c++ - 比较指向成员的指针？

Question

我很困惑为什么不能使用二元运算符比较指向成员的指针<

class Point3d{
  protected:
      //..
 public:
     float x;
      static list<Point3d*> *freeList;
 public:
     float y;
     static const int chunkSize = 250;
 public:
    float z;

};
和一个模板:

template< class class_type, class data_type1, class data_type2 >

char* access_order(data_type1 class_type:: *mem1, data_type2 class_type:: *mem2)
{

    return
      mem1 < mem2 ?
         "member 1 accurs first":
         "member 2 accurs first";
}  

当我像下面这样调用 access_order 时:

access_order(&Point3d::z, &Point3d::y);

g++ 报告:

"invalid operands of types ‘float Point3d::*’ and ‘float Point3d::*’ to binary ‘operator<’"

有没有办法比较成员的指针，我的意思是不相等的比较，如何？

Answer 1

您可以比较对象成员的地址:

A a;
if (std::less<void*>()(&a.a, &a.b))
    std::cout << "a precedes b\n";
else
    std::cout << "a follows b\n";