std::errc

#include <filesystem>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <string>
#include <system_error>
#include <thread>
 
void print_error(const std::string& details, std::error_code error_code)
{
    std::string value_name;
    if (error_code == std::errc::invalid_argument)
        value_name = "std::errc::invalid_argument";
    if (error_code == std::errc::no_such_file_or_directory)
        value_name = "std::errc::no_such_file_or_directory";
    if (error_code == std::errc::is_a_directory)
        value_name = "std::errc::is_a_directory";
    if (error_code == std::errc::permission_denied)
        value_name = "std::errc::permission_denied";
 
    std::cout << details << ":\n  "
              << std::quoted(error_code.message())
              << " (" << value_name << ")\n\n";
}
 
void print_errno(const std::string& details, int errno_value = errno)
{
    print_error(details, std::make_error_code(std::errc(errno_value)));
}
 
int main()
{
    std::cout << "脱离不是线程的线程...\n";
    try
    {
        std::thread().detach();
    }
    catch (const std::system_error& e)
    {
        print_error("脱离空线程时报错", e.code());
    }
 
    std::cout << "打开不存在的文件...\n";
    std::ifstream nofile{"nonexistent-file"};
    if (!nofile.is_open())
        print_errno("为读取打开不存在的文件时报错");
 
    std::cout << "将目录当作文件进行读取...\n";
    std::filesystem::create_directory("dir");
    std::ifstream dir_stream{"dir"};
    [[maybe_unused]] char c = dir_stream.get();
    if (!dir_stream.good())
        print_errno("从目录读取数据时报错");
 
    std::cout << "为写入打开只读文件...\n";
    std::fstream{"readonly-file", std::ios::out};
    std::filesystem::permissions("readonly-file", std::filesystem::perms::owner_read);
    std::fstream write_readonly("readonly-file", std::ios::out);
    if (!write_readonly.is_open())
        print_errno("为写入打开只读文件时报错");
}