bool is_open() const;

Checks if the file stream has an associated file.

Effectively calls rdbuf()->is_open().



Return value

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


#include <string>
#include <fstream>
#include <iostream>
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.open(filename, std::ios::out);
        std::cout << "fs.is_open() = " << fs.is_open() << '\n';

Possible output:

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 365 C++98 is_open was not declared with const qualifier declared with const qualifier

See also

opens a file and associates it with the stream
(public member function)
closes the associated file
(public member function)

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