BigInt problem with hexdigit string

[Thanh-Binh]

BigInt header only does not work with hexdigit string like
std::string hexStr = "123456789abcdef123456789abcdef";
It is very kind of you to extend it.

Best thanks

[whym1here]

int get_val(char c) {
    assert(('0' <= c and c <= '9') or ('a' <= c and c <= 'f'));
    if ('0' <= c and c <= '9') 
        return c - '0';
    else
        return c - 'a' + 10;
}

BigInt hex_to_BigInt(const std::string& hex) {
    assert(!hex.empty());
    BigInt res = 0, p = 1;
    for (auto it = hex.rbegin(); it != hex.rend(); it++) {
        res += p * get_val(*it);
        p *= 16;
    }
    return res;
}

I had something similar to this in mind. Do we have any library specific optimizations?
Obviously we would need to handle lots of edge cases in original implementation.