输入流、输出流: operator<<(ostream& ostream, Widget const&)

一般地, 定义为友元函数.

简易版

#include <istream>  // for std::istream
#include <ostream>  // for std::ostream

class Widget {
 public:
  // 将类成员写入流中
  friend std::ostream& operator<<(std::ostream& ostream, Widget const& widget) {
    ostream << widget.value_;
    return ostream;  // 按引用返回输出流
  }

  // 从流中读取数据
  friend std::istream& operator>>(std::istream& istream, Widget& widget) {
    istream >> widget.value_;
    return istream;  // 按引用返回输入流
  }

 private:
  int value_;
};

复杂版

提示

相比于进行这样的复杂实现来序列化, 你可能更想用第三方库进行序列化.

以按 (real,imaginary) 格式读入复数为例:

定义 operator<<

#include <iomanip>  // for std::setprecision
#include <ostream>  // for std::ostream
#include <sstream>  // for std::ostringstream

class Complex {
 public:
  // 将复数以保留两位小数的 (real,imaginary) 写入流中
  friend std::ostream& operator<<(std::ostream& ostream,
                                  Complex const& complex) {
    // 为避免污染 `ostream` 的格式, 新建一个 `std::ostringstream`
    std::ostringstream oss;

    oss << std::fixed << std::setprecision(2);
    oss << '(' << complex.real_ << ',' << complex.imaginary_ << ')';

    return ostream << oss.str();
  }

 private:
  double real_;
  double imaginary_;
};

定义 operator>>

#include <istream>  // for std::istream

class Complex {
 public:
  // 从流中读取 (real,imaginary) 为复数
  friend std::istream& operator>>(std::istream& istream, Complex& complex) {
    bool is_good = false;  // 记录输入是否成功

    // 从流中读取特定格式的数据, 并暂存在变量中
    double real      = 0;
    double imaginary = 0;
    if (istream >> std::ws  // 忽略输入中最前面的所有空白符
        && istream.get() == '('
        && istream >> real
        && istream.get() == ','
        && istream >> imaginary
        && istream.get() == ')') {
      is_good = true;  // 输入成功
    }

    if (is_good) {
      // 如果读取完全成功, 更新 complex
      complex.real_      = real;
      complex.imaginary_ = imaginary;
    } else {
      // 如果读取失败, 设置 istream 为失败状态
      istream.setstate(std::ios_base::failbit);
    }

    return istream;
  }

 private:
  double real_;
  double imaginary_;
};