【Geekband】Homework 2

题目为 Rectangle 类实现构造函数,拷贝构造函数,赋值操作符,析构函数。

class Shape
{                  
   int no;
};             
class Point
{
   int x;
   int y;
};             
class Rectangle: public Shape
{
   int width;
   int height;
   Point * leftUp;
public:
   Rectangle(int width, int height, int x, int y);
   Rectangle(const Rectangle& other);
   Rectangle& operator=(const Rectangle& other);
   ~Rectangle();        
};

设计思路

01. Big Three Function Design - 三大函数设计

1.1 General Constructor - 构造函数

• private could be omit at the begging of a class.

class MyClass
{
    // for private variables
public:
    // for public interface
protected:
    // for protected functions
}

• We would prefer to use this

Rectangle(int width, int height)
{
    this->width = width;
    this->height = height;  
}

• There is a point-to-class pointer and how to build a constructor

// Class
class Point
{
   int x;
   int y;
public:
    Point (int x  = 0, int y =0)
    {
        this->x = x;
        this->y = y;
    }
    int get_x() const {return x;}
    int get_y() const {return y;}
};              
class Rectangle: public Shape
{
   int width;
   int height;
   Point* leftUp; 
public:
   Rectangle(int width, int height, int x, int y);
    ... 
};
// Constructor
Rectangle::Rectangle(int width, int height, int x, int y)
{
     this->width = width;
     this->height = height;
     this->leftUp = new Point(x, y);
}

1.2 Copy Constructor

• Step 1: 父类Shape的创建在函数列表中完成，不能使用this->no进行处理。
• Step 2: 子类的non-pointer成员，也可以在函数列表中完成
• Step 3: 子类的pointer成员，必须先检查是否为nullptr，在依次进行赋值，即深拷贝。

Rectangle::Rectangle(const Rectangle& other):
    Shape(other),  // Step 1: father class member
    width_(other.width_),  // Step 2: non-pointer member
    height_(other.height_)
{ 
    if (other.leftUp_ != nullptr)  // Step 3: Check whether other is nullptr
    {
        this->leftUp_ = new Point(*(other.leftUp_));  // Point copy constructor; other.leftUp_ is a (Point* ); so *(other.leftUp_) is a Point.     
    }
    else
        this->leftUp_= nullptr;
}

• 【思考】此处最对*(other.leftUp_)不是很理解，后续请教他人后更深入的理解：*就是dereference
  • other.leftUp_ is a Point *
  • *(other.leftUp_) is a Point
  • new Point(?)的?是可以接收const Pointer&的内容
  • 相比于int等价于

  int number = 1;
  int *number_ptr  = &number;
  int dereferenced = *number_ptr;
  

1.3 Copy Assignment

• Step 1：先检查是否是self-assignment
• Step 2：父类程序按的处理。（这里经过查询书籍，可以知道，直接使用系统默认的copy assignment函数即可完成任务）
• Step 3：子类non-pointer成员直接赋值
• Step 4：检查pointer等号左右两侧是否为nullptr，并分别处理

Rectangle& Rectangle::operator=(const Rectangle& other)
{
    if (this == &other) // Step 1:  Self Assignment Check
    {
        return *this;
    }   
    Shape::operator=(other);    // Step 2: Default copy assignment
    this->width_ = other.width_;  // Step 3: General member variables copy
    this->height_ = other.height_; 
    if( other.leftUp_ != nullptr )   // Step 4: Check pointer member
    {
       if( this->leftUp_ != nullptr )
       {
          *(this->leftUp_) = *(other.leftUp_);
       }
       else
       {
          this->leftUp_ = new Point(*other.leftUp_);
       }
    }
    else
    {
       delete this->leftUp_;
       this->leftUp_ = nullptr;
    } 
    return *this;
}

1.4 Destructor

• General idea is to delete the pointer directly.

Rectangle::~Rectangle()
{
   delete leftUp;
}

• More safe solution

Rectangle::~Rectangle()
{
   delete leftUp;  // Just release the memory space, but not the value is still an address
   leftUp = nullptr;
}

1.5 inline function

• The body of an inline fucntion should be in the .h file, otherwise you will get

undefined reference to

• 参考
• http://stackoverflow.com/questions/4769479/c-inlining-class-methods-causes-undefined-reference
• https://isocpp.org/wiki/faq/inline-functions#inline-member-fns

总结

• 首先理清类之间的相互关系，一边后续设计三大函数时，明白各个类成员的来历。
• 严格按照既定的设计思路书写，不能随心所欲，以免漏写或画蛇添足。
• 处理指针问题要格外小心。

Reference

• https://github.com/GeekBand/GeekBand-CPP-1501-Homework/blob/master/G2015010269%2FMakefile
• http://www.cnblogs.com/gaobaoru-articles/p/5236282.html
• http://www.jianshu.com/p/629c743b0fd3