Simple program to change the display resolution on Windows

changeres.cpp

// prebuilt for VC14/VS2015 here:
// https://dl.dropboxusercontent.com/u/267889/changeres.exe

#include <iostream>
#include <sstream>
#include <Windows.h>

int main(int argc, char **argv)
{
  bool reset = false;
  bool keep = false;
  bool error = true;
  DWORD width = 0, height = 0, refresh = 0;

  if (argc > 1 && strcmp(argv[1], "reset") == 0)
    reset = true;
  else if (argc >= 3)
  {
    int base = 1;
    if (argc > 3 && strcmp(argv[1], "keep") == 0)
    {
      keep = true;
      ++base;
    }

    // std::stoul throws exceptions, and streams have better error indications,
    // so put everything in a stream
    std::stringstream ss;
    for (int i = base; i < argc; ++i)
      ss << argv[i] << ' ';

    ss >> width >> height;
    if (argc > base + 2)
      ss >> refresh;
    error = ss.fail();
  }

  if (error && !reset)
  {
    std::cout << "usage: " << argv[0] << " (reset | [keep] <width> <height> [<hz>])" << std::endl;
    return 1;
  }

  DEVMODE mode;
  
  if (!reset)
  {
    // search for a matching setting
    DEVMODE iter;
    iter.dmSize = sizeof(iter);
    iter.dmDriverExtra = 0;

    DWORD maxRefresh = 0;
    DWORD maxBpp = 0;
    for (DWORD i = 0; EnumDisplaySettings(NULL, i, &iter); ++i)
    {
      if (iter.dmPelsWidth == width && iter.dmPelsHeight == height)
      {
        // check fixed refresh rate
        if (refresh != 0 && iter.dmDisplayFrequency != refresh)
          continue;
        // try to maximize color depth (and refresh rate)
        if (iter.dmBitsPerPel >= maxBpp && iter.dmDisplayFrequency >= maxRefresh)
        {
          maxRefresh = iter.dmDisplayFrequency;
          maxBpp = iter.dmBitsPerPel;
          mode = iter;
        }
      }
    }

    if (maxRefresh == 0 || maxBpp == 0)
    {
      std::cerr << "no matching display mode found" << std::endl;
      return 1;
    }
    else
    {
      std::cout << "found display mode: " << mode.dmPelsWidth << "x" << mode.dmPelsHeight
                << " at " << mode.dmDisplayFrequency << " hz " << mode.dmBitsPerPel << " bpp"
                << std::endl;
    }
  }

  // maybe these were overwritten
  mode.dmSize = sizeof(mode);
  mode.dmDriverExtra = 0;

  auto res = ChangeDisplaySettings((reset ? nullptr : &mode), (keep ? CDS_UPDATEREGISTRY : 0));
  if (res == DISP_CHANGE_SUCCESSFUL)
    return 0;

  // text from MSDN
  switch (res)
  {
    case DISP_CHANGE_BADDUALVIEW: std::cerr << "The settings change was unsuccessful because the system is DualView capable." << std::endl; break;
    case DISP_CHANGE_BADFLAGS:    std::cerr << "An invalid set of flags was passed in." << std::endl; break;
    case DISP_CHANGE_BADMODE:     std::cerr << "The graphics mode is not supported." << std::endl; break;
    case DISP_CHANGE_BADPARAM:    std::cerr << "An invalid parameter was passed in.This can include an invalid flag or combination of flags." << std::endl; break;
    case DISP_CHANGE_FAILED:      std::cerr << "The display driver failed the specified graphics mode." << std::endl; break;
    case DISP_CHANGE_NOTUPDATED:  std::cerr << "Unable to write settings to the registry." << std::endl; break;
    case DISP_CHANGE_RESTART:     std::cerr << "The computer must be restarted for the graphics mode to work." << std::endl; break;
    default:                      std::cerr << "Unknown error code " << res << std::endl; break;
  }

  return 1;
}