std::basic_fstream<CharT,Traits>::is_open

#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';
    }
}

Возможный вывод:

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