C++向量迭代器N\u元素编译错误

下面的代码将无法编译。从第2行到最后一行(第n\u元素…)有错误。这似乎与比较器有关。编译器声称“该项的计算结果不是一个包含2个参数的函数”。如何修复编译错误?

    struct Result {
        Result(unsigned int id, double result);
        bool cmp(const Result &a, const Result &b)  const;

        unsigned int id;
        double result;
    };


Result::Result(unsigned int id, double result) {
    this->id = id;
    this->result = result;
}

bool Result::cmp(const Result &a, const Result &b)  const {
    if(a.result < b.result) {
        return true;
    }
    return false;
}

    //25th-percentile
    int index = (int) ((buffer.size()+1.0)/4.0 - 0.499);
    vector<Result>::iterator itrindex = buffer.begin() + index;
    nth_element(buffer.begin(), itrindex, buffer.end(), &Result::cmp);
    double twentyfifthperc = buffer[index].result;