c++ - What's wrong with this screen capture code? -

i found screen capture code on here, when tried build it wouldn't build, fixed code myself , builds don't think works because when debugging vs says can't read hbitmap (no data?). i'm novice programmer have no idea @ point... , didn't fix code properly...

i appreciate help.

#include <windows.h>  int main() {     // device context of screen     hdc hscreendc = createdc(l"display", null, null, null);          // , device context put in     hdc hmemorydc = createcompatibledc(hscreendc);      int x = getdevicecaps(hscreendc, horzres);     int y = getdevicecaps(hscreendc, vertres);      // maybe worth checking these positive values     hbitmap hbitmap = createcompatiblebitmap(hscreendc, x, y);      // new bitmap     hbitmap holdbitmap = (hbitmap)selectobject(hmemorydc, hbitmap);      bitblt(hmemorydc, 0, 0, 640, 480, hscreendc, 0, 0, srccopy);     hbitmap = (hbitmap)selectobject(hmemorydc, holdbitmap);     // image held in hbitmap. can save or whatever } 

short answer: there's nothing wrong.

hbitmap contains handle bitmap has screencap data retrieved via bitblt. when hover on hbitmap in visual studio, merely informing hbitmap not valid pointer memory, correct report- windows handles tokens resolve structures memory location , implementation managed privately windows api.

to demonstrate code indeed pulling screen, try writing file. helpful use gdi+ write file, save lot of initialization code have write hand.

here's quick console app emit png file using code , a helper function getencoderclsid png encoder.

#include "stdafx.h" #include <windows.h> #include <gdiplus.h>  using namespace gdiplus;  int getencoderclsid(const wchar* format, clsid* pclsid) {   uint  num = 0;          // number of image encoders   uint  size = 0;         // size of image encoder array in bytes    imagecodecinfo* pimagecodecinfo = null;    getimageencoderssize(&num, &size);   if (size == 0)     return -1;  // failure    pimagecodecinfo = (imagecodecinfo*)(malloc(size));   if (pimagecodecinfo == null)     return -1;  // failure    getimageencoders(num, size, pimagecodecinfo);    (uint j = 0; j < num; ++j)   {     if (wcscmp(pimagecodecinfo[j].mimetype, format) == 0)     {       *pclsid = pimagecodecinfo[j].clsid;       free(pimagecodecinfo);       return j;  // success     }   }    free(pimagecodecinfo);   return -1;  // failure }  int _tmain(int argc, _tchar* argv[]) {   gdiplusstartupinput gdiplusstartupinput;   ulong_ptr gdiplustoken;   gdiplusstartup(&gdiplustoken, &gdiplusstartupinput, null);    // device context of screen   hdc hscreendc = createdc(l"display", null, null, null);   // , device context put in   hdc hmemorydc = createcompatibledc(hscreendc);    int x = getdevicecaps(hscreendc, horzres);   int y = getdevicecaps(hscreendc, vertres);    // maybe worth checking these positive values   hbitmap hbitmap = createcompatiblebitmap(hscreendc, x, y);    // new bitmap   hbitmap holdbitmap = (hbitmap)selectobject(hmemorydc, hbitmap);    bitblt(hmemorydc, 0, 0, 640, 480, hscreendc, 0, 0, srccopy);   hbitmap = (hbitmap)selectobject(hmemorydc, holdbitmap);   // image held in hbitmap. can save or whatever    clsid pngclsid;   getencoderclsid(l"image/png", &pngclsid);    bitmap *bmp = new bitmap(hbitmap, null);   bmp->save(l"desktop_slice.png", &pngclsid, null);   delete bmp;    gdiplusshutdown(gdiplustoken);   return 0; } 

make sure add gdiplus.lib list of source libraries in project settings. running create file called "desktop_slice.png" emitted.

if need additional work after retrieving bitmap containing screen data, should select compatible dc , call additional gdi functions dc, or perform other modifications hmemorydc before swapping out bitmap selectobject.

if need lower-level work @ pixel level, should @ creating dib section known pixel format meets needs, , working pointer returned ppvbits argument.

createdibsection @ msdn