factory工廠模式是所有模式中最有用的模式之一。它強制用一個factory工廠來創建對象，程序中所有需創建對象的代碼都在這個factory中執行，它是Factory Method模式的變體。示例代碼：
#include <iostream>
#include <stdexcept>
#include <cstddef>
#include <string>
#include <vector>
#include "../pruge.h"
using namespace std;

class Shape{
   virtual void draw()=0;
   virtual void erase()=0;
   virtual ~Shape();
   class BadShapeCreation:public logic_error{
public:
   BadShapeCreation(string type):logic_error("Cann't create type"+typed){}
};
   static Shape* factory(const string& type)
   throw(BadShapeCreation);
};

class Circle:public Shape{
   Circle(){};//private constructor
   friend class Shape;
public:
   void draw(){cout<<"Circle::draw"<<endl;}
   void erase(){cout<<"Circle::erase"<<endl;}
   ~Circle(){cout<<"Circle::~Circle" <<endl;}
};

class Square:public Shape{
   Square(){};//private constructor
   friend class Shape;
public:
   void draw(){cout<<"Square::draw"<<endl;}
   void erase(){cout<<"Square::erase"<<endl;}
   ~Square(){cout<<"Square::~Circle" <<endl;}
};

Shape* Shape::factory(const string& type)
throw(Shape::BadShapeCreation){
if(type== "Circle") return new Circle;
if(type=="Square") return new Square;
throw BadShapeCreation(type);
}

char* sl[] = {"Circle","Square",Square","Circle","Circle","Circle",Square"};

int main(){
vector<Shape*> shape;
try{
   for(size_t i = 0; i<sizeof(sl)/sizeof(sl[0]); ++i){
         shape.push_back(shape::factory(sl[i]));
   }catch(Shape::BadShpaeCreation e){
   cout << e.what()<<endl;
   purge(shapes);
   return EXIT_FAILURE;
   }
for(size_t i = 0; i < shapes.size(); i++){
   shapes[i]->draw();
   shapes[i]->erase();
   }
pruge(shapes);
}
備注：factory()允許一個參數來決定何種類型的Shape對象，這里參數也可以是string。添加新的Shape類型時，只要修改factory（）就可以。
為確保只能創建在factory（）中，Shape把構造聲明為私有，Shape有聲明為友元（也可以把Shape::factory()聲明為友元函數），這樣factory（）可以訪問它的構造函數。這樣做的缺點：一旦新類型被加入到這種結構來時，必須更新基類。