string 替换字符的问题、、求大神看看

int main()
{
    string s = "a tho b thru c tho";
    string oldval = "tho";
    string newval = "through";

    auto curr = s.begin();

    while (curr <= (s.end() - oldval.size()))
    {
        string s1(curr, curr + oldval.size());
        if (s1 == oldval)
        {
            curr = s.replace(curr,oldval.size(),newval);******************这步一直报错，说参数有问题
            curr += newval.size();
            
        }
        else curr++;
    }cout << s << endl;
//    hanshu(s, oldval, newval);


    return 0;
}