C++ LANGUAGE OPENING A FILE

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C++ LANGUAGE OPENING A FILE

Opening a File 


A file must be opened before you can read from it or 
write to it. Either ofstream or fstream object may 
be used to open a file for writing. And ifstream 
object is used to open a file for reading purpose only. 

Following is the standard syntax for open() function,
 which is a member of fstream, ifstream, and ofstream objects. 
void open(const char *filename, ios::openmode mode); 




Mode Flag  Description 


ios::app  Append mode. All output to that 
                file to be appended to the end. 


ios::ate  Open a file for output and move the read/write 
                control to the end of the file. 

ios::in  Open a file for reading. 

ios::out  Open a file for writing. 

ios::trunc  If the file already exists, its contents 
                will be truncated before opening the file. 

You can combine two or more of these values by ORing them together. 
For example if you want to open a file in write mode and want to 
truncate it in case that already exists, following will be the syntax: 

ofstream outfile; 

outfile.open("file.dat", ios::out | ios::trunc ); 

Similar way, you can open a file for reading and writing purpose as follows: 

fstream  afile; 

afile.open("file.dat", ios::out | ios::in );