C++ Memory management failing when deleting Bitmap and CLSID objects using GDI+ - c++

Question

I am unable to manage memory for Bitmap and CLSID objects I have created in a screenshot object class. Both of these are from the GDI+ library. The header lists the following private variables in Screenshot.h
#include <gdiplus.h>
#include <iostream>
#include <fstream>
#include <string>
#include "windows.h"
#pragma once
#pragma comment(lib, "gdiplus.lib")
using namespace std;
using namespace Gdiplus;
class Screenshot
{
private:
HDC dc, memdc, fontdc;
HBITMAP membit;
Bitmap* bmpPtr;
CLSID clsid;
ULONG_PTR gdiplusToken;
int GetEncoderClsid(const WCHAR* format, CLSID* pClsid);
public:
Screenshot();
~Screenshot();
void TakeScreenshot(string userAction, string winName, long xMousePos, long yMousePos, long long tStamp);
void SaveScreenshot(string filename);
void memoryManagement();
};
Then when my main program takes a screenshot, the values are filled in with TakeScreenshot(), but not yet saved to disk
void Screenshot::TakeScreenshot(//redacted for readibility) {
GdiplusStartupInput gdiplusStartupInput;
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
HWND hwnd = GetDesktopWindow();
dc = ::GetDC(0);
int scaleHeight, scaleWidth = 0;
int Height = GetSystemMetrics(SM_CYVIRTUALSCREEN);
int Width = GetSystemMetrics(SM_CXVIRTUALSCREEN);
scaleHeight = Height + (0.1 * Height);
memdc = CreateCompatibleDC(dc);
membit = CreateCompatibleBitmap(dc, Width, scaleHeight);
HBITMAP bmpContainer = (HBITMAP)SelectObject(memdc, membit);
BitBlt(memdc, 0, 0, Width, Height, dc, 0, 0, SRCCOPY);
//Other code that adds fonts, etc. Does not invoke bmpPtr
bmpPtr = new Bitmap(membit, NULL);
GetEncoderClsid(L"image/jpeg", &clsid);
If the screenshot is saved, another function SaveScreenshot() uses bmpPtr->Save() and Gdiplus shutdown is called inside of it. However, some of the screenshots get popped off from a queue (STL queue) and out of memory instead of saved, as follows:
void ManageQueue(Screenshot& ssObj)
{
//If queue contains 30 screenshots, pop off first element and push new object
//Else just push new object
if (screenshotQueue.size() == MAX_SCREENSHOTS)
{
screenshotQueue.front().memoryManagement();
screenshotQueue.pop();
screenshotQueue.push(ssObj);
}
else
{
screenshotQueue.push(ssObj);
}
}
I wrote a MemoryManagement() function to perform the necessary releases and deletes before the Screenshot is popped off. This function is not called if the screenshot has been saved:
void Screenshot::memoryManagement()
{
delete bmpPtr;
delete &clsid;
ReleaseDC(NULL, memdc);
DeleteObject(fontdc);
DeleteObject(memdc);
DeleteObject(membit);
}
When either the delete on bmpPtr or clsid is called, whether it is from this function or in the deconstructor, the program is crashing. I am experiencing significant memory leaks with the program now and without running a windows equivalent of Valgrind I'm assuming it's coming from here. How can I successfully delete these objects? I will credit any answer in my source code as a contributing programmer. Please leave any suggestions for improving my question if needed.

Answer 1

scaleHeight = Height + (0.1 * Height);
This seems to be an attempt to fix the problem with DPI scaling. It will work if DPI settings is at 10%, but that's usually not the case. You have to make your program DPI aware through the manifest file. Use SetProcessDPIAware for a quick fix.
Don't declare dc, memdc, etc. as class members. These are GDI handles (not GDI+) which you can hold for a short time, usually during the duration of the function. You have to release them as soon as possible.
Also other variables like clsid don't need to be declared as class members. You can declare them as class member if you wish, but there is nothing to gain.
If you have a multi-monitor setup you also need SM_XVIRTUALSCREEN/Y to get the top-left corner of the monitor setup.
//call this once on start up
SetProcessDPIAware();
HDC dc = ::GetDC(0);
int x = GetSystemMetrics(SM_XVIRTUALSCREEN);
int y = GetSystemMetrics(SM_YVIRTUALSCREEN);
int Height = GetSystemMetrics(SM_CYVIRTUALSCREEN);
int Width = GetSystemMetrics(SM_CXVIRTUALSCREEN);
HDC memdc = CreateCompatibleDC(dc);
HBITMAP membit = CreateCompatibleBitmap(dc, Width, Height);
HBITMAP bmpContainer = (HBITMAP)SelectObject(memdc, membit);
BitBlt(memdc, 0, 0, Width, Height, dc, x, y, SRCCOPY);
Bitmap* bmpPtr = new Bitmap(membit, NULL);
// or just Bitmap bmp(membit, NULL);
CLSID clsid;
GetEncoderClsid(L"image/jpeg", &clsid);
bmpPtr->Save(L"output.jpg", &clsid);
//cleanup:
delete bmpPtr;
SelectObject(memdc, bmpContainer);
DeleteObject(membit);
DeleteDC(memdc);
ReleaseDC(0, dc);

Answer 2

The solution for this problem was to use the namespace delete instead of regular delete. Switching to this prevented the break point trigger during debug and has sealed the memory leak.
void Screenshot::memoryManagement()
{
::delete bmpPtr;
ReleaseDC(NULL, memdc);
DeleteObject(fontdc);
DeleteObject(memdc);
DeleteObject(membit);
GdiplusShutdown(gdiplusToken);
}