小编Bha*_*rya的帖子

#include <iostream>
using namespace std;

class Complex
{
private:
    int real, imag;
public:
    Complex(int r = 0, int i =0)
    {  real = r;   imag = i; }
    **friend ostream & operator << (ostream &out, const Complex &c);
    friend istream & operator >> (istream &in,  Complex &c);**
};

ostream & operator << (ostream &out, const Complex &c)
{
    out << c.real;
    out << "+i" << c.imag << endl;
    return out;
}

istream & operator >> (istream &in,  Complex &c)
{ …

int n = 5;    
if(2<=n<=20) 
 {
  cout << "hello";
 }

在上面的代码中，它没有给出错误，它成功运行并且给出“ hello”作为输出。

但是我们必须使用&&这种方程式。
谁能解释一下？