Understanding: conversion from long_ptr to bool possible loss of data - winapi

Question

I am fresh off from learning C++ and to learn winapi, I am doing the Forgers Win32 API tutorial.
Here is my code:
//MyControl.h
#pragma once
#include <windows.h>
#include "resource.h"
BOOL CALLBACK DlgProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam);
BOOL BrushExists(HBRUSH hBrush);
-------------------------------------------------------
//MyControl.cpp
#include "MyControl.h"
HBRUSH g_hbrBackground = NULL;
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow)
{
return (int)DialogBox(hInstance, MAKEINTRESOURCE(IDD_MAIN), NULL, (DLGPROC)DlgProc);
}
BOOL BrushExists(HBRUSH hBrush) // I added this to better understand what is happening
{
if (hBrush)
return TRUE;
else
return FALSE;
}
BOOL CALLBACK DlgProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
switch (Message)
{
case WM_INITDIALOG:
{
g_hbrBackground = CreateSolidBrush(RGB(0, 0, 0));
//... Other code skipped copy pasting ...
}
break;
case WM_COMMAND:
//... Other code skipped copy pasting ...
break;
case WM_CTLCOLORDLG:
return (LONG_PTR)g_hbrBackground; // Where casting happens
// return BrushExists(g_hbrBackground); // Tried this to understand things
break;
case WM_CTLCOLORSTATIC:
{
HDC hdcStatic = (HDC)wParam;
SetTextColor(hdcStatic, RGB(255, 255, 255));
SetBkMode(hdcStatic, TRANSPARENT);
return (LONG_PTR)g_hbrBackground; // Where casting happens
// return BrushExists(g_hbrBackground); // Tried this to understand things
}
break;
//... Other code skipped copy pasting ...
}
The code compiles and gives the warning: C4244: 'return': conversion from 'LONG_PTR' to 'BOOL', possible loss of data
The casting works and the dialog box is colored right as expected:
Black colored dialog.
My question: What happens in the background when HBRUSH is returned as a LONG_PTR from a function (DLGPROC) that returns BOOL? My assumption from reading C++ was that if the LONG_PTR is non-zero then the BOOL returned is TRUE and if LONG_PTR is zero then the BOOL returned is FALSE. To check this I created the function BOOL BrushExists(HBRUSH hBrush);// See code. Using this function to check for the HBRUSH and return TRUE, compiles without warnings and runs without error. But the coloring of the Dialog box does not happen:
Black color missing.
So my assumption is wrong. The LONG_PTR seems to be evaluated by the win32 API as a number instead of as a BOOL. Can someone explain to me how this is happening?

Answer 1

Note that it's BOOL and not bool. With the Windows SDK, BOOL is a typedef for int, which can store a 32-bit value - it's not a simple boolean that can only store true or false.
This is a kludge in the API basically. In 32 bit Windows, a brush handle fits in a 32 bit value, and even though it's ugly and potentially confusing, it's "safe" to return a brush handle cast as a BOOL (and you had to cast it, since that's what DialogProc was defined as returning).
Once Windows gained 64 bit support this situation obviously wasn't acceptable - in this example, brush handles (which are pointers) are 64 bits in size - casting them to a 32-bit type isn't safe.
Therefore, the definition of DialogProc was changed to return INT_PTR rather than BOOL. This is typedefed to 32-bits in x86 and 64-bits in x64. The example code you've used obviously pre-dates this change, but all new code should use the correct definition of DialogProc as returning INT_PTR.

Answer 2

BOOL is nothing but int typedef'd.
LONG_PTR may be either long int or __int64 - depending on platform chosen.
From header:
#ifdef _WIN64
typedef __int64 LONG_PTR;
#else
typedef long LONG_PTR;
#endif
You seem to be building the project as x64 compiler, and hence BOOL to LONG_PTR (essentially int to int64) is giving you warning.
Note that int and long are essentially same on Visual C++/Windows.