c++ - "undefined reference to"C++运算符重载

Question

我想创建类Q(有理数)，它具有以下功能:

1)print(): 以:numerator/denominator 的形式打印有理数

2)cancel():执行取消，如果可能的话:(3/9 = 1/3)

3)value():有理数的取值为double类型；

4)运算符重载加法、减法、除法、乘法。

这是我的代码:

#include <iostream>
#include <vector>
#include <string>
#include <stdexcept>
using namespace std;


class Q{
private:
    int num;
    int denom;

public:
    Q();
    Q(int num,int denom);
    void print();
    Q cancel();
    void value();
    friend Q operator*(Q lhs,Q rhs);
    friend Q operator/(Q lhs,Q rhs);
    friend Q operator+(Q lhs,Q rhs);
    friend Q operator-(Q lhs,Q rhs);
};

Q::Q(int numerator,int denominator){
    num = numerator;
    denom = denominator;
}


void Q::print(){
    cout << to_string(num) + "/" + to_string(denom);
}

Q Q::cancel(){
    vector<int> canc_val = {2,3,5,7};
    while(true){
        if(num % canc_val[0] == 0 && denom % canc_val[0] == 0){
            num /= canc_val[0];
            denom /= canc_val[0];
        } else if(num % canc_val[1] == 0 && denom % canc_val[1] == 0){
            num /= canc_val[1];
            denom /= canc_val[1];
        } else if(num % canc_val[2] == 0 && denom % canc_val[2] == 0){
            num /= canc_val[2];
            denom /= canc_val[2];
        }else if(num % canc_val[3] == 0 && denom % canc_val[3] == 0){
            num /= canc_val[3];
            denom /= canc_val[3];
        }else{
            break;
        }
    }
    return Q(num,denom);
}

void Q::value(){
    double res;
    res = num/denom;
    cout << res;
}

Q operator*(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.num;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

Q operator/(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom;
    temp.denom = lhs.denom * rhs.num;
    return temp;
}

Q operator+(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom + rhs.num*lhs.denom;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

Q operator-(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom - rhs.num*lhs.denom;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

int main(){
    Q number_1(3,7);
    Q number_2(3,9);
    Q number_3(15,125);

    number_1.print();
    cout << "\n";
    number_2.print();
    number_2.cancel();
    cout << "\n";
    number_2.print();
    cout << "\n";
    number_3.value();
    cout << "\n";
    number_3.cancel();
    cout << "\n";
    number_3.print();

    Q number_4 = number_1 + number_2;
    number_4.print();
    number_4.cancel();
    cout << "\n";
    number_4.print();

    Q number_5 = number_2 - number_3;
    number_5.print();
    number_5.cancel();
    cout << "\n";
    number_5.print();


    Q number_6 = number_1 * number_1;
    number_6.print();
    number_6.cancel();
    cout << "\n";
    number_6.print();

    Q number_7 = number_2 / number_3;
    number_7.print();
    number_7.cancel();
    cout << "\n";
    number_7.print();

}

但是编译器给我一个错误:

/tmp/cce4zPM4.o: In function `operator*(Q&, Q&)':
:(.text+0x170): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator/(Q&, Q&)':
:(.text+0x1b0): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator+(Q&, Q&)':
:(.text+0x1f0): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator-(Q&, Q&)':
:(.text+0x230): undefined reference to `Q::Q()'
collect2: error: ld returned 1 exit status

我不知道如何修复它。你能帮帮我吗？

Answer 1

Q::Q() 没有定义，只有声明，但您必须提供定义，如下所示:

Q::Q() :
    num(0),
    denom(1)
{
}

不要在没有初始化列表的情况下定义它，因为这意味着您的数据成员具有不确定的值，最终导致未定义的行为。