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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../system_category.html below:

std::system_category - cppreference.com

Obtains a reference to the static error category object for errors reported by the operating system. The object is required to override the virtual function std::error_category::name() to return a pointer to the string "system". It is also required to override the virtual function std::error_category::default_error_condition() to map the error codes that match POSIX errno values to std::generic_category.

(none)

A reference to the static object of unspecified runtime type, derived from std::error_category.

On Windows, system_category() typically maps some Windows error codes to POSIX ones. On POSIX, system_category() tends to be equivalent to std::generic_category() except for the name.

#include <iomanip>
#include <iostream>
#include <string>
#include <system_error>
 
int main()
{
    for (int const code : {EDOM, 10001})
    {
        const std::error_condition econd =
            std::system_category().default_error_condition(code);
 
        std::cout << "Category: " << econd.category().name() << '\n'
                  << "Value:    " << econd.value() << '\n'
                  << "Message:  " << econd.message() << "\n\n";
    }
}

Possible output:

Category: generic
Value:    33
Message:  Numerical argument out of domain
 
Category: system
Value:    10001
Message:  Unknown error 10001

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