system_error.cpp

#include <winerror.h>
#include <system_error>
#include <iostream>
#include <string>

int main()
{
    std::system("chcp 932");
    std::error_code ec{ERROR_FILE_EXISTS, std::system_category()};
    std::system_error se{ec};
    auto const str = se.what();
    std::cout << str << std::endl;
}

win_system_error.cpp

#include <Windows.h>
#include <system_error>
#include <clocale>

LRESULT CALLBACK procedure(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg) {
        case WM_DESTROY:
        {
            UpdateWindow(nullptr);
            std::setlocale(LC_CTYPE, "jpn");
            wchar_t dest[1024];
            std::size_t size;
            mbstowcs_s(
                &size,
                dest,
                std::system_error{std::error_code{(int) GetLastError(), std::system_category()}}.what(),
                _TRUNCATE
            );
            MessageBox(nullptr, dest, L"", MB_OK);
            PostQuitMessage(0);
            return 0;
        }
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);;
}

int WINAPI WinMain(HINSTANCE hInst, HINSTANCE, LPSTR, int)
{
    WNDCLASSEX wc;
    wc.cbSize = sizeof(WNDCLASSEX);
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = &procedure;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInst;
    wc.hIcon = static_cast<HICON>(LoadImage(nullptr, IDI_APPLICATION, IMAGE_ICON, 0, 0, LR_DEFAULTSIZE | LR_SHARED));
    wc.hCursor = static_cast<HCURSOR>(LoadImage(nullptr, IDC_ARROW, IMAGE_CURSOR, 0, 0, LR_DEFAULTSIZE | LR_SHARED));
    wc.hbrBackground = static_cast<HBRUSH>(GetStockObject(WHITE_BRUSH));
    wc.lpszMenuName = nullptr;
    wc.lpszClassName = TEXT("main window");
    wc.hIconSm = static_cast<HICON>(LoadImage(nullptr, IDI_APPLICATION, IMAGE_ICON, 0, 0, LR_DEFAULTSIZE | LR_SHARED));

    RegisterClassEx(&wc);

    auto hwnd = CreateWindowEx(
        WS_EX_OVERLAPPEDWINDOW,
        TEXT("main window"),
        TEXT("RxCpp x Win32API"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        640, 480,
        nullptr,
        nullptr,
        hInst,
        nullptr);
    ShowWindow(hwnd, SW_SHOW);
    UpdateWindow(hwnd);
    MSG msg;
    BOOL ret;
    while ((ret = GetMessage(&msg, hwnd, 0, 0)) != 0) {
        if (ret == -1) {
            break;
        }
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return 0;
}