#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
#define W cout <<

template <class T>
struct Dyn
{
    T (*f)(int x);
    Dyn (auto _f) { f = _f; }
    T operator [] (int x) { return f(x); }
};

int main()
{
    Dyn<int> mul2( [](int x){return 2*x;} );
    W mul2[2] endl;
    return 0;
}