分數類，實現加減乘除操作符的重載（未考慮約數）

1. #include <stdlib.h>
2. #include <iostream>
4. using namespace std;
6. class Fraction
7. {
8. public:
9. Fraction(){}
10. Fraction(int num, int deno): numerator(num), denominator(deno){}
11. Fraction(const Fraction &copy)
12. {
13. this->numerator = copy.numerator;
14. this->denominator = copy.denominator;
15. }
17. Fraction& operator=(const Fraction &copy)
18. {
19. if (this == &copy)
20. {
21. return *this;
22. }
24. this->numerator = copy.numerator;
25. this->denominator = copy.denominator;
27. return *this;
28. }
30. ~Fraction(){}
32. friend ostream& operator<<(ostream &os, const Fraction &a)
33. {
34. os << a.numerator << "/" << a.denominator << endl;
35. return os;
36. }
38. friend istream& operator>>(istream &in, Fraction &a)
39. {
40. in >> a.numerator >> a.denominator;
41. return in;
42. }
44. Fraction operator+(const Fraction &a)
45. {
46. Fraction tmp(this->numerator * a.denominator + a.numerator * this->denominator, this->denominator * a.denominator);
47. return tmp;
48. }
50. Fraction operator-(const Fraction &a)
51. {
52. Fraction tmp(this->numerator * a.denominator - a.numerator * this->denominator, this->denominator * a.denominator);
53. return tmp;
54. }
56. Fraction operator*(const Fraction &a)
57. {
58. Fraction tmp(this->numerator * a.numerator, this->denominator * a.denominator);
59. return tmp;
60. }
62. Fraction operator/(const Fraction &a)
63. {
64. Fraction tmp(this->numerator * a.denominator, this->denominator * a.numerator);
65. return tmp;
66. }
68. private:
69. int numerator;
70. int denominator;
72. };
74. int main()
75. {
76. Fraction a(5, 31);
77. Fraction b(3, 10);
79. Fraction c(a - b);
80. cout << c << endl;
81. return 0;
82. }

期間遇到如下錯誤

2. [test@ls test]$ g++ -g test.cpp -o test
3. test.cpp: In function 『std::istream& operator>>(const istream&, Fraction&)』:
4. test.cpp:39:17: error: passing 『const istream {aka const std::basic_istream<char>}』 as 『this』 argument of 『std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>]』 discards qualifiers [-fpermissive]
5. in >> a.numerator >> a.denominator;
6. ^
7. test.cpp:40:16: error: invalid initialization of reference of type 『std::istream& {aka std::basic_istream<char>&}』 from expression of type 『const istream {aka const std::basic_istream<char>}』
8. return in;
9. ^

原因為輸入操作符重載時，參數添加了const關鍵字。由於要對輸入流進行修改，const操作符不應存在

2. friend istream& operator>>(const istream &in, Fraction &a)
3. {
4. in >> a.numerator >> a.denominator;
5. return in;
6. }

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