为什么operator <是非成员函数？(Why should operator< be non-member function?)

 我记得C++ Primer告诉我们operator<应该non-member function ，我总是遵守规则。 但现在我想知道原因。 
 
 我写了以下代码： 
 
#include <iostream>
using std::cout;
using std::endl;

struct Point1
{
  int x, y;
  Point1(const int a, const int b): x(a), y(b) { }
};
inline bool operator<(const Point1& lhs, const Point1& rhs)
{
  return lhs.x < rhs.x || (lhs.x == rhs.x && lhs.y < rhs.y);
}

struct Point2
{
  int x, y;
  Point2(const int a, const int b): x(a), y(b) { }
  bool operator<(const Point2& rhs)
  {
    return x < rhs.x || (x == rhs.x && y < rhs.y);
  }
};

int main()
{
  Point1 a(1, 2), b(1, 3);
  cout << (a < b) << " " << (b < a) << endl;
  Point2 c(2, 3), d(2, 4);
  cout << (c < d) << " " << (d < c) << endl;
}
 
 在这种情况下，似乎它们没有区别， member函数似乎更简单。 
 
 但在这种情况下： 
 
#include <iostream>
using std::cout;
using std::endl;

 // Usually I write it for comparing floats
class Float1
{
  long double _value;
public:
  static const long double EPS = 1e-8;
  Float1(const long double value): _value(value) { }
  const long double Get() const { return _value; }
};
inline bool operator<(const Float1& lhs, const Float1& rhs)
{
  return rhs.Get() - lhs.Get() > Float1::EPS;
}
inline bool operator<(const Float1& lhs, const long double rhs)
{
  return rhs - lhs.Get() > Float1::EPS;
}

class Float2
{
  long double _value;
public:
  static const long double EPS = 1e-8;
  Float2(const long double value): _value(value) { }
  const long double Get() const { return _value; }
  bool operator<(const Float2& rhs)
  {
    return rhs._value - _value > Float2::EPS;
  }
  bool operator<(const long double rhs)
  {
    return rhs - _value > Float2::EPS;
  }
};

int main()
{
  Float1 x(3.14);
  Float2 y(2.17);
  long double zero = .0;
  cout << (x < zero) << " " << (zero < x) << endl;
  //cout << (y < zero) << " " << (zero < y) << endl; Compile Error!
}
 
 （x <零）和（零<x）都有效！ （ long double转换为Float ？） 
 
 但是（零<y）不，因为零不是Float 。 
 
 您可以看到，在第一种情况下， member function成本较少，而在第二种情况下， non-member function使得比较更容易。 所以我想知道 
 
 
 
 在第一种情况下，我应该使用member function而不non-member function吗？ 
 
 
 为什么C++ Primer建议binary operator non-member function ？ 
 
 
 member function和non-member function有什么区别吗？ 
 
 
 谢谢你的帮助！ 

I remeber C++ Primer tells us operator< should be non-member function, and I always obey the rule. But now I want to know the reason.
 
I wrote following code:
 
#include <iostream>
using std::cout;
using std::endl;

struct Point1
{
  int x, y;
  Point1(const int a, const int b): x(a), y(b) { }
};
inline bool operator<(const Point1& lhs, const Point1& rhs)
{
  return lhs.x < rhs.x || (lhs.x == rhs.x && lhs.y < rhs.y);
}

struct Point2
{
  int x, y;
  Point2(const int a, const int b): x(a), y(b) { }
  bool operator<(const Point2& rhs)
  {
    return x < rhs.x || (x == rhs.x && y < rhs.y);
  }
};

int main()
{
  Point1 a(1, 2), b(1, 3);
  cout << (a < b) << " " << (b < a) << endl;
  Point2 c(2, 3), d(2, 4);
  cout << (c < d) << " " << (d < c) << endl;
}
 
In this case, It seems they don't make difference and member function seems much simpler.
 
But in this case:
 
#include <iostream>
using std::cout;
using std::endl;

 // Usually I write it for comparing floats
class Float1
{
  long double _value;
public:
  static const long double EPS = 1e-8;
  Float1(const long double value): _value(value) { }
  const long double Get() const { return _value; }
};
inline bool operator<(const Float1& lhs, const Float1& rhs)
{
  return rhs.Get() - lhs.Get() > Float1::EPS;
}
inline bool operator<(const Float1& lhs, const long double rhs)
{
  return rhs - lhs.Get() > Float1::EPS;
}

class Float2
{
  long double _value;
public:
  static const long double EPS = 1e-8;
  Float2(const long double value): _value(value) { }
  const long double Get() const { return _value; }
  bool operator<(const Float2& rhs)
  {
    return rhs._value - _value > Float2::EPS;
  }
  bool operator<(const long double rhs)
  {
    return rhs - _value > Float2::EPS;
  }
};

int main()
{
  Float1 x(3.14);
  Float2 y(2.17);
  long double zero = .0;
  cout << (x < zero) << " " << (zero < x) << endl;
  //cout << (y < zero) << " " << (zero < y) << endl; Compile Error!
}
 
Both (x < zero) and (zero < x) work! (is long double converted to Float?)
 
But (zero < y) don't, because zero is not a Float.
 
You see, in first case, member function costs less code length, and in second case, non-member function makes comparing easier. So I want to know
 
 
 
In first case, should I use member function instead of non-member function?
 
 
Why C++ Primer suggests binary operators be non-member function?
 
 
Is there any other case that member function and non-member function make difference?
 
 
Thanks for helping!

原文：https://stackoverflow.com/questions/12047174