Creating Recursive Lambdas

Yes, a lambda can call itself recursively. However, because lambdas don't have a name by default, you need to use a special technique to achieve this.

The trick is to capture a reference to the lambda itself using a std::function. Here's an example that calculates the factorial of a number using a recursive lambda:

1#include <iostream>
2#include <functional>
3
4int main() {
5  std::function<int(int)> factorial{
6    [&factorial](int n) {
7      if (n <= 1) {
8        return 1;
9      } else {
10        return n * factorial(n - 1);
11      }
12    }
13  };
14
15  std::cout << "Factorial of 5: "
16    << factorial(5) << '\n';
17}

1Factorial of 5: 120

Here's how this works:

1. We declare a std::function named factorial. This will hold our lambda.
2. We initialize factorial with a lambda that takes an integer n and returns an integer.
3. Inside the lambda, we check the base case (n <= 1). If this is true, we return 1.
4. If the base case is not met, we return n * factorial(n - 1). This is where the recursion happens.
5. Importantly, factorial is captured by reference in the lambda. This allows the lambda to call itself through the factorial variable.

When we call factorial(5), it will recursively compute the factorial of 5, which is 120.

This pattern can be used to create recursive lambdas for various tasks. However, it's important to ensure that your recursive lambda has a base case that stops the recursion, otherwise you'll get infinite recursion and a stack overflow.

Also note that recursive lambdas can be less efficient than iterative solutions due to the overhead of function calls. However, they can be useful in situations where a recursive solution is more clear and concise.

Remember that this technique requires capturing the lambda by reference. Be careful not to let the reference outlive the lambda, as this will result in undefined behavior.