为 + 运算符重载 C++

Question

我目前正在学习如何在 C++ 中进行运算符重载，我在网上找到了一些在运行以下时有效的代码。

class Complex {
 private:
  int real, imag;

 public:
  Complex(int r = 0, int i = 0) {
    real = r;
    imag = i;
  }

  // This is automatically called when '+' is used with
  // between two Complex objects
  Complex operator+(Complex const &obj) {
    Complex res;
    res.real = real + obj.real;
    res.imag = imag + obj.imag;
    return res;
  }
  void print() { cout << real << " + i" << imag << endl; }
};

int main() {
  Complex c1(10, 5), c2(2, 4);
  Complex c3 = c1 + c2;  // An example call to "operator+"
  c3.print();
}

然而，当我尝试类似的结构时，我收到以下错误：未找到默认构造函数和 << 我不熟悉的错误。

class Chicken {
 private:
  int i;
  int k;
  int s;
  int total = 0;

 public:
  Chicken(int index, int kentucky, int sign) {
    i = index;
    k = kentucky;
    s = sign;
  }
  Chicken operator+(Chicken obj) {
    Chicken Chicky;
    Chicky.total = i + obj.i;
    return Chicky;
  }
};

int main() {
  Chicken P(1, 2, 3);
  Chicken U(2, 2, 2);
  Chicken W = P + U;

  cout << W;
}

enter image description here

Answer 1

Below 是更正后的例子。首先，您收到错误是因为在 operator+ 中您默认构造了一个 Chicken 类型的对象，但您的 class 没有任何默认构造函数。 解决方法是添加默认构造函数

#include <iostream>
class Chicken{
    //needed for cout<<W; to work
    friend std::ostream& operator<<(std::ostream& os, const Chicken &rhs);
    private:
      int i = 0;
      int k = 0;
      int s = 0;
      int total = 0; 

    public:

    

    //default constructor 
    Chicken() = default;
    
    
    //use constructor initializer list instead of assigning inside the body
    Chicken(int index, int kentucky, int sign): i(index), k(kentucky), s(sign){

        //i = index;
        //k = kentucky;
        //s = sign;
    }
    Chicken operator + (Chicken obj){
        Chicken Chicky;//this needs/uses default constructor
        Chicky.total = i + obj.i;
        return Chicky;
    }
   
};
std::ostream& operator<<(std::ostream& os, const Chicken &rhs)
{
    os << rhs.i << rhs.k << rhs.s << rhs.total ;
    return os;
}
int main(){
    
   
    
    Chicken P(1,2,3); //this uses parameterized constructor
    Chicken U(2,2,2); //this uses parameterized constructor
    Chicken W = P+U;  //this uses overloaded operator+
    
    std::cout << W;   //this uses oveloaded operator<<
    
}

其次，您使用了语句 cout<<W; 但没有重载 operator<<。要使 std::cout<< W; 正常工作，您需要重载 operator<<，如我在上面的代码中所示。

您可以查看工作示例 here。

第三，您 can/should 使用 构造函数初始化列表 而不是如上所示在构造函数体内赋值。

为 + 运算符重载 C++

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