c++ - C++ 如何确定重载运算符的参数？

Question

我重载了 I/O 运算符:

struct Time {
  int hours;
  int minutes;
};

ostream &operator << ( ostream &os, Time &t ) {
  os << setfill('0') << setw( 2 ) << t.hours;
  os << ":";
  os << setfill('0') << setw( 2 ) << t.minutes;
  return os;
}

istream &operator >> ( istream &is, Time &t ) { 
  is >> t.hours;
  is.ignore(1, ':');
  is >> t.minutes;
  return is;
}

我想知道当我调用 cin >> time 时，编译器如何确定 is &is 参数。这是我的 main() 程序:

operator>>( cin, time );
cout << time << endl;

cin >> (cin , time);
cout << time << endl;

cin >> time;                     //Where is cin argument???
cout << time << endl;

Answer 1

cin >> time;

这是运算符 >>> 有两个操作数。如果发现重载运算符函数为非成员，则左操作数成为第一个参数，右操作数成为第二个参数。于是就变成了:

operator>>(cin, time);

所以 cin 参数只是运算符的第一个操作数。

参见标准的 §13.5.2:

A binary operator shall be implemented either by a non-static member function (9.3) with one parameter or by a non-member function with two parameters. Thus, for any binary operator @, x@y can be interpreted as either x.operator@(y) or operator@(x,y).

如果您想知道这如何应用于链式运算符，请看:

cin >> time >> something;

这相当于:

(cin >> time) >> something;

这也等同于:

operator>>(operator>>(cin, time), something);