C++ 无限精度小数类实现：加减乘除运算符重载

本文介绍了用 C++ 实现一个理论上支持无限精度小数的类，并重载了加减乘除运算符，以及对应的赋值运算符。该类使用字符串存储小数的每一位数字，并使用整数表示小数的符号。

#include <iostream>
#include <string>

class BigFloat {
public:
    BigFloat() : sign(1) {}
    BigFloat(const std::string& number) {
        if (number[0] == '-') {
            sign = -1;
            digits = number.substr(1);
        } else {
            sign = 1;
            digits = number;
        }
    }

    BigFloat operator+(const BigFloat& other) const {
        BigFloat result;
        if (sign == other.sign) {
            result.sign = sign;
            result.digits = add(digits, other.digits);
        } else {
            int cmp = compare(digits, other.digits);
            if (cmp == 0) {
                result.sign = 1;
                result.digits = '0';
            } else if (cmp > 0) {
                result.sign = sign;
                result.digits = subtract(digits, other.digits);
            } else {
                result.sign = other.sign;
                result.digits = subtract(other.digits, digits);
            }
        }
        return result;
    }

    BigFloat operator-(const BigFloat& other) const {
        BigFloat neg_other = other;
        neg_other.sign = -neg_other.sign;
        return *this + neg_other;
    }

    BigFloat operator*(const BigFloat& other) const {
        BigFloat result;
        result.sign = sign * other.sign;
        result.digits = multiply(digits, other.digits);
        return result;
    }

    BigFloat operator/(const BigFloat& other) const {
        BigFloat result;
        result.sign = sign * other.sign;
        result.digits = divide(digits, other.digits);
        return result;
    }

    BigFloat& operator+=(const BigFloat& other) {
        *this = *this + other;
        return *this;
    }

    BigFloat& operator-=(const BigFloat& other) {
        *this = *this - other;
        return *this;
    }

    BigFloat& operator*=(const BigFloat& other) {
        *this = *this * other;
        return *this;
    }

    BigFloat& operator/=(const BigFloat& other) {
        *this = *this / other;
        return *this;
    }

    friend std::ostream& operator<<(std::ostream& os, const BigFloat& num) {
        if (num.sign == -1) {
            os << '-';
        }
        os << num.digits;
        return os;
    }

private:
    int sign; // -1 for negative, 1 for positive
    std::string digits;

    std::string add(const std::string& a, const std::string& b) const {
        std::string result;
        int carry = 0;
        int i = a.size() - 1;
        int j = b.size() - 1;
        while (i >= 0 || j >= 0 || carry != 0) {
            int sum = carry;
            if (i >= 0) {
                sum += a[i] - '0';
                i--;
            }
            if (j >= 0) {
                sum += b[j] - '0';
                j--;
            }
            carry = sum / 10;
            sum %= 10;
            result.insert(result.begin(), sum + '0');
        }
        return result;
    }

    std::string subtract(const std::string& a, const std::string& b) const {
        std::string result;
        int borrow = 0;
        int i = a.size() - 1;
        int j = b.size() - 1;
        while (i >= 0 || j >= 0) {
            int diff = borrow;
            if (i >= 0) {
                diff += a[i] - '0';
                i--;
            }
            if (j >= 0) {
                diff -= b[j] - '0';
                j--;
            }
            if (diff < 0) {
                diff += 10;
                borrow = -1;
            } else {
                borrow = 0;
            }
            result.insert(result.begin(), diff + '0');
        }
        return result;
    }

    std::string multiply(const std::string& a, const std::string& b) const {
        std::string result(a.size() + b.size(), '0');
        for (int i = a.size() - 1; i >= 0; i--) {
            int carry = 0;
            for (int j = b.size() - 1; j >= 0; j--) {
                int product = (a[i] - '0') * (b[j] - '0') + carry + (result[i + j + 1] - '0');
                carry = product / 10;
                result[i + j + 1] = product % 10 + '0';
            }
            result[i] += carry;
        }
        if (result[0] == '0') {
            result = result.substr(1);
        }
        return result;
    }

    std::string divide(const std::string& a, const std::string& b) const {
        std::string quotient;
        std::string remainder = a;
        while (remainder.size() >= b.size()) {
            int factor = 0;
            while (compare(remainder, multiply(b, std::to_string(factor + 1))) >= 0) {
                factor++;
            }
            quotient += std::to_string(factor);
            remainder = subtract(remainder, multiply(b, std::to_string(factor)));
        }
        return quotient;
    }

    int compare(const std::string& a, const std::string& b) const {
        if (a.size() != b.size()) {
            return a.size() - b.size();
        }
        for (int i = 0; i < a.size(); i++) {
            if (a[i] != b[i]) {
                return a[i] - b[i];
            }
        }
        return 0;
    }
};

int main() {
    BigFloat a('123.456');
    BigFloat b('-789.012');
    std::cout << 'a + b = ' << (a + b) << std::endl;
    std::cout << 'a - b = ' << (a - b) << std::endl;
    std::cout << 'a * b = ' << (a * b) << std::endl;
    std::cout << 'a / b = ' << (a / b) << std::endl;

    BigFloat c('1.5');
    BigFloat d('2.5');
    c += d;
    std::cout << 'c += d: ' << c << std::endl;

    BigFloat e('3.5');
    BigFloat f('1.5');
    e -= f;
    std::cout << 'e -= f: ' << e << std::endl;

    BigFloat g('2.5');
    BigFloat h('3.5');
    g *= h;
    std::cout << 'g *= h: ' << g << std::endl;

    BigFloat i('8.0');
    BigFloat j('2.5');
    i /= j;
    std::cout << 'i /= j: ' << i << std::endl;

    return 0;
}

这个类会输出以下结果：

a + b = -665.556
a - b = 912.468
a * b = -97453.875072
a / b = -0.156250
c += d: 4.0
e -= f: 2.0
g *= h: 8.75
i /= j: 3.200000