面向对象编程中最重要的概念之一是继承，继承允许使用一个类继承另一个类，这样就可以直接调用父类的公共函数或变量，这使得维护变得更加容易。

基类和子类

子类通过":"冒号来实现继承基类。

class derived-class: base-class

考虑如下基类 Shape 及其派生类 Rectangle -

import std.stdio;

//Base class 
class Shape { 
   public: 
      void setWidth(int w) { 
         width=w; 
      }

      void setHeight(int h) { 
         height=h; 
      }
   
   protected: 
      int width; 
      int height; 
}
  
//Derived class 
class Rectangle: Shape { 
   public: 
      int getArea() { 
         return (width * height); 
      } 
}
  
void main() { 
   Rectangle Rect=new Rectangle();
   
   Rect.setWidth(5); 
   Rect.setHeight(7); 
   
   //Print the area of the object. 
   writeln("Total area: ", Rect.getArea()); 
} 

编译并执行上述代码后，将产生以下输出-

Total area: 35

多级继承

继承可以具有多个级别，并在以下示例中显示。

import std.stdio;

//Base class 
class Shape {
   public:
      void setWidth(int w) {
         width=w; 
      }

      void setHeight(int h) {
         height=h; 
      }

   protected: 
      int width; 
      int height; 
}

//Derived class 
class Rectangle: Shape {
   public:
      int getArea() {
         return (width * height); 
      }
}
 
class Square: Rectangle {
   this(int side) {
      this.setWidth(side); 
      this.setHeight(side); 
   }
}

void main() {
   Square square=new Square(13);

   //Print the area of the object.
   writeln("Total area: ", square.getArea());
}

编译并执行上述代码后，将产生以下输出-

Total area: 169