00001 #include <string> 00002 #include <iostream> 00003 00004 class Image //abstract 00005 { 00006 protected: 00007 Image() {} 00008 00009 public: 00010 virtual ~Image() {} 00011 00012 virtual void displayImage() = 0; 00013 }; 00014 00015 class RealImage : public Image 00016 { 00017 public: 00018 explicit RealImage(std::string filename) 00019 { 00020 this->filename = filename; 00021 loadImageFromDisk(); 00022 } 00023 00024 00025 void displayImage() 00026 { 00027 std::cout<<"Displaying "<<filename<<std::endl; 00028 } 00029 00030 private: 00031 void loadImageFromDisk() 00032 { 00033 // Potentially expensive operation 00034 // ... 00035 std::cout<<"Loading "<<filename<<std::endl; 00036 } 00037 00038 std::string filename; 00039 }; 00040 00041 class ProxyImage : public Image 00042 { 00043 public: 00044 explicit ProxyImage(std::string filename) 00045 { 00046 this->filename = filename; 00047 this->image = NULL; 00048 } 00049 ~ProxyImage() { delete image; } 00050 00051 void displayImage() 00052 { 00053 if (image == NULL) { 00054 image = new RealImage(filename); // load only on demand 00055 } 00056 00057 image->displayImage(); 00058 } 00059 00060 private: 00061 std::string filename; 00062 Image* image; 00063 }; 00064 00065 int main(int argc, char* argv[]) 00066 { 00067 std::cout<<"main"<<std::endl; 00068 00069 Image* image1 = new ProxyImage("HiRes_10MB_Photo1"); 00070 Image* image2 = new ProxyImage("HiRes_10MB_Photo2"); 00071 Image* image3 = new ProxyImage("HiRes_10MB_Photo3"); 00072 00073 image1->displayImage(); // loading necessary 00074 image2->displayImage(); // loading necessary 00075 image1->displayImage(); // no loading necessary; already done 00076 // The third image "HiRes_10MB_Photo3" never loaded - time saved! 00077 00078 delete image1; 00079 delete image2; 00080 delete image3; 00081 00082 return 0; 00083 } 00084 00085 /*Output 00086 main 00087 Loading HiRes_10MB_Photo1 00088 Displaying HiRes_10MB_Photo1 00089 Loading HiRes_10MB_Photo2 00090 Displaying HiRes_10MB_Photo2 00091 Displaying HiRes_10MB_Photo1 00092 */