如何在代码中使用性病而不包括“使用命名空间性病”？

Question

在此运算符重载代码中，我不想在需要的地方写“ using namespace std”，而是想包含“ std ::”。

在cout和cin之后添加“ std ::”后，我仍然在其他包含“ std ::”的地方出现错误。

#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 &, const Complex &);
  friend istream & operator >> (istream &, Complex &);
};

ostream & operator << (ostream &out, Complex &obj)
{
  out<<obj.real<<" "<<obj.imag;
  return out;
}
istream & operator >> (istream &in, const Complex &obj)
{
  in>>obj.real>>obj.imag;
  return in;
}

int main()
{
  Complex obj;
  std::cin>>obj;
  std::cout<<obj;
  return 0;
}

它应该使用istream运算符输入两个数字，并使用ostream运算符输出两个数字。

Answer 1

添加std::到ostream和istream

它们来自标头<istream>，<ostream>并且在<iosfwd>

#include<iostream>
//using namespace std;
class Complex
{
private:
    int real, imag;
public:

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

    friend std::ostream& operator<<(std::ostream& out, const Complex& obj);
    friend std::istream& operator>>(std::istream& in, Complex& obj);
};

std::ostream& operator<<(std::ostream &out, const Complex &obj)
{
    out << obj.real << " " << obj.imag;
    return out;
}
std::istream& operator>>(std::istream &in, Complex &obj)
{
    in >> obj.real >> obj.imag;
    return in;
}

int main()
{
    Complex obj;
    std::cin >> obj;
    std::cout << obj;
    return 0;
}

（与std::问题无关）您也可以friend使用get / set成员函数在类的外部访问私有变量而无需声明。感谢@aschepler指出我在可访问性方面的错误。

#include<iostream>
class Complex
{
private:
    int real, imag;
public:
    int get_real() const {
        return real;
    }
    void set_real(int real) {
        this->real = real;
    }
    int get_imag() const {
        return imag;
    }
    void set_imag(int imag) {
        this->imag = imag;
    }

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

};

std::ostream& operator<<(std::ostream &out, const Complex &obj)
{
    out << obj.get_real() << " " << obj.get_real();
    return out;
}
std::istream& operator>>(std::istream &in, Complex &obj)
{
    int real, imag;
    in >> real >> imag;
    obj.set_real(real);
    obj.set_imag(imag);
    return in;
}

int main()
{
    Complex obj;
    std::cin >> obj;
    std::cout << obj;
    return 0;
}