详解C++ 共享数据保护机制

例：

class A

{

 public:

 A(int i,int j) {x=i; y=j;}

      ...

 private:

 int x,y;

};

A const a(3,4); //a是常对象，不能被更新

//常成员函数举例

#include<iostream>

using namespace std;

class R {

public:

 R(int r1, int r2) : r1(r1), r2(r2) { }

 void print();

 void print() const;

private:

 int r1, r2;

};



void R::print() {

 cout << r1 << ":" << r2 << endl;

}

void R::print() const {

 cout << r1 << ";" << r2 << endl;

}

int main() {

 R a(5,4);

 a.print(); //调用void print()

 const R b(20,52);

 b.print(); //调用void print() const

  return 0;

}

//常数据成员举例

#include <iostream>

using namespace std;

class A {

public:

  A(int i);

  void print();

private:

  const int a;

  static const int b; //静态常数据成员

};



const int A::b=10;

A::A(int i) : a(i) { }

void A::print() {

 cout << a << ":" << b <<endl;

}

int main() {

//建立对象a和b，并以100和0作为初值，分别调用构造函数，

//通过构造函数的初始化列表给对象的常数据成员赋初值

 A a1(100), a2(0);

 a1.print();

 a2.print();

 return 0;

}

//常引用作形参

#include <iostream>

#include <cmath>

using namespace std;

class Point { //Point类定义

public:   //外部接口

  Point(int x = 0, int y = 0)

 : x(x), y(y) { }

  int getX() { return x; }

  int getY() { return y; }

  friend float dist(const Point &p1,const Point &p2);

private:   //私有数据成员

  int x, y;

};



float dist(const Point &p1, const Point &p2) {

  double x = p1.x - p2.x;

  double y = p1.y - p2.y;

  return static_cast<float>(sqrt(x*x+y*y));

}



int main() { //主函数

  const Point myp1(1, 1), myp2(4, 5);

  cout << "The distance is: ";

  cout << dist(myp1, myp2) << endl;

  return 0;

}