c++ - 为什么我的 vector 大小一直重置为 0？ (目前正在使用类(class))

Question

我一直在尝试制作一个多项式类，我已经完成了将近一半。但是，我的 vector 在“多项式 q(coeff, expo);”之后一直重置为 0，谁能告诉我为什么？

class polynomial {
  public:
    polynomial();
    polynomial(vector<float> coefficient, vector<int> degree);

    friend ostream& operator<<(ostream& os, const polynomial& y);

  private:
    vector<float> coeff1;
    vector<int> expo1;
};

polynomial::polynomial(){
  coeff1.clear();
  expo1.clear();
  coeff1.push_back(1);
}

polynomial::polynomial(vector<float> coefficient, vector<int> degree){
  if (coefficient.size() != degree.size()){
    cout << "Error. The number of coefficients are not the same as the number of exponents. Polynomial will be set to 1." << endl;
    polynomial();
  }
  else {

    for (int b = 0; b<degree.size(); b++) {
      for (int c = 0; c<b; c++){
        if (degree[b] > degree[c]){
          int holder = degree[b];
          degree[b] = degree[c];
          degree[c] = holder;

          float holder1 = coefficient[b];
          coefficient[b] = coefficient[c];
          coefficient[c] = holder1;
        }
      }
    }


    for (int a = 0; a<coefficient.size(); a++) {
      coeff1.push_back (coefficient[a]);
      expo1.push_back (degree[a]);
    }
  }
}


ostream& operator<<(ostream& os, const polynomial& y){
  if (y.coeff1.size() != y.expo1.size()) {
    os << 1;
    return os;
  }
  else {
    for (int x = 0; x<y.coeff1.size(); x++){
      if (y.coeff1[x] != y.coeff1[y.coeff1.size() - 1]) {
        if (y.expo1[x] == 1){
          os << y.coeff1[x] << "x" << " + ";
        }
        else if(y.expo1[x] == 0) {
          os << y.coeff1[x];
        }
        else {
          os << y.coeff1[x] << "x^" << y.expo1[x] << " + ";
        }
      }
      else {
        if (y.expo1[x] == 1){
          os << y.coeff1[x] << "x";
        }
        else if(y.expo1[x] == 0) {
          os << y.coeff1[x];
        }
      }

    }

    return os;
  }
}

int main()
{
  vector<float> coeff;
  vector<int> expo;

  coeff.push_back(3);
  coeff.push_back(16);
  coeff.push_back(10);
  //    coeff.push_back(7);

  expo.push_back(4);
  expo.push_back(1);
  expo.push_back(2);
  expo.push_back(3);


  polynomial p;
  cout << "The polynomial is: " << p << endl;
  polynomial q(coeff, expo);
  cout << "The polynomial is: " << q << endl;
  return 0;
}

[有几行无用的代码，因为我想检查我的 vector 大小在哪里变为 0]

Answer 1

这一行:

polynomial();

创建一个未命名的对象并立即销毁它。它具有与以下相同的效果:

{
    polynomial x;
}

我猜你是想“调用构造函数”。然而，这是不可能的，构造函数是特殊的，只能通过尝试创建对象来“调用”；或来自 ctor-initializer 列表。

---

我建议重新设计您的构造函数。首先，复制构造函数是伪造的(它不复制除 ptr 之外的任何字段，而且 int *ptr 也没有被普通构造函数初始化)。事实上 int *ptr; 应该完全删除。

在构造函数 polynomial() 中，对 clear() 的调用是多余的。 vector 一开始是空的，因为这是一个构造函数，所以它只在第一次创建的多项式上被调用。

我建议对带有两个参数的构造函数执行此操作:

polynomial::polynomial(vector<float> coefficient, vector<int> degree)
{
    if (coefficient.size() != degree.size())
    {
    // typically it would be better to just throw an exception here, the caller will not expect a 1-polynomial 
        std::cerr << "Error. The number of coefficients are not the same as the number of exponents. Polynomial will be set to 1." << std::endl;
        coeff1.push_back(1);
        wala();
    }
    else 
    {
        set_polynomial(coefficient, degree);
    }
}

并将用于对系数和度数进行排序的其他逻辑移至名为 set_polynomial 的私有(private)函数中。这使您的构造函数易于阅读并使您的代码整洁；您可能希望稍后使用此函数以允许用户更改多项式。

专业提示:查看 std::tie，您实际上可以使用单个命令对 coefficient 和 degree 进行排序。