CODEWITHSUNDEEP
c++
Nate
Nate

Reputation: 241

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

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;

}

Upvotes: 24

Views: 139279

Answers (3)

Nekuromento
Nekuromento

Reputation: 2235

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

Upvotes: 5

Code_So1dier
Code_So1dier

Reputation: 942

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();

Upvotes: 48

user258808
user258808

Reputation: 734

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

Upvotes: 2

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