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。