memoize_last.cpp

// Special case of memoization. Only last value is cached.

#include <iostream>
#include <memory>

template<typename R, typename Arg, typename Comp = std::equal_to<Arg>, typename Fun>
std::function<R(const Arg&)> memoize_last(Fun f)
{
	struct Cache
	{
		R result;
		Arg x;
		bool init = false;
	};
	
	auto cache = std::make_shared<Cache>();
	
	return [=](Arg x) -> R
	{
		if (cache->init && Comp()(cache->x, x))
		{
			return cache->result;
		}
		
		cache->init = true;
		cache->x = x;
		cache->result = f(x);
		return cache->result;
	};
}

int do_complex_calc(int x)
{
	std::cout << "COMPLEX CALC: " << x << std::endl;
	return x * x;
}

int main()
{
	auto calc = memoize_last<int, int>(do_complex_calc);
	
	calc(5);
	calc(5);
	calc(5);
	calc(10);
	calc(10);
	calc(10);
	calc(5);

	return 0;
}