [&]() {  // captures everything by reference
        for (int n : nums) std::cout << n << ' ';
        std::cout << '\n';
    };
    print_all();

    // Init capture (C++14) — create new variables in the capture
    auto counter = [count = 0]() mutable { return ++count; };
    std::cout << counter() << '\n';  // 1
    std::cout << counter() << '\n';  // 2
    std::cout << counter() << '\n';  // 3
}

Generic and Template Lambdas

C++14 introduced generic lambdas with auto parameters. C++20 extended this with explicit template parameter lists, allowing full template power in lambdas.

generic_lambdas.cpp

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

int main() {
    // C++14: generic lambda — auto parameters
    auto print = [](const auto& x) {
        std::cout << x << '\n';
    };
    print(42);           // int
    print(3.14);         // double
    print("hello");      // const char*

    // C++14: generic lambda for comparisons
    auto max_of = [](const auto& a, const auto& b) {
        return (a > b) ? a : b;
    };
    std::cout << max_of(3, 7) << '\n';             // 7
    std::cout << max_of("abc", "xyz") << '\n';     // xyz (lexicographic)

    // C++20: template lambda — explicit template parameters
    auto add = []<typename T>(T a, T b) -> T {
        return a + b;
    };
    std::cout << add(1, 2) << '\n';                // 3
    std::cout << add(1.5, 2.5) << '\n';            // 4.0

    // C++20: constrained template lambda with concepts
    auto integral_only = []<std::integral T>(T x) {
        return x * 2;
    };
    std::cout << integral_only(5) << '\n';         // 10
    // integral_only(3.14);  // ERROR: double doesn't satisfy std::integral

    // Using generic lambdas with algorithms
    std::vector<std::string> words = {"banana", "apple", "cherry"};
    std::sort(words.begin(), words.end(),
              [](const auto& a, const auto& b) {
                  return a.size() < b.size();  // sort by length
              });
    for (const auto& w : words) std::cout << w << ' ';  // apple banana cherry
    std::cout << '\n';
}

std::function and std::invoke

std::function is a type-erased callable wrapper that can hold any callable with a matching signature — lambdas, function pointers, functors, member function pointers. std::invoke is a uniform way to call any callable.

std_function.cpp

#include <functional>
#include <iostream>
#include <string>
#include <vector>

// Regular function
int add(int a, int b) { return a + b; }

struct Multiplier {
    int factor;
    int operator()(int x) const { return x * factor; }
    int multiply(int x) const { return x * factor; }
};

int main() {
    // std::function can hold different callable types
    std::function<int(int, int)> op;

    op = add;                                        // function pointer
    std::cout << op(3, 4) << '\n';                   // 7

    op = [](int a, int b) { return a * b; };         // lambda
    std::cout << op(3, 4) << '\n';                   // 12

    // Store callables in a collection
    std::vector<std::function<int(int)>> transforms;
    transforms.push_back([](int x) { return x + 1; });
    transforms.push_back([](int x) { return x * 2; });
    transforms.push_back(Multiplier{10});            // functor

    int val = 5;
    for (auto& fn : transforms) {
        val = fn(val);
    }
    std::cout << val << '\n';  // ((5+1)*2)*10 = 120

    // std::invoke — uniform call syntax for any callable
    Multiplier m{3};
    std::cout << std::invoke(add, 10, 20)       << '\n';  // 30
    std::cout << std::invoke(m, 7)              << '\n';  // 21
    std::cout << std::invoke(&Multiplier::multiply, m, 7) << '\n';  // 21
    std::cout << std::invoke(&Multiplier::factor, m)      << '\n';  // 3 (data member)
}

The IIFE Pattern

IIFE (Immediately Invoked Function Expression) is a pattern borrowed from JavaScript. You define a lambda and call it immediately. This is useful for complex initialization of const variables:

``cpp const auto config = [&]() { Config c; c.host = read_env("HOST"); c.port = parse_int(read_env("PORT")); if (c.port <= 0) c.port = 8080; c.debug = is_debug_build(); return c; }(); // <-- note the () — invoked immediately``

Without IIFE, you would either need a non-const variable (losing immutability guarantees) or a separate factory function. IIFE keeps the initialization logic local and the result const.

Pitfall

Capturing by reference is dangerous when the lambda outlives the captured variable:

``cpp std::function make_counter() { int count = 0; return [&count]() { return ++count; }; // BUG: count is destroyed! }``

The returned lambda holds a reference to a local variable that no longer exists. This is undefined behavior. Fix it by capturing by value (with mutable if needed):

``cpp std::function make_counter() { int count = 0; return [count]() mutable { return ++count; }; // OK: owns a copy }``

Rule of thumb: capture by reference for short-lived lambdas (algorithm predicates), capture by value for long-lived lambdas (callbacks, stored in data structures).

Best Practice

std::function has overhead: it allocates memory (for large closures), uses virtual dispatch, and prevents inlining. For algorithm predicates and other cases where the callable is used immediately, pass the lambda directly — the compiler can inline it completely.

Use std::function only when you need to:
- Store callables for later use
- Type-erase different callable types into a uniform container
- Pass callables across ABI boundaries

In templates, use auto or concepts to accept callables without type erasure:

``cpp template F> void apply(F&& fn, int value) { fn(value); // no std::function overhead }``

Key Takeaways

Lambdas are zero-overhead anonymous function objects — use them freely with STL algorithms
Capture by value [x] for safety with long-lived lambdas; capture by reference [&x] for short-lived ones
C++14 generic lambdas (auto params) and C++20 template lambdas give full generic programming power
std::function type-erases callables but has overhead — prefer auto or templates when possible
The IIFE pattern [&]() { ... }() enables complex initialization of const variables
Init captures ([x = expr]) allow creating new variables in the closure (C++14)

Quiz — Test Your Knowledge

(15 XP)

1. What does the `mutable` keyword do on a lambda?

2. What is the danger of capturing a local variable by reference in a lambda that is returned from a function?

3. What does `[count = 0]() mutable { return ++count; }` demonstrate?

STL Algorithms: The Power of <algorithm>

C++20 Ranges: A New Paradigm