建立一个分数类。分数类的数据成员包括分子和分母，成员函数包括显示、输入、约分、通分、比较、加、减、乘、除、求相反数。

好的，建立一个分数类的代码如下：

#include <iostream>
#include <algorithm>

using namespace std;

class Fraction {
public:
    Fraction(int num = 0, int den = 1) : numerator(num), denominator(den) {}
    void display() const {
        cout << numerator << "/" << denominator << endl;
    }
    void input() {
        cin >> numerator >> denominator;
    }
    void reduce() {
        int gcd = __gcd(numerator, denominator);
        numerator /= gcd;
        denominator /= gcd;
    }
    void to_common(Fraction &f) {
        int lcm = denominator / __gcd(denominator, f.denominator) * f.denominator;
        numerator *= lcm / denominator;
        denominator = lcm;
        f.numerator *= lcm / f.denominator;
        f.denominator = lcm;
    }
    bool operator < (const Fraction &f) const {
        return numerator * f.denominator < denominator * f.numerator;
    }
    bool operator > (const Fraction &f) const {
        return numerator * f.denominator > denominator * f.numerator;
    }
    bool operator == (const Fraction &f) const {
        return numerator * f.denominator == denominator * f.numerator;
    }
    Fraction operator + (const Fraction &f) const {
        Fraction res(numerator * f.denominator + f.numerator * denominator, denominator * f.denominator);
        res.reduce();
        return res;
    }
    Fraction operator - (const Fraction &f) const {
        Fraction res(numerator * f.denominator - f.numerator * denominator, denominator * f.denominator);
        res.reduce();
        return res;
    }
    Fraction operator * (const Fraction &f) const {
        Fraction res(numerator * f.numerator, denominator * f.denominator);
        res.reduce();
        return res;
    }
    Fraction operator / (const Fraction &f) const {
        Fraction res(numerator * f.denominator, denominator * f.numerator);
        res.reduce();
        return res;
    }
    Fraction operator - () const {
        return Fraction(-numerator, denominator);
    }

private:
    int numerator;
    int denominator;
};

int main() {
    Fraction a(2, 3), b(4, 6);
    a.display(); // 2/3
    b.display(); // 4/6
    b.reduce();
    b.display(); // 2/3
    a.to_common(b);
    a.display(); // 4/6
    b.display(); // 4/6
    if (a < b) cout << "a < b" << endl; // a < b
    if (a > b) cout << "a > b" << endl;
    if (a == b) cout << "a == b" << endl; // a == b
    (a + b).display(); // 8/6
    (a - b).display(); // 0/6
    (a * b).display(); // 8/18
    (a / b).display(); // 8/12
    (-a).display(); // -4/6
}

这个代码实现了分数类，包括了显示、输入、约分、通分、比较、加、减、乘、除、求相反数等成员函数。