Std::is_error_code_enum (3) Linux Manual Page

std::is_error_code_enum<std::io_errc> – std::is_error_code_enum<std::io_errc>

Synopsis

Defined in header<ios>

    template <>
    (since C++ 11)

        struct is_error_code_enum<std::io_errc> : public std::true_type {
};

This specialization of std::is_error_code_enum informs other library components that values of type std::io_errc are enumerations that hold error codes, which makes them implicitly convertible and assignable to objects of type std::error_code.
Inherited from std::integral_constant

Member constants

value true
(public static member constant)
[static]

Member functions

converts the object to bool, returns value
operator bool (public member function)

operator() returns value
(public member function)
(C++14)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

Example

The comparison between e.code() and std::io_errc::stream compiles because std::is_error_code_enum<std::io_errc>::value == true
// Run this code

#include <iostream>

#include <fstream>

int main()

{
    std::ifstream f("doesn't exist");
    try {
        f.exceptions(f.failbit);
    } catch (const std::ios_base::failure &e) {
        std::cout << "Caught an ios_base::failure.\n";
        if (e.code() == std::io_errc::stream)
            std::cout << "The error code is std::io_errc::stream\n";
    }
}

Output:

Caught an ios_base::failure.
The error code is std::io_errc::stream

std::is_error_code_enum (3) Linux Manual Page