File Handling

1. File Handling

2. Read data from file and display it on screen #include<iostream.h> #include<fstream.h> int main() { /* fin object is created with parameterzied constructor */ ifstreamfin("in.txt"); //file will be searched in current directory char ch; while(fin) { fin>>ch; cout<<ch; } return 0; }

3. Read data from file and display it on screen #include<iostream.h> #include<fstream.h> int main() { ifstreamfin(“c:\\in.txt"); //give full path of file char ch; while(fin.eof() == 0) { fin>>ch; cout<<ch; } return 0; }

4. Read data from file and display it on screen #include<iostream.h> #include<fstream.h> int main() { /* fin is created with default constructor so use open() to open file */ ifstreamfin; fin.open(“in.txt”); char ch; while(fin.eof() == 0) { fin>>ch; cout<<ch; } return 0; }

5. Read data from one file and save it on two different file depending on condition while(fin) { fin>>ch; if(ch >=65 && ch <=124) fout1<<ch; else if(ch >=48 && ch <=57) fout2<<ch; cout<<ch; }  fin.close(); fout1.close(); fout2.close(); return 0; } #include<iostream.h> #include<fstream.h> int main() { ofstream fout1,fout2; fout1.open("alpha.txt"); fout2.open("digits.txt"); ifstream fin; fin.open("in.txt"); char ch;

6. Take data from user store it in file again read data from file and display it on screen. f.open("item.txt",ios::in); f>>name; f>>cost; cout<<"\n Item name :"; cout<<name; cout<<"\n Cost :"; cout<<cost; f.close(); return 0; } #include<iostream.h> #include<fstream.h> int main() { fstream f; f.open("item.txt",ios::out); char name[20]; float cost; cout<<"\n Enter item name"; cin>>name; f<<name<<"\n"; cout<<"\n Enter cost"; cin>>cost; f<<cost<<"\n"; f.close();

7. Use of getline function #include<iostream.h> #include<fstream.h> int main() { ifstream fin("item.txt"); char line[80]; while(fin) { fin.getline(line,80); cout<<endl<<line; } return 0; }

8. Functions for manipulation of file pointers • seekg() Moves get pointer to a specified location • seekp() Moves put pointer to a specified location. • tellg() Gives the current position of the get pointer • tellp() Gives the current position of the put pointer • E.g. fin.seekg(10); int p = fout.tellp();

9. Example of seekg() /*moves get pointer to 1st location*/ f.seekg(0); f>>name; f>>cost; cout<<"\n Item name :"; cout<<name; cout<<"\n Cost :"; cout<<cost; return 0; } int main() { fstream f; f.open("item.txt",ios::in | ios::out); char name[20]; float cost; cout<<"\n Enter item name"; cin>>name; f<<name<<"\n"; cout<<"\n Enter cost"; cin>>cost; f<<cost<<"\n";

10. Example of seekg() /*moves get pointer to 3rd location*/ f.seekg(3); f>>name; f>>cost; cout<<"\n Item name :"; cout<<name; cout<<"\n Cost :"; cout<<cost; return 0; } int main() { fstream f; f.open("item.txt",ios::in | ios::out); char name[20]; float cost; cout<<"\n Enter item name"; cin>>name; f<<name<<"\n"; cout<<"\n Enter cost"; cin>>cost; f<<cost<<"\n";

11. Functions with offset • seekg(offset,refposition) • Seekp(offset,refposition) • Offset represents number of bytes the file pointer is to be moved from the location specified by the parameter refposiotion. • Refposition takes one of the form specified in ios class (constant) • ios::beg beginning of file • ios::curcureent position of the pointer • ios::end end of the file

12. Pointer offset calls

13. Reading and Writing Class object • read() is used to read object data from file • Syntax read( (char *) &obj_name , sizeof(obj_name)) • write() is used to write object data to the file • Syntax write((char *) &obj_name, sizeof(obj_name))

14. Example int main() { item ob[3]; fstream f; f.open("stock.dat",ios::in | ios::out); cout<<"\n enter details for items"; for(int i=0;i<3;i++) { ob[i].readdata(); f.write( (char *) &ob[i],sizeof(ob[i])); } cout<<"\n Output : "; for(i=0;i<3;i++) { f.read((char*) &ob[i] , sizeof(ob[i])); ob[i].writedata(); } return 0; } #include<iostream.h> #include<fstream.h> #include<iomanip.h> class item { char name[20]; float cost; public: void readdata(); void writedata(); }; void item::readdata() { cout<<"\n Enter item name: "; cin>>name; cout<<"\n Enter item Cose: "; cin>>cost; } void item::writedata() { cout<<setiosflags(ios::left)<<setw(10)<<name; cout<<endl<<setprecision(2)<<cost; }

15. Error handling during file operations • eof() Returns true if end of file encountered • fail() Returns true when input or output operation has failed • bad() Returns true invalid option is attempted(used for fatal errors) • good() Returns true if no error has occurred .

16. Command line Argument #include<iostream.h> #include<fstream.h> int main(int argc,char *argv[]) { if(argc < 2) cout<<“\n Insufficient argunents “; else { ifstream fin(argv[1]); char ch; while(fin) { fin>>ch; cout<<ch; } return 0; }

17. Advanced features in c++ • Mutable keyword • Namespace • RTTI (runtime type identification) 1. using typeid(ptr name).name() 2. dynamic_cast e.g mp = dynamic_cast<myclass*>(b); • Smart Pointers

18. Typecasting in c++ 3.Conversion from void pointer void *vptr; float *fptr; fptr = static_cast<float*> vptr; 4. Upcasting safe Base *bptr; Bptr = static_cast<base *>(&d); 5. Conversion Use traditional casting for converting from one class to another. Sample *s; s= static_cast<sample *>(bptr) //error s= (sample *) bptr; • Static_cast 1.Castless conversion int i=10; float f; f=i; Explicit conversion f = static_cast<float> i; 2. Narrowing conversion i= static_cast<int> f;

19. Cont…. • Const_cast const int a =10; int *p ; p= const_cast<int *> (&a); If we want to change data member in const function e.g. (const_cast<sample *>(this))->data = 30; • reinterprete_cast Used to change from pointer to integer and vice versa int *iptr = reinterprete_cast<int *>a;