How to check if a file exists and is readable in C++?

Question

I\'ve got a fstream my_file(\"test.txt\"), but I don\'t know if test.txt exists. In case it exists, I would like to know if I can read it, too. How to do that?

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Answer 1

C++17, cross-platform: Check file existence with std::filesystem::exists and readability with std::filesystem::status & std::filesystem::perms:

#include <iostream>
#include <filesystem> // C++17
namespace fs = std::filesystem;

/*! \return True if owner, group and others have read permission,
            i.e. at least 0444.
*/
bool IsReadable(const fs::path& p)
{
    std::error_code ec; // For noexcept overload usage.
    auto perms = fs::status(p, ec).permissions();
    if ((perms & fs::perms::owner_read) != fs::perms::none &&
        (perms & fs::perms::group_read) != fs::perms::none &&
        (perms & fs::perms::others_read) != fs::perms::none
        )
    {
        return true;
    }
    return false;
}

int main()
{
    fs::path filePath("path/to/test.txt");
    std::error_code ec; // For noexcept overload usage.
    if (fs::exists(filePath, ec) && !ec)
    {
        if (IsReadable(filePath))
        {
            std::cout << filePath << " exists and is readable.";
        }
    }
}

Consider also checking for the file type.

Answer 2

Concerning the use of fstat in windows, I am not sure if it is what you want. From Microsoft the file must be already open. Stat should work for you.