C++ Lambda Expressions: Syntax, Closures, Capture Modes & Generic Lambdas

A Lambda Expression is an anonymous (unnamed) inline function object that can capture variables from its enclosing scope. Introduced in C++11, lambdas are indispensable when writing concise callbacks and algorithms.

Capture Modes Breakdown

• []: Empty capture clause (accesses only its arguments).
• [=]: Captures all local variables in scope by value (read-only copy).
• [&]: Captures all local variables in scope by reference (can modify originals).
• [this]: Captures the current class object instance pointer.
• [x, &y]: Captures variable x by value and variable y by reference.

Generic Lambda (C++14) with STL Algorithm

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

int main() {
    std::vector<int> numbers = {5, 12, 18, 7, 25, 30};
    int threshold = 15;

    // Capture threshold by value, take element by const reference
    int count = std::count_if(numbers.begin(), numbers.end(), [threshold](int n) {
        return n > threshold;
    });

    std::cout << "Numbers greater than " << threshold << ": " << count << "\n"; // 3

    // Generic auto lambda (C++14):
    auto printer = [](const auto& item) {
        std::cout << "[ITEM]: " << item << "\n";
    };
    printer(42);
    printer("Modern C++");

    return 0;
}