std::ifstream can't open file?

Question

I'm writing a program for my laboratory work using C++. But I have a strange problem that I see first time. I can't open my text file using the ifstream::open method.

This simple code doesn't work too:

int main() {
    ifstream file;
    file.open("numbers.txt");
    if (file.is_open())
        cout << "fstream.open(c:\\numbers.txt): it's ok." << endl;
    else
        cout << "fstream.open(c:\\numbers.txt): error: can't open this file." << endl;
    // output: fstream.open(c:\\numbers.txt): error: can't open this file.
    file.close();
    return 0;
}

It's no compiler error but the program can't open the text file. This file exists, but I can't open it from my program. I tried to move it to "C:/" and to the "<visual studio 2022 project folder>/x64/Debug" but it didn't help.

I tried to compile it with a g++ on Linux and it's all right there. In Windows I can open that file with Notepad, but my program can't open it. What can be wrong?

Answer 1

As someone in the comments said, you are using relative path. Try checking where executable is running by using this command:

std::filesystem::path cwd = std::filesystem::current_path();
std::cout << "Current path is: " << cwd << "\n";

If your file is not present in this directory, try using actual path of file, for example: "D:/documents/numbers.txt". Also, you can use ifstream's constructor to open a file directly without using ifstream::open() method like this:

std::ifstream file("numbers.txt");

File can read by std::getline() function like this:

std::string line;
while(getline(file, line))
{
    std::cout << line << "\n";
}

getline function reads file's line until there is any left and saves it in a string which is outputted every time loop executes. Note that here, std::getline was called without writing namespace's name because of ADL(argument-dependent lookup).

std::fstream is also a good way of manipulating with files, providing read and write functionality in only one class. Reading file with fstream would look like that:

std::fstream file("numbers.txt", std::ios::in);
std::string line;
while(getline(file, line))
{
    std::cout << line << "\n";
}

std::ios::in indicates that stream will be used for reading, and std::ios::out indicates that it will be used for writing into file.

Hope it helps.