从C到C++要注意的33件事（3）

翻译 于 2016-11-28 20:49:20 发布 · 474 阅读 · 1
文章标签：

#c++ #c语言 #class #namespace

本文介绍了C++中的类和对象概念，包括常量成员、继承、多态、文件操作及格式化输出等内容。

25 类的变量也可以是常量，就像静态变量一样

using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;
   const static double pi = 3.1415927;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double cylinder_volume ()
   {
      return x * x / 4 * pi * y;
   }
};

int main()
{
   cout << "The value of pi: " << vector::pi << endl << endl;

   vector k (3, 4);

   cout << "Result: " << k.cylinder_volume() << endl;

   return 0;
}
Output
The value of pi: 3.14159

Result: 28.2743

26 一个类可以从其他的类派生过来，这个新的类可以继承原有类的所有成员和方法，并且可以增加新的成员和方法


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module()
   {
      return sqrt (x*x + y*y);
   }

   double surface()
   {
       return x * y;
   }
};

class trivector: public vector    // trivector is derived from vector
{
public:
   double z;                      // added to x and y from vector

   trivector (double m=0, double n=0, double p=0): vector (m, n)
   {
      z = p;                      // Vector constructor will
   }                              // be called before trivector
                                  // constructor, with parameters
                                  // m and n.

   trivector (vector a)           // What to do if a vector is
   {                              // cast to a trivector
      x = a.x;
      y = a.y;
      z = 0;
   }

   double module ()               // define module() for trivector
   {
      return sqrt (x*x + y*y + z*z);
   }

   double volume ()
   {
       return this->surface() * z;         // or x * y * z
   }
};

int main ()
{
   vector a (4, 5);
   trivector b (1, 2, 3);

   cout << "a (4, 5)    b (1, 2, 3)    *r = b" << endl << endl;

   cout << "Surface of a: " << a.surface() << endl;
   cout << "Volume of b: " << b.volume() << endl;
   cout << "Surface of base of b: " << b.surface() << endl;

   cout << "Module of a: " << a.module() << endl;
   cout << "Module of b: " << b.module() << endl;
   cout << "Module of base of b: " << b.vector::module() << endl;

   trivector k;
   k = a;               // thanks to trivector(vector) definition
                        // copy of x and y,       k.z = 0
   vector j;
   j = b;               // copy of x and y.       b.z leaved out

   vector *r;
   r = &b;

   cout << "Surface of r: " << r->surface() << endl;
   cout << "Module of r: " << r->module() << endl;

   return 0;
}


a (4, 5)    b (1, 2, 3)    *r = b

Surface of a: 20
Volume of b: 6
Surface of base of b: 2
Module of a: 6.40312
Module of b: 3.74166
Module of base of b: 2.23607
Surface of r: 2

Module of r: 2.23607


27  在上面的程序里面，r->module 计算vector的模型，这样做为什么可以是因为r被声明为vector类的指针。但是我们要记住，如果你想程序检查这种对象的指针或者使用相关的方法，那么我们必须把类里面的方法声明为虚函数。

（如果继承的基类里面有至少一个方法被声明为虚函数的话，那么每一个实例都会被加上事实上的4个Byte的指针，当然，64位有可能是8个Byte）。

using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   virtual double module()
   {
      return sqrt (x*x + y*y);
   }
};

class trivector: public vector
{
public:
   double z;

   trivector (double m = 0, double n = 0, double p = 0)
   {
      x = m;                  // Just for the game,
      y = n;                  // here I do not call the vector
      z = p;                  // constructor and I make the
   }                          // trivector constructor do the
                              // whole job. Same result.

   double module ()
   {
      return sqrt (x*x + y*y + z*z);
   }
};

void test (vector &k)
{
    cout << "Test result:          " << k.module() << endl;
}

int main ()
{
   vector a (4, 5);
   trivector b (1, 2, 3);

   cout << "a (4, 5)    b (1, 2, 3)" << endl << endl;

   vector *r;

   r = &a;
   cout << "module of vector a: " << r->module() << endl;

   r = &b;
   cout << "module of trivector b: " << r->module() << endl;

   test (a);

   test (b);

   vector &s = b;

   cout << "module of trivector b: " << s.module() << endl;

   return 0;
}
Output
a (4, 5)    b (1, 2, 3)

module of vector a: 6.40312
module of trivector b: 3.74166
Test result:          6.40312
Test result:          3.74166
module of trivector b: 3.74166

28 你可能想知道一个类是否可以同时继承多个类， 那么答案是对的

using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double surface()
   {
      return fabs (x * y);
   }
};

class number
{
public:

   double z;

   number (double a)
   {
      z = a;
   }

   int is_negative ()
   {
      if (z < 0) return 1;
      else       return 0;
   }
};

class trivector: public vector, public number
{
public:

   trivector(double a=0, double b=0, double c=0): vector(a,b), number(c)
   {
   }           // The trivector constructor calls the vector
               // constructor, then the number constructor,
               // and in this example does nothing more.

   double volume()
   {
      return fabs (x * y * z);
   }
};

int main ()
{
   trivector a(2, 3, -4);

   cout << a.volume() << endl;
   cout << a.surface() << endl;
   cout << a.is_negative() << endl;

   return 0;
}
Output
24
6
1

29 类的继承可以让你依据基类设计更负责的类，而另外一个用处则是可以让程序员写出一些通用的函数。

当我们写了一个没有任何变量的基类的时候，在你的程序里面是没有办法使用这个类的。也就是说当你想继承这个类的时候，就一定要让成员函数正确的在类里被定义，这样才能合理的继承基类

using namespace std;
#include <iostream>
#include <cmath>

class octopus
{
public:

   virtual double module() = 0;  // = 0 implies function is not
                                 // defined. This makes instances
                                 // of this class cannot be declared.
};

double biggest_module (octopus &a, octopus &b, octopus &c)
{
    double r = a.module();
    if (b.module() > r) r = b.module();
    if (c.module() > r) r = c.module();
    return r;
}

class vector: public octopus
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module()
   {
      return sqrt (x * x + y * y);
   }
};

class number: public octopus
{
public:

   double n;

   number (double a = 0)
   {
      n = a;
   }

   double module()
   {
      if (n >= 0) return n;
      else        return -n;
   }
};

int main ()
{
    vector k (1,2), m (6,7), n (100, 0);
    number p (5),   q (-3),  r (-150);

    cout << biggest_module (k, m, n) << endl;
    cout << biggest_module (p, q, r) << endl;

    cout << biggest_module (p, q, n) << endl;

   return 0;
}
Output
100
150
100

30 public：这些变量或者函数能够被外部程序方位

   protect:仅能被类本身或者继承类访问

   private:只能被自己类的方法所访问。

   

using namespace std;
#include <iostream>

class vector
{
protected:

   double x;
   double y;

public:

   void set_x (int n)
   {
      x = n;
   }
   void set_y (int n)
   {
      y = n;
   }

   double surface ()
   {
      double s;
      s = x * y;
      if (s < 0) s = -s;
      return s;
   }
};

int main ()
{
   vector a;

   a.set_x (3);
   a.set_y (4);

   cout << "The surface of a: " << a.surface() << endl;

   return 0;
}


Output
The
 surface of a: 12






31 让我们看看C++是怎么读写文件的


这是写一个文件的 程序


using namespace std;
#include <iostream>
#include <fstream>

int main ()
{
   fstream f;

   f.open("test.txt", ios::out);

   f << "This is a text output to a file." << endl;

   double a = 345;

   f  << "A number: " << a << endl;

   f.close();

   return 0;
}






Content of
 file test.txt
This is a text output to a file. A number: 345




下面是怎么读文件


using namespace std;
#include <iostream>
#include <fstream>

int main ()
{
   fstream f;
   char c;

   cout << "What's inside the test.txt file" << endl;
   cout << endl;

   f.open("test.txt", ios::in);

   while (! f.eof() )
   {
      f.get(c);                          // Or c = f.get()
      cout << c;
   }

   f.close();

   return 0;





32 有时候我们可以像文件一样操作字符数组，这对我们在转换和管理内存数组的时候很有用


using namespace std;
#include <iostream>
#include <strstream>
#include <cstring>
#include <cmath>

int main ()
{
   char a[1024];
   ostrstream b(a, 1024);

   b.seekp(0);                           // Start from first char.
   b << "2 + 2 = " << 2 + 2 << ends;     // ( ends, not endl )
                                         // ends is simply the
                                         // null character   '\0'
   cout << a << endl;

   double v = 2;

   strcpy (a, "A sinus: ");

   b.seekp(strlen (a));
   b << "sin (" << v << ") = " << sin(v) << ends;

   cout << a << endl;

   return 0;
}

Output
2
 + 2 = 4
A sinus: sin (2) = 0.909297





33 C++可以格式化输出用两个不同的方式，要注意到，width（）和setws()只对下一个要输出的对象起作用。


using namespace std;
#include <iostream>
#include <iomanip>

int main ()
{
   int i;

   cout << "A list of numbers:" << endl;
   for (i = 1; i <= 1024; i *= 2)
   {
      cout.width (7);
      cout << i << endl;
   }

   cout << "A table of numbers:" << endl;
   for (i = 0; i <= 4; i++)
   {
      cout << setw(3) << i << setw(5) << i * i * i << endl;
   }

   return 0;
}

Output
A
 list of numbers:       1       2       4       8      16      32      64     128     256     512    1024 A table of numbers:   0    0   1    1   2    8   3   27   4   64


以上是一些基本的C++的知识，主要是针对一些C用户转换到C++需要注意的一些点，没什么复杂的。其实翻译这个系列文章的目的也是自己现在正处于这个挑战中，也希望通过这个积累和构造自己的知识体系吧。同时也能和各位同学共勉。
如果同学们在看文章的过程中发现错误和有任何问题，意见，欢迎发表评论。我个人很愿意和大家一起讨论，进步。





                

        

    

  



    



            

  



          




        



    





    



      

      

        
      

      





	

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