value becoming 0 for no obvious reason

for some reason, the value mod i am creating is becoming 0 in keygen despite being correct when it is made and after keygen is run. i dont get why. can anyone tell me?

class RC5{
    private:
        uint64_t w, r, b;
        uint128_t mod;
        std::string mode;
        std::vector <uint64_t> S;

    public:
        RC5(std::string KEY, std::string MODE, uint64_t W = 32, uint64_t R = 12, uint64_t B = 16){
            uint128_t mod = 1;
            mod <<= W;
            mode = MODE;
            w = W;
            r = R;
            b = B;
            std::cout << mod << std::endl;         // 1 << 32
            keygen(KEY);
            std::cout << mod << std::endl;         // 1 << 32 
        }

        void keygen(std::string key){
            std::cout << mod << std::endl;         // 0
            // lots of commented out stuff
        }
};

i am sure that uint128_t is开发者_开发问答 written correctly, so this doesnt seem to make sense. if necessary, uint128_t can be found here.

erm, because in the ctor you are modifying a local variable called mod which shadows your class member.

This part of RC5

uint128_t mod = 1;
mod <<= W; 

creates a new local variable mod that hides the class member. In keygen you use the other mod.