effective c++ 条款18 make interface easy to use correctly and hard to use incorrectly

class Date
{
private:
  int month;
  int day;
  int year;
public:
Date(int month,int day,int year)
{
  this->month = month;
  ...  
}
}

//wrong example
Date date(30,3,1995);//should be 3,30
Date date(2,30,1995);//should be 3,30

class Month
{
  ... 
};
class Day
{
  ... 
};
class Year
{
  ... 
};

class Date
{
private:
  int month;
  int day;
  int year;
public:
Date(Month month,Day day,Year year)
{
  this->month = month;
  ...  
}
}