bitblt and bitmaps

[g3RC4n]

i want to make a bitmap class where i can draw a bitmap on screen

but i also want to be able to undraw the bitmap, so therefore i also want to know whats underneath the bitmap

#pragma once

#include <windows.h>

class bitmap{
public:
	bitmap(HWND hwnd,HINSTANCE hinstance, LPCTSTR str):
	    _hwnd(hwnd),
		_hdc(GetDC(_hwnd)),
		_hbitmap(LoadBitmap(hinstance,str)),
		_hdcmem(CreateCompatibleDC(_hdc)),
		_hdcunder(CreateCompatibleDC(_hdc)),
		x_pos(0),
		y_pos(0){
		SelectObject(_hdcmem,_hbitmap);
		BITMAP bitmap;
		GetObject(_hbitmap,sizeof(BITMAP),&bitmap);
		x_size = bitmap.bmWidth;
		y_size = bitmap.bmHeight;
	}
	~bitmap(){
		DeleteDC(_hdcmem);
		ReleaseDC(_hwnd,_hdc);
	}
	void draw(int x, int y){
		BitBlt(_hdcunder,x_pos=x,y_pos=y,x_size,y_size,_hdc,0,0,SRCCOPY);
		BitBlt(_hdc,x_pos,y_pos,x_size,y_size,_hdcmem,0,0,SRCCOPY);
	}
	void undraw(){
		BitBlt(_hdc,x_pos,y_pos,x_size,y_size,_hdcunder,0,0,SRCCOPY);
	}
private:
	HWND _hwnd; //windows handle
	HDC _hdc;   //dc for painting
	HBITMAP _hbitmap; //bitmap handle
	HDC _hdcmem;  //bitmap memory
	HDC _hdcunder; //memory fro what was under the bitmap that i drew
	int x_size, //metrics of bitmap
		y_size;
	int x_pos,//position of bitmap for undrawing it
		y_pos;
};

/what i'm trying to do is store what is underneath the position where the bitmap will be drawin to be put in _hdcunder, the draw the bitmap, then when undraw() is called it will putback the original imiage, why doesn't this work?