类的构造函数：

类的构造函数是，每当我们创建该类的新对象时执行一类特殊的成员函数。

构造函数都会有完全相同的名字作为类，它没有任何返回类型可言，甚至没有作废。构造可以为某些成员变量设置的初始值非常有用。

下面的示例说明构造函数的概念：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line();  // This is the constructor
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line(void)
{
    cout << "Object is being created" << endl;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line;
 
   // set line length
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

当上述代码被编译和执行时，它产生了以下结果：

Object is being created
Length of line : 6

带参数的构造函数：

默认的构造函数没有任何参数，但如果需要，构造函数可以有参数。这有助于在其创建时的初始值赋给一个对象作为显示，如下面的例子：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line(double len);  // This is the constructor
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line( double len)
{
    cout << "Object is being created, length = " << len << endl;
    length = len;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line(10.0);
 
   // get initially set length.
   cout << "Length of line : " << line.getLength() <<endl;
   // set line length again
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

当上述代码被编译和执行时，它产生了以下结果：

Object is being created, length = 10
Length of line : 10
Length of line : 6

使用初始化列表来初始化字段：

使用初始化列表来初始化字段：

Line::Line( double len): length(len)
{
    cout << "Object is being created, length = " << len << endl;
}

上面的语法等于如下语法：

Line::Line( double len)
{
    cout << "Object is being created, length = " << len << endl;
    length = len;
}

如果对于一个C类，则具有多个字段的X，Y，Z等，进行初始化，然后用可以使用相同的语法和由逗号分隔字段如下：

C::C( double a, double b, double c): X(a), Y(b), Z(c)
{
  ....
}

类析构函数：

析构函数是执行一个类的特殊成员函数时，它的类的对象超出范围或当删除表达式应用到一个指向类的对象。

析构函数将有完全相同的名称作为前缀类带有波浪号（〜），它可以没有返回值，也不能采取任何参数。析构函数可以释放资源，跳出程序很像在关闭文件之前，释放内存等非常有用

以下举例说明析构函数的概念：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line();   // This is the constructor declaration
      ~Line();  // This is the destructor: declaration
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line(void)
{
    cout << "Object is being created" << endl;
}
Line::~Line(void)
{
    cout << "Object is being deleted" << endl;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line;
 
   // set line length
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

让我们编译和运行上面的程序，这将产生以下结果：

Object is being created
Length of line : 6
Object is being deleted

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