c++ - C++ 运算符重载失败

Question

我如下定义了一个 Fraction 类，重载了“=”和“+”运算符。

我使这个程序尽可能简单以显示问题。

#include <iostream>

class Fraction {
private:
    int nominator;
    int denominator;
public:    
    Fraction ();
    Fraction (int, int);

    Fraction & operator = (Fraction &);

    friend Fraction operator + (Fraction &, Fraction &);

    // static function:
    // find the Greatest Common Divisor of two numbers
    int static GCD(int x, int y);

};

int Fraction::GCD(int x, int y) {
    if (y == 0) {
        return x;
    } else {
        return GCD (y, x % y);
    }
}

Fraction::Fraction () {
    nominator = NULL;
    denominator = NULL;
}

Fraction::Fraction (int num_1, int num_2) {
    int divisor = Fraction::GCD (num_1, num_2);
    nominator = num_1 / divisor;
    denominator = num_2 / divisor;
}

Fraction & Fraction::operator = (Fraction &A) {
    nominator = A.nominator;
    denominator = A.denominator;
    return *this;
}

Fraction operator + (Fraction &A, Fraction &B) {
    int nominator = A.nominator * B.denominator + B.nominator * A.denominator;
    int denominator = A.denominator * B.denominator;
    int divisor = Fraction::GCD (nominator, denominator);
    return Fraction (nominator / divisor, denominator / divisor);
}

在 Main() 函数中，我有三个测试用例

int main(int argc, const char * argv[]) {

    Fraction frac_a = Fraction(1, 3);
    Fraction frac_b = Fraction(1, 4);

    // test 1: no compile error 
    frac_a + frac_b;
    frac_a = frac_b;

    // test 2: no compile error
    Fraction frac_c = frac_a + frac_b;

    // test 3: Error: No viable overloaded '='
    Fraction frac_d;   
    frac_d = frac_a + frac_b;

    return 0;
}

问题是，为什么“测试 3”出现“No viable overloaded '='”错误？

Answer 1

您的赋值运算符被声明为引用非常量 Fraction。您的加法运算符按值返回，因此您试图将临时分配给 frac_c。临时对象不能绑定(bind)到非常量引用。

解决方法是让你的赋值运算符通过 const-reference 获取:

Fraction & operator = (const Fraction &);