GDI+ - Cannot do anything with Gdiplus::Graphics (C++) - c++

Question

I'm new to GDI+ and I've no idea what the problem is so I'll just post all of the code. I'm trying to simply draw an image but in my debugger I can see that graphics which I try to use in WM_PAINT is NULL. I've seen many people do pretty much the exact same thing which I'm trying to do so I'm pretty confused as to what's going on.
#include "stdafx.h"
#include "GUI.h"
#include <objidl.h>
#include <gdiplus.h>
#include <iostream>
using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.lib")
#define MAX_LOADSTRING 100
// Global Variables:
HINSTANCE hInst; // current instance
WCHAR szTitle[MAX_LOADSTRING]; // The title bar text
WCHAR szWindowClass[MAX_LOADSTRING]; // the main window class name
// Forward declarations of functions included in this code module:
ATOM MyRegisterClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM);
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPWSTR lpCmdLine,
_In_ int nCmdShow)
{
UNREFERENCED_PARAMETER(hPrevInstance);
UNREFERENCED_PARAMETER(lpCmdLine);
// Initialize global strings
LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
LoadStringW(hInstance, IDC_GUI, szWindowClass, MAX_LOADSTRING);
MyRegisterClass(hInstance);
// Perform application initialization:
if (!InitInstance (hInstance, nCmdShow))
{
return FALSE;
}
HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_GUI));
MSG msg;
// Main message loop:
while (GetMessage(&msg, nullptr, 0, 0))
{
if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return (int) msg.wParam;
}
ATOM MyRegisterClass(HINSTANCE hInstance)
{
WNDCLASSEXW wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_GUI));
wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);
wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wcex.lpszMenuName = MAKEINTRESOURCEW(IDC_GUI);
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
return RegisterClassExW(&wcex);
}
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
hInst = hInstance; // Store instance handle in our global variable
HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);
if (!hWnd)
{
return FALSE;
}
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
return TRUE;
}
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
PAINTSTRUCT ps;
HDC hdc;
switch (message)
{
case WM_COMMAND:
{
int wmId = LOWORD(wParam);
// Parse the menu selections:
switch (wmId)
{
case IDM_ABOUT:
DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
break;
case IDM_EXIT:
DestroyWindow(hWnd);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
break;
case WM_PAINT:
{
hdc = BeginPaint(hWnd, &ps);
Graphics graphics(hdc);
Image image(L"C:\\light.png");
graphics.DrawImage(&image, 0, 0);
EndPaint(hWnd, &ps);
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}
// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
UNREFERENCED_PARAMETER(lParam);
switch (message)
{
case WM_INITDIALOG:
return (INT_PTR)TRUE;
case WM_COMMAND:
if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
{
EndDialog(hDlg, LOWORD(wParam));
return (INT_PTR)TRUE;
}
break;
}
return (INT_PTR)FALSE;
}

Answer 1

You are missing the calls to GdiplusStartup() and GdiplusShutdown().
From the reference of GdiplusStartup:
You must call GdiplusStartup before you create any GDI+ objects, and
you must delete all of your GDI+ objects (or have them go out of
scope) before you call GdiplusShutdown.
... and from GdiplusShutdown:
The GdiplusShutdown function cleans up resources used by Windows GDI+.
Each call to GdiplusStartup should be paired with a call to
GdiplusShutdown.
I'm using a RAII class like this to simplify the task:
class GdiPlusInit
{
public:
GdiPlusInit()
{
Gdiplus::GdiplusStartupInput startupInput;
Gdiplus::GdiplusStartup( &m_token, &startupInput, NULL );
// NOTE: For brevity I omitted error handling, check function return value!
}
~GdiPlusInit()
{
if( m_token )
Gdiplus::GdiplusShutdown( m_token );
}
// Class is non-copyable.
GdiPlusInit( const GdiPlusInit& ) = delete;
GdiPlusInit& operator=( const GdiPlusInit& ) = delete;
private:
ULONG_PTR m_token = 0;
};
Usage:
Create an instance of the class at the beginning of the scope where you want to use GDI+ functions (for performance reasons I wouldn't do that in a function that is called frequently). I usually create it as member variable of window classes or other classes that use GDI+ so clients of my code don't need to be told to initialize GDI+. It doesn't matter if clients already call GdiplusStartup() and GdiplusShutdown() on their own, because the calls can be nested, if they are properly paired.
In your case:
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPWSTR lpCmdLine,
_In_ int nCmdShow)
{
UNREFERENCED_PARAMETER(hPrevInstance);
UNREFERENCED_PARAMETER(lpCmdLine);
GdiPlusInit gdiplus; // calls GdiplusStartup() and stores the returned token
// ... remaining code of your application ...
// When the scope ends, the destructor of GdiPlusInit calls GdiplusShutdown(),
// passing the stored token.
}