I'm doing Elgamal Signature Scheme and I need to use the decimal hash value from the message to compute S for signature generation.

    string hash = md5(message);
    cout << hash << endl;
    NTL::ZZ msgHash = strtol(hash.c_str(), NULL, 16);
    cout << msgHash << endl;

There are no integer large enough to contain the value of 32 byte hexadecimal hash, and so I tried big integer from NTL library but it didn't work out because you cannot assign long integer to NTL::ZZ type. Is there any good solution to this? I'm doing this with visual C++ in Visual Studio 2013.