How to read and write to a text file in C++? - c++

Question

Hey everyone, I have just started to learn C++ and I wanted to know how to read and write to a text file. I have seen many examples but they have all been hard to understand/follow and they have all varied. I was hoping that someone here could help. I am a total beginner so I need clear instructions. Here is an example of what i'm trying to do:
#include <iostream>
#include <fstream>
using namespace std;
string usreq, usr, yn, usrenter;
int start ()
{
cout << "Welcome..."
int main ()
{
cout << "Is this your first time using TEST" << endl;
cin >> yn;
if (yn == "y")
{
ofstream iusrfile;
ofstream ousrfile;
iusrfile.open("usrfile.txt", "w");
iusrfile >> usr;
cout << iusrfile;
iusrfile.close();
cout << "Please type your Username. \n";
cin >> usrenter;
if (usrenter == usr)
{
start ();
}
}
else
{
cout << "THAT IS NOT A REGISTERED USERNAME.";
}
return 0;
}

Answer 1

Header files needed:
#include <iostream>
#include <fstream>
declare input file stream:
ifstream in("in.txt");
declare output file stream:
ofstream out("out.txt");
if you want to use variable for a file name, instead of hardcoding it, use this:
string file_name = "my_file.txt";
ifstream in2(file_name.c_str());
reading from file into variables (assume file has 2 int variables in):
int num1,num2;
in >> num1 >> num2;
or, reading a line a time from file:
string line;
while(getline(in,line)){
//do something with the line
}
write variables back to the file:
out << num1 << num2;
close the files:
in.close();
out.close();

Answer 2

Default c++ mechanism for file IO is called streams.
Streams can be of three flavors: input, output and inputoutput.
Input streams act like sources of data. To read data from an input stream you use >> operator:
istream >> my_variable; //This code will read a value from stream into your variable.
Operator >> acts different for different types. If in the example above my_variable was an int, then a number will be read from the strem, if my_variable was a string, then a word would be read, etc.
You can read more then one value from the stream by writing istream >> a >> b >> c; where a, b and c would be your variables.
Output streams act like sink to which you can write your data. To write your data to a stream, use << operator.
ostream << my_variable; //This code will write a value from your variable into stream.
As with input streams, you can write several values to the stream by writing something like this:
ostream << a << b << c;
Obviously inputoutput streams can act as both.
In your code sample you use cout and cin stream objects.
cout stands for console-output and cin for console-input. Those are predefined streams for interacting with default console.
To interact with files, you need to use ifstream and ofstream types.
Similar to cin and cout, ifstream stands for input-file-stream and ofstream stands for output-file-stream.
Your code might look like this:
#include <iostream>
#include <fstream>
using namespace std;
int start()
{
cout << "Welcome...";
// do fancy stuff
return 0;
}
int main ()
{
string usreq, usr, yn, usrenter;
cout << "Is this your first time using TEST" << endl;
cin >> yn;
if (yn == "y")
{
ifstream iusrfile;
ofstream ousrfile;
iusrfile.open("usrfile.txt");
iusrfile >> usr;
cout << iusrfile; // I'm not sure what are you trying to do here, perhaps print iusrfile contents?
iusrfile.close();
cout << "Please type your Username. \n";
cin >> usrenter;
if (usrenter == usr)
{
start ();
}
}
else
{
cout << "THAT IS NOT A REGISTERED USERNAME.";
}
return 0;
}
For further reading you might want to look at c++ I/O reference

Answer 3

To read you should create an instance of ifsteam and not ofstream.
ifstream iusrfile;
You should open the file in read mode.
iusrfile.open("usrfile.txt", ifstream::in);
Also this statement is not correct.
cout<<iusrfile;
If you are trying to print the data you read from the file you should do:
cout<<usr;
You can read more about ifstream and its API here