binary – How to convert large bitsets ie bitset into hexadecimal in C++?

I have a large bitset<256> that I want to convert to hexadecimal but common solutions of to_ulong() and to_ullong() throws overflow error.

        bitset<256> bitresult = to_bitset(hexToBinary(x2)) ^ to_bitset(hexToBinary(random[i-1]));
            stringstream res;

            bitset<64> b1;
            bitset<64> b2;
            bitset<64> b3;
            bitset<64> b4;

            for (int i1=0; i1<64; i1++)
            b1[i1]=bitresult[i1];
            for (int i1=0; i1<64; i1++)
            b2[i1]=bitresult[i1+64];
            for (int i1=0; i1<64; i1++)
            b3[i1]=bitresult[i1+128];
            for (int i1=0; i1<64; i1++)
            b4[i1]=bitresult[i1+192];

            res<< hex << b1.to_ullong();
            res<< hex << b2.to_ullong();
            res<< hex << b3.to_ullong();
            res<< hex << b4.to_ullong();
            string hashresult = res.str();
            cout << hashresult <<endl;
                 return sha256(hashresult);

However I am having issues returning them back to Hexadecimal, due to the large size, to.ullong() throws overflow error. The code above is my attempt but for me seems very flawed, is there a better solution to this ?