Fixing `std::functional`.

functional.h

#include <functional>
#include <memory>
#include <stdlib.h>

#if defined(USE_ALTERNATIVE)

template<class FunctionSignature>
class function;

template<class Result, class...Args>
class function<Result(Args...)> {
public:
    template<class Functor>
    function(Functor const& functor) : o(new D<Functor>(functor)) {}
    Result operator()(Args&&...params) const {
        return (*o)(std::forward<Args>(params)...);
    }
    function& operator=(std::nullptr_t) { o.reset(); return *this; }
private:
    struct B {
        virtual ~B() = default;
        virtual Result operator()(Args&&...) const = 0;
    };
    template<class Functor>
    struct D : B {
        Functor f;
        D(const Functor& input) : f(input) {}
        Result operator()(Args&&...params) const override {
            return f(std::forward<Args>(params)...);
        }
    };
    std::unique_ptr<B> o;
};
#else

using std::function;

#endif

int main(int argb,char**args) {
    int a = atoi(args[0]);
    int b = atoi(args[1]);
    int c = atoi(args[2]);
    int d = atoi(args[3]);
    int e = atoi(args[4]);
    auto f1 = function<int(int)>([=](int n) -> int {
        while (n != c * a) {
            args[n] += c - b * d / e;
        }
        return *args[3];
    });
    if (b&1) {
        f1 = nullptr;
    }

    auto x = f1(15);
    return x;
}