Question

我是C ++学习的初学者，我对＆＃39;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆＃和＆＃39;＆gt;＆gt;＆＃39;。为什么cout的结果不正确？同时，输入c的坐标后，程序冻结。代码：

class Vector
{friend istream &operator >> (istream &is,Vector &vec );
friend ostream &operator << (ostream &os,Vector &vec );
private:
    int num;
    double *cor;
public:
    Vector(int n=0,double *c=NULL);//
};
int main()
{   double b[5]={1,2,3,4,5};
    Vector a(5,b);
    cout<<a;
    Vector c(2);
    cin>>c;
}
Vector::Vector(int n,double *c)
{
    num=n;
    double *cor=new double[num];
    if (c) {
        for (int i=0;i<n;i++) {cor[i]=c[i];cout<<cor[i]<<endl;}
    }
}
istream &operator >> (istream &is,Vector &vec )
{   cout<<"Input the coordinate：";
    for (int i=0;i<vec.num;i++)
        is>>vec.cor[i];
    return is;
}
ostream &operator << (ostream &os,Vector &vec )
{
    for (int i=0;i<vec.num;i++){
        os<<vec.cor[i];}
    return os;
}

Answer 1

double *cor=new double[num];

您正在声明名为cor的本地变量，而不是初始化成员cor。它应该是：

cor = new double[num];

但是在实际代码中，您会使用unique_ptr到double数组，delete自动生成数组（没有额外开销）：

#include <iostream>
#include <memory> // for unique_ptr

using namespace std;

class Vector {
public:
    Vector(int = 0, double* = nullptr);

private:
    friend istream& operator>>(istream&, Vector& vec);
    friend ostream& operator<<(ostream&, Vector& vec);

    int num;
    unique_ptr<double[]> array;
};

int main() {
    double b[] = {1, 2, 3, 4, 5};

    Vector a(5, b);
    cout << a;

    Vector c(2);
    cin >> c;
}

Vector::Vector(int n, double* c) {
    num = n;
    array = make_unique<double[]>(n); // in C++14
    //array = unique_ptr<double[]>(new double[n]); // in C++11

    if (!c) return;

    for (int i = 0; i < n; i++) {
        array[i] = c[i];
        cout << array[i] << endl;
    }
}

istream& operator>>(istream& is, Vector& vec) {
    cout << "Input the coordinates: ";
    for (int i = 0; i < vec.num; i++)
        is >> vec.array[i];
    return is;
}

ostream& operator<<(ostream& os, Vector& vec) {
    for (int i = 0; i < vec.num; i++)
        os << vec.array[i];
    return os;
}

Answer 2

小而重要的提示：使用'const'：

> ostream &operator << (ostream &os,const Vector &vec)

因为vec是通过引用收到的，你不会改变它。

关于C ++运算符重载的问题

2 个答案: