如何确保文件存在并已打开

Question

I am trying to make a program which checks if a file exists, otherwise it creates and opens it. This is what I tried doing:

fstream file("data.txt");
if (!file.is_open())
{
    ofstream file("data.txt");
    file.close();
}
fstream file("data.txt");

The program tries to open the data.txt file and if it does not exist, it creates it, but then it is a local, so I have to close and open it in the main function. If the file already exists the I get the multiple instances error. I tried closing the file before opening it but it still gives me the same error. Code:

fstream file("data.txt");
if (!file.is_open())
{
    ofstream file("data.txt");
    file.close();
}
file.close();
fstream file("data.txt");

Is there a better way to do it than that? Thx for your time =) (yes, I have searched for answers before asking.)

Answer 1

Simply create a file stream without specifying without telling which file to open.

fstream file;
file.open("data.txt");

Then use .fail() to check if it has FAILED in opening the file

if (fileStream.fail()) 
{
    ofstream file("data.txt");
}
file.open("data.txt");

Then open it again. fstream (Filestream) is just simply a child of iostream which does not required to be initialized and can be declared you can read more about it over here

Answer 2

Also this is c and POSIX - maybe it is of interest, cause it is a fast and handy check if file exists.

#include <unistd.h>
if(access(filename, F_OK ) == 1) // file exists
...