请求各位OIer大佬帮忙检查并改改下面的代码，

我改来改去总是报两种错误：

[Error] no matching function for call to 'operator-(const bigint&, const bigint&)'

[Error] no matching function for call to 'operator-(const bigint&, bigint&)'

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>

using namespace std;

// 定义大整数结构体
struct bigint {
    vector<int> num;
    bool isNegative = false;

    bigint() {}
    bigint(const string& str) {
        if (str[0] == '-') {
            isNegative = true;
            for (int i = str.size() - 1; i > 0; --i) {
                num.push_back(str[i] - '0');
            }
        } else {
            for (int i = str.size() - 1; i >= 0; --i) {
                num.push_back(str[i] - '0');
            }
        }
        // 去除前导零
        while (!num.empty() && num.back() == 0) {
            num.pop_back();
        }
        // 如果结果为 0，去掉负号
        if (num.empty()) {
            isNegative = false;
        }
    }
};

// 输出大整数
void bigout(const bigint& res) {
    if (res.isNegative && !res.num.empty()) {
        cout << "-";
    }
    if (res.num.empty()) {
        cout << "0";
    } else {
        for (int i = res.num.size() - 1; i >= 0; --i) {
            cout << res.num[i];
        }
    }
    cout << endl;
}

// 比较两个大整数的绝对值大小
bool absGreater(const bigint& a, const bigint& b) {
    if (a.num.size() != b.num.size()) {
        return a.num.size() > b.num.size();
    }
    for (int i = a.num.size() - 1; i >= 0; --i) {
        if (a.num[i] != b.num[i]) {
            return a.num[i] > b.num[i];
        }
    }
    return false;
}

// 判断两个大整数是否相等
bool operator==(const bigint& a, const bigint& b) {
    return a.isNegative == b.isNegative && a.num == b.num;
}

// 判断大整数 a 是否小于大整数 b
bool operator<(const bigint& a, const bigint& b) {
    if (a.isNegative != b.isNegative) {
        return a.isNegative;
    }
    if (a.isNegative) {
        return absGreater(a, b);
    }
    return absGreater(b, a);
}

// 判断大整数 a 是否大于大整数 b
bool operator>(const bigint& a, const bigint& b) {
    return b < a;
}

// 判断大整数 a 是否小于等于大整数 b
bool operator<=(const bigint& a, const bigint& b) {
    return !(a > b);
}

// 判断大整数 a 是否大于等于大整数 b
bool operator>=(const bigint& a, const bigint& b) {
    return !(a < b);
}

// 大整数加法
bigint operator+(const bigint& a, const bigint& b) {
    if (a.isNegative == b.isNegative) {
        bigint res;
        res.isNegative = a.isNegative;
        res.num.resize(max(a.num.size(), b.num.size()) + 1);
        for (int i = 0; i < a.num.size(); ++i) {
            res.num[i] += a.num[i];
        }
        for (int i = 0; i < b.num.size(); ++i) {
            res.num[i] += b.num[i];
        }
        for (int i = 0; i < res.num.size(); ++i) {
            if (res.num[i] >= 10) {
                res.num[i + 1]++;
                res.num[i] %= 10;
            }
        }
        while (!res.num.empty() && res.num.back() == 0) {
            res.num.pop_back();
        }
        return res;
    } else {
        if (a.isNegative) {
            return b - (-a);
        } else {
            return a - (-b);
        }
    }
}

// 取负操作
bigint operator-(const bigint& a) {
    bigint res = a;
    res.isNegative = !res.isNegative;
    return res;
}

// 大整数减法
bigint operator-(const bigint& a, const bigint& b) {
    if (a.isNegative == b.isNegative) {
        if (absGreater(a, b)) {
            bigint res;
            res.isNegative = a.isNegative;
            res.num.resize(a.num.size());
            for (int i = 0; i < a.num.size(); ++i) {
                res.num[i] = a.num[i];
            }
            for (int i = 0; i < b.num.size(); ++i) {
                res.num[i] -= b.num[i];
            }
            for (int i = 0; i < res.num.size(); ++i) {
                if (res.num[i] < 0) {
                    res.num[i] += 10;
                    res.num[i + 1]--;
                }
            }
            while (!res.num.empty() && res.num.back() == 0) {
                res.num.pop_back();
            }
            return res;
        } else {
            bigint res = b - a;
            res.isNegative = !res.isNegative;
            return res;
        }
    } else {
        if (a.isNegative) {
            bigint res = a + (-b);
            res.isNegative = true;
            return res;
        } else {
            return a + (-b);
        }
    }
}

// 大整数乘法
bigint operator*(const bigint& a, const bigint& b) {
    bigint res;
    res.isNegative = a.isNegative != b.isNegative;
    res.num.resize(a.num.size() + b.num.size());
    for (int i = 0; i < a.num.size(); ++i) {
        for (int j = 0; j < b.num.size(); ++j) {
            res.num[i + j] += a.num[i] * b.num[j];
        }
    }
    for (int i = 0; i < res.num.size(); ++i) {
        if (res.num[i] >= 10) {
            res.num[i + 1] += res.num[i] / 10;
            res.num[i] %= 10;
        }
    }
    while (!res.num.empty() && res.num.back() == 0) {
        res.num.pop_back();
    }
    return res;
}

// 大整数和整数的乘法
bigint operator*(const bigint& a, int b) {
    bigint res;
    res.isNegative = (a.isNegative && b > 0) || (!a.isNegative && b < 0);
    b = abs(b);
    res.num.resize(a.num.size() + 10);
    for (int i = 0; i < a.num.size(); ++i) {
        res.num[i] += a.num[i] * b;
    }
    for (int i = 0; i < res.num.size(); ++i) {
        if (res.num[i] >= 10) {
            res.num[i + 1] += res.num[i] / 10;
            res.num[i] %= 10;
        }
    }
    while (!res.num.empty() && res.num.back() == 0) {
        res.num.pop_back();
    }
    return res;
}

// 大整数和整数的加法
bigint operator+(const bigint& a, int b) {
    if (b < 0) {
        bigint temp;
        temp.num.push_back(-b);
        return a - temp;
    }
    bigint res = a;
    res.num[0] += b;
    for (int i = 0; i < res.num.size(); ++i) {
        if (res.num[i] >= 10) {
            if (i + 1 == res.num.size()) {
                res.num.push_back(0);
            }
            res.num[i + 1] += res.num[i] / 10;
            res.num[i] %= 10;
        }
    }
    while (!res.num.empty() && res.num.back() == 0) {
        res.num.pop_back();
    }
    return res;
}

// 大整数除法和取模的公共处理函数
pair<bigint, bigint> div_mod(const bigint& a, const bigint& b) {
    bigint quotient, remainder;
    quotient.isNegative = a.isNegative != b.isNegative;
    remainder.isNegative = a.isNegative;
    quotient.num.clear();
    remainder.num.clear();
    bigint absA = a;
    absA.isNegative = false;
    bigint absB = b;
    absB.isNegative = false;

    if (absB.num.empty()) {
        // 除数为 0 的情况
        cerr << "Error: division by zero" << endl;
        return {quotient, remainder};
    }

    for (int i = absA.num.size() - 1; i >= 0; --i) {
        remainder = remainder * 10 + absA.num[i];
        int q = 0;
        while (remainder >= absB) {
            remainder = remainder - absB;
            q++;
        }
        quotient.num.insert(quotient.num.begin(), q);
    }

    while (!quotient.num.empty() && quotient.num.back() == 0) {
        quotient.num.pop_back();
    }
    while (!remainder.num.empty() && remainder.num.back() == 0) {
        remainder.num.pop_back();
    }

    return {quotient, remainder};
}

// 大整数除法
bigint operator/(const bigint& a, const bigint& b) {
    return div_mod(a, b).first;
}

// 大整数取模
bigint operator%(const bigint& a, const bigint& b) {
    return div_mod(a, b).second;
}

int main() {
    string strA, strB;
    cin >> strA >> strB;
    bigint a(strA), b(strB);
    // 输出和
    bigout(a + b);
    // 输出差
    bigout(a - b);
    // 输出积
    bigout(a * b);
    // 输出商
    bigout(a / b);
    // 输出余
    bigout(a % b);
    return 0;
}

谢谢了！！！！！！！！