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Showing content from http://en.cppreference.com/w/cpp/io/basic_streambuf/../basic_filebuf/../basic_fstream/is_open.html below:

std::basic_fstream<CharT,Traits>::is_open - cppreference.com

Checks if the file stream has an associated file.

Effectively calls rdbuf()->is_open().

(none)

true if the file stream has an associated file, false otherwise.

#include <fstream>
#include <iostream>
#include <string>
 
int main()
{
    std::string filename = "some_file";
 
    std::fstream fs(filename, std::ios::in);
 
    std::cout << std::boolalpha;
    std::cout << "fs.is_open() = " << fs.is_open() << '\n';
 
    if (!fs.is_open())
    {
        fs.clear();
        fs.open(filename, std::ios::out);
        std::cout << "fs.is_open() = " << fs.is_open() << '\n';
    }
}

Possible output:

fs.is_open() = false
fs.is_open() = true

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