fstream doesn't resolve path

问题:

i am new to c++ and trying to write a simple function, that saves a string to a file. The function works, when i pass the full path to fstream, but it doesn't resolve relative paths.

Here is the relevant part of my code

#include <iostream>
#include <fstream>

void writeToFile ()
{
    std::fstream fs;
    fs.open ("/home/blabla/Documents/test.txt", std::fstream::in | std::fstream::out | std::fstream::app);
    fs << " test content";

    fs.close();
}

This works fine, but i would like to create the file in the folder, where my program is executed, so i tried this

fs.open ("./test.txt", std::fstream::in | std::fstream::out | std::fstream::app);

I also tried

fs.open ("~/Documents/test.txt", std::fstream::in | std::fstream::out | std::fstream::app);

Neither of them created a new file and i did not get any error message.

I found this post, which suggests, that i can pass relative paths to fstream but only gives windows examples. How to use fstream objects with relative path?

I work on Linux Mint, the target environment is debian.

I am thankful for any hints or suggestions, Michael

回答1:

Your program

#include <iostream>
#include <fstream>

void writeToFile () {
    std::fstream fs;
    fs.open ("./test.txt", std::fstream::in | std::fstream::out | std::fstream::app);
    fs << " test content";

    fs.close();
}

int main() {
    writeToFile();
}

works perfectly fine for me on Coliru.

As you can see from the ls output, the file was created

a.out      << The executable program
main.cpp   << The source
test.txt   << The created text file

and cat at least dumps the content of the file

 test content

As mentioned in my comment, a path like ~/Documents/test.txt is evaluated by the shell, while ./test.txt should work for openting an existing file, if you're running your program in the same directory where the file exists.

"Neither of them created a new file and i did not get any error message."

You'll never get any kind of error message, if std::fstream::open() failed for some reason.
You have to check for the stream state after that call, like e.g.

 if(!fs) { std::cerr << "Could not open file" << std::endl; }

You may e.g. not have rights to create a file in this directory.

Another option is to use the std::basic_ios::exceptions() function of std::fstream(), to trigger an exception when the stream state runs into an error condition.

回答2:

Relative paths do work with streams. You have two interesting cases though. The tilde (~) is a special character that some shells interpret. I suspect that fstream doesn't do that interpretation. As to the example of "./test.txt", I think the previous comment is correct - that file has been created - it's just not where you expected it.