lambda.recursive.org

Lambda recursive

” … You can store it in a variable and reference that variable (although you cannot declare the type of that variable as auto, you would have to use an std::function object instead) … ”

Source code from: https://stackoverflow.com/questions/14531993/can-lambda-functions-be-recursive/14532044

http://cpptruths.blogspot.com/2013/10/creating-recursive-lambdas-and.html

http://www.riptutorial.com/cplusplus/example/8508/recursive-lambdas

*this vs this in C++

this is a pointer, and *this is a dereferenced pointer.

source code example

If you had a function that returned this, it would be a pointer to the current object, while a function that returned *this would be a “clone” of the current object, allocated on the stack – unless you have specified the return type of the method to return a reference.

*source code: * https://stackoverflow.com/questions/2750316/this-vs-this-in-c#2750322

*/

#include <iostream>
#include <functional>

using namespace std;

std::function<int(int)> create() {
	int foo = 20;
	std::function<int(int)> f = [=](int n) mutable {
         std::function<int(int)> recurse = [&](int n) { 
            	foo = 10;
            	return (n<=2)? 1 : recurse(n-1) + recurse(n-2); 
         	};  
			return recurse(n);
	};  
  	return f;
}

template < class F >
struct y_combinator {
	F f;
	// forward operator()
	template <class... Args>
	decltype(auto) operator()(Args&&... args) const {
        	return f(*this, std::forward<Args>(args)...);
    }
};

//helper functions for deduce thr type of th lamnda
template < class F >
y_combinator< std::decay_t <F>> make_y_combinator (F&& f){
	return {std::forward<F>(f)};
};
	
int main()
{
	std::function<int (int)> factorial = [&] (int i) 
	{ 
		return (i == 1) ? 1 : i * factorial(i - 1); 
	};
	
	//ERROR
	//When the function ends, so does the fib object and consequently, the reference inside the closure becomes invalid
	std::function<int(int)> fib = [&fib](int n)
  	{
		return (n <= 2)? 1 : fib(n-1) + fib(n-2);
  	};
	
	cout << factorial(5) << endl;
	cout << fib(10) << endl;	
	auto a = create();
	cout << a(10) << endl;
	
	auto gcd = make_y_combinator(
		[](auto&& gcd, int a, int b){
			return b == 0 ? a : gcd(b, a%b);
  		}
	);
	
	
	return 0;
}