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  C++ 类构造函数和析构函数

  • 类构造函数

    类构造函数是类的特殊成员函数，只要我们创建该类的新对象，该构造函数便会自动执行。构造函数将具有与类完全相同的名称，并且根本没有任何返回类型，甚至没有void。构造函数对于为某些成员变量设置初始值非常有用。以下示例解释了构造函数的概念- 
    
#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;
}
    尝试一下
    编译并执行上述代码后，将产生以下结果- 
    
Object is being created
Length of line : 6

  • 构造函数参数

    默认构造函数没有任何参数，但是如果需要，构造函数可以具有参数。这有助于您在创建对象时为对象分配初始值，如以下示例所示- 
    
#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;
}
    尝试一下
    编译并执行上述代码后，将产生以下结果- 
    
Object is being created, length = 10
Length of line : 10
Length of line : 6

  • 使用初始化列表初始化字段

    如果使用参数化构造函数，则可以使用以下语法初始化字段- 
    
Line::Line( double len): length(len) {
   cout << "Object is being created, length = " << len << endl;
}
    上面的语法等于下面的语法- 
    
Line::Line( double len) {
   cout << "Object is being created, length = " << len << endl;
   length = len;
}
    尝试一下
    如果对于C类，您有多个字段X，Y，Z等要初始化，则可以使用相同的语法并以逗号分隔字段，如下所示- 
    
C::C( double a, double b, double c): X(a), Y(b), Z(c) {
   ....
}

  • 类析构函数

    析构函数是类的特殊成员函数，每当其类的对象超出作用域或将delete表达式应用于该类的对象的指针时，都执行该析构函数。析构函数的名称与以波浪号（~）为前缀的类的名称完全相同，并且既不能返回值，也不能采用任何参数。析构函数对于在程序退出之前释放资源非常有用，例如关闭文件，释放内存等。以下示例解释了析构函数的概念- 
    
#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;
}
    尝试一下
    上面的代码编译并执行后，返回以下结果- 
    
Object is being created
Length of line : 6
Object is being deleted
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