Fstream

fstream is a standard C++ library that handles reading from and writing to files either in text or in binary formats. It is an object oriented alternative to C's FILE from the C standard library.

ofstream

fstream can both input and output to files in several modes. The following example creates a file called 'file.txt' and puts the text 'Hello World' followed by a newline into it.

#include <fstream>

int main()
{
    std::ofstream file;// can be merged to std::ofstream file("file.txt");
    file.open("file.txt");
    file << "Hello world!\n";
    file.close();// is not necessary because the destructor closes the open file by default
    return 0;
}

ifstream

ifstream is the C++ standard library class that provides an interface to read data from files as input streams. The input stream may open a file in the constructor, just like an output stream. ifstream inf("input.dat", ifstream::in); //ifstream::in is a default, so it could be omitted

or afterwards:

ifstream inf; inf.open("input.dat");

To close a stream, one uses the close method: inf.close();

The ifstream destructor will close the file cleanly as well. It is perfectly acceptable to allow the ifstream object to fall out of scope without calling close. This is often a desirable style, as it simplifies the "clean up" process when an exception is thrown or an error is otherwise encountered.

This would be a basic cat utility:

#include <iostream>
#include <fstream>
#include <string>

using std::cin;
using std::cout;
using std::cerr;

void print(std::istream& in)
{
		try{
			std::string tmp;
			while(1){
				std::getline(in,tmp);
				tmp+='\n';
				cout.write(tmp.c_str(),tmp.length());
			}
		}
		catch(std::ifstream::failure e){
			if(!in.eof())
				cerr << e.what() <<'\n';
		}

}

int main(int argc,char** argv)
{

	std::ifstream in;
	in.exceptions(  std::ifstream::eofbit | std::ifstream::failbit | std::ifstream::badbit );
	cin.exceptions( std::ifstream::eofbit | std::ifstream::failbit | std::ifstream::badbit );

	if(argc==1){
		print(cin);
	}
	try{
		for(int i=1;i<argc;i++){

			if(argv[i]==std::string("-")){
				print(cin);
			}
			else{
				in.open(argv[i]);
				print(in);
			}
		}
	}
	catch(std::ifstream::failure e){
		cerr << e.what();
	}

}

External references

• fstream reference