Open a file

[FireCoder]

How do you open a file with c++?

[cnoeval]

I generally take 3 or so seconds to look it up.

http://www.cplusplus.com/doc/tutorial/files/

Seems silly to wait around for a forum to answer such a super basic question.

[megatron 0]

There are a couple of ways one could open a file:

- Open for read
- Open for write
- Open for read and write

In order to open a file, you must include the fstream header, like this:

#include <fstream>

This allows you to use the fstream object, like this:

std::fstream open_for_read("file.txt", std::ios::in);
std::fstream open_for_write("file.txt", std::ios::out);
std::fstream open_for_read_and_write("file.txt", std::ios::in | std::ios::out); // Use the bitwsie OR operator or "pipe" to allow for different modes 

It is more common to use the basic fstream objects:

std::ifstream open_for_read("file.txt");
std::ofstream open_for_write("file.txt");

Immediately after creating the object, it is good to check if it actually opened:

if(fstream_object)
{
    // File exists and is ready for processing
}

Then when the file is open, you may extract or insert data using the bit shift operator, "<<" for output and ">>" for input, just like cout and cin:

std::string line = "Hello, World!";
fstream_object_out << line;
int my_var;
fstream_object_in >> my_var;

This will also work with user defined objects, providing you define how it should work:

class Account
{
private:
    std::string name;
    int balance;
public:
    // This also works for cout and cin too!
    inline friend std::ostream& operator<<(std::ostream& os, Account& a)
    {
         os << a.name << " " << a.balance;
         return os;
    }
    inline friend std::istream& operator>>(std::istream& is, Account& a)
    {
        is >> a.name >> a.balance;
        return is;
    }
}

The last issue with oening files is of course, closing them. Most of the time this isn't a problem when you isolate your file manipulation because the destructor of the fstream objects closes them on destruction ( Going "out of scope" ) But, to be on the safe side:

fstream_object.close();

And well.. that's all there is to it!

Hope this helped and good luck.

EDIT:

I copy and pasted so I can reuse for similar questions.