std::function<int(int)> Memoize(std::function<int(int)> fn)
    {
        std::map<int, int> memo;
        std::function<int(int)> helper = [=](int pos)
        {
            if (memo.count(0) == 0)
            {
                memo[pos] = fn(pos);
            }
            return memo[pos];
        };
        return helper;
    }

1>Source1.cpp(24): error C2678: binary '[' : no operator found which takes a left-hand operand of type 'const std::map<_Kty,_Ty>' (or there is no acceptable conversion)

std::function<int(int)> Memoize(std::function<int(int)> fn)
    {
        std::map<int, int> memo;
        std::function<int(int)> helper = [=](int pos) mutable // <== HERE!!
        {
            if (memo.count(0) == 0)
            {
                memo[pos] = fn(pos);
            }
            return memo[pos];
        };
        return helper;
    }

class Helper
{
public:
    int operator()(int) const;
private:
    std::map<int, int> memo;
    std::function<int(int)> fn;
};

std::function<int(int)> helper = [=](int pos) mutable
// etc