从C到C++要注意的33件事（2）

20 如果你不想用inline来声明一个方法，或者你指向在类的定义里面包含最少的内容（或者你想用.hpp和.cpp来隔离源代码和声明），那么你只需要在类定义里面声明一下方法，然后在该类下面实现它就可以了。

using namespace std;#include <iostream>class vector{public:   double x;   double y;   double surface();         // The ; and no {} show it is a prototype};double vector::surface(){   double s = 0;   for (double i = 0; i < x; i++)   {      s = s + y;   }   return s;}int main (){   vector k;   k.x = 4;   k.y = 5;   cout << "Surface: " << k.surface() << endl;   return 0;}

OutputSurface: 20

对于一个刚开始接触C++的人来说，如果你想用头文件和源文件来隔离代码，那么你可以参考下面的例子：

A header file vector.h:

class vector{public:   double x;   double y;   double surface();};

A source file vector.cpp:

using namespace std;#include "vector.h"

double vector::surface()
{
   double s = 0;

   for (double i = 0; i < x; i++)
   {
      s = s + y;
   }

   return s;
}

And another source file main.cpp:

using namespace std;#include <iostream>#include "vector.h"
int main (){   vector k;   k.x = 4;   k.y = 5;   cout << "Surface: " << k.surface() << endl;   return 0;}

你可以用下面的命令来编译vector.cpp，可以生成对应的.o文件

g++ -c vector.cpp

每次如果你修改main.cpp文件，你可以把它编译成一个可执行文件，我们可以起名为test：

g++ main.cpp vector.o -o test

然后我们可以执行这个可执行文件

./test

这样做有以下好处：

1.vector.cpp只需要编译一次，在大型工程中这样节约很多时间。

2.你可以把vector.h文件和.o文件给其他用户，这样他们可以使用你的.o，但是不能修改你的代码

21 当一个方法被一个实例应用的时候，这个方法可以使用这个实例的变量，并且修改，或者运算。但是有些时候，我们还是需要知道实例的地址，那么this这个关键字就派上用场了

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);   }   void set_length (double a = 1)   {      double length;      length = this->module();      x = x / length * a;      y = y / length * a;   }};int main (){   vector c (3, 5);   cout << "The module of vector c: " << c.module() << endl;   c.set_length(2);            // Transforms c in a vector of size 2.   cout << "The module of vector c: " << c.module() << endl;   c.set_length();             // Transforms b in an unitary vector.   cout << "The module of vector c: " << c.module() << endl;   return 0;}

The module of vector c: 5.83095
The module of vector c: 2
The module of vector c: 1

22.在C++ 中我们也可以声明一个对象的数组

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);   }};int main (){   vector s [1000];   vector t[3] = {vector(4, 5), vector(5, 5), vector(2, 4)};   s[23] = t[2];   cout << t[0].module() << endl;   return 0;}

Output6.40312

23.下面是一个完整的类的实例

using namespace std;#include <iostream>#include <cmath>class vector{public:   double x;   double y;   vector (double = 0, double = 0);   vector operator + (vector);   vector operator - (vector);   vector operator - ();   vector operator * (double a);   double module();   void set_length (double = 1);};vector::vector (double a, double b){   x = a;   y = b;}vector vector::operator + (vector a){   return vector (x + a.x, y + a.y);}vector vector::operator - (vector a){   return vector (x - a.x, y - a.y);}vector vector::operator - (){   return vector (-x, -y);}vector vector::operator * (double a){   return vector (x * a, y * a);}double vector::module(){   return sqrt (x * x + y * y);}void vector::set_length (double a){   double length = this->module();   x = x / length * a;   y = y / length * a;}ostream& operator << (ostream& o, vector a){   o << "(" << a.x << ", " << a.y << ")";   return o;}int main (){   vector a;   vector b;   vector c (3, 5);   a = c * 3;   a = b + c;   c = b - c + a + (b - a) * 7;   c = -c;   cout << "The module of vector c: " << c.module() << endl;   cout << "The content of vector a:  " <<  a << endl;   cout << "The opposite of vector a: " << -a << endl;   c.set_length(2);            // Transforms c in a vector of size 2.   a = vector (56, -3);   b = vector (7, c.y);   b.set_length();             // Transforms b in an unitary vector.   cout << "The content of vector b: " << b << endl;   double k;   k = vector(1, 1).module();  // k will contain 1.4142.   cout << "k contains: " << k << endl;   return 0;}

The module of vector c: 40.8167
The content of vector a: (3, 5)
The opposite of vector a: (-3, -5)
The content of vector b: (0.971275, 0.23796)
k contains: 1.41421

我们也可以定义一个函数来让两个vector类相加，不论这个函数是否在类里面。要注意，这不是一个方法，就是一个函数。

vector operator + (vector a, vector b){   return vector (a.x + b.x, a.y + b.y);}

24，如果变量在类里面被定义为static,那么它们会被所有的类的实例共享。

using namespace std;#include <iostream>class vector{public:   double x;   double y;   static int count;   vector (double a = 0, double b = 0)   {      x = a;      y = b;      count++;   }   ~vector()   {      count--;   }};int vector::count = 0;int main (){   cout << "Number of vectors:" << endl;   vector a;   cout << vector::count << endl;   vector b;   cout << vector::count  << endl;   vector *r, *u;   r = new vector;   cout << vector::count << endl;   u = new vector;   cout << a.count << endl;   delete r;   cout << vector::count << endl;   delete u;   cout << b.count << endl;   return 0;}

Output1
2
3
4
3
2