SendMessage fails to send custom message to main window - c++

Question

Before beginning to read the problem please note i read Petzold's book (windows programming 5th edition)!
In form.h i have declared the custom message named CMessage (it stand for "custom message", which later calls the method for creating a button), in formbutton.cpp i am sending message from its constructor to the main window which is created in form.cpp. I found out from getLastError method in my helper class that send message fails because of invalid window handle, it is strange because if i use isWindow(forms.mainWindowHandle) before sending the message i got messagebox that shows "operation completed successfully". The same design worked before splitting whole code into multiple headers and cpps, now its broken.
******
step by step debugging :
form myform(hInstance);
form::form(HINSTANCE hInstanceForm);
HWND form::Createform(HWND handle, HINSTANCE hInstance, int xPos , int yPos , int xSize , int ySize);
Formbutton::Formbutton(BOOL create);
form::form();
SendMessage(forms.mainWindowHandle, CMessage,0 ,0);//error
//rest of the code and main window is shown but not the button
******
formbutton.cpp
#pragma once
#include "FormButton.h"
#include "Form.h"
#include "helper.h"
Formbutton::Formbutton(){}
Formbutton::Formbutton(BOOL create)
{
helper help;
form forms;
if (create)
{
SendMessage(forms.mainWindowHandle, CMessage,0 ,0); //same with PostMessage
help.getLastError();
}
}
HWND Formbutton::createButton(HWND button, HWND parent)
{
button = CreateWindow(TEXT("Button"), // window class name
TEXT("Button"), // window caption
WS_CHILD | WS_VISIBLE | BS_DEFPUSHBUTTON | WS_TABSTOP, // window style
500, // initial x position
500, // initial y position
300, // initial x size
500, // initial y size
parent, // parent window handle
NULL, // window menu handle
(HINSTANCE)GetWindowLongPtr(parent, -6), // program instance handle
NULL); // creation parameters
ShowWindow(button, SW_SHOW);
return button;
}
form.h
#pragma once
#include <Windows.h>
#define CMessage (WM_USER+0x0001)
class form
{
private:
HWND Createform(HWND handle, HINSTANCE hInstance, int xPos = CW_USEDEFAULT, int yPos = CW_USEDEFAULT, int xSize = GetSystemMetrics(SM_CXFULLSCREEN), int ySize = GetSystemMetrics(SM_CYFULLSCREEN));
MSG msg;
WNDCLASS wndclass;
protected:
static LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
public:
HWND mainWindowHandle;
form::form();
form::form(HINSTANCE hInstanceForm);
form::~form();
WPARAM updateForm();
};
form.cpp
#pragma once
#include "Form.h"
#include "FormButton.h"
LRESULT CALLBACK form::WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
PAINTSTRUCT ps;
RECT rect;
Formbutton fb;
switch (message)
{
case WM_CREATE:
return 0;
case WM_PAINT:
InvalidateRect(hwnd, &rect, TRUE);
hdc = BeginPaint(hwnd, &ps);
SelectObject(hdc, GetStockObject(SYSTEM_FIXED_FONT));
SetBkMode(hdc, TRANSPARENT);
EndPaint(hwnd, &ps);
return 0;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
case CMessage: //this is the custom message
fb.createButton(fb.buttonHandle, hwnd);
default:
return DefWindowProc(hwnd, message, wParam, lParam);
}
return 0;
}
HWND form::Createform(HWND handle, HINSTANCE hInstance, int xPos , int yPos , int xSize , int ySize)
{
wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hInstance = hInstance;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
wndclass.lpszMenuName = NULL;
wndclass.lpszClassName = TEXT("Framework");
if (!RegisterClass(&wndclass))
{
MessageBox(NULL, TEXT("This program requires Windows NT!"),
TEXT("Framework"), MB_ICONERROR);
}
handle = CreateWindow(TEXT("Framework"), // window class name
TEXT("The Hello Program"), // window caption
WS_OVERLAPPEDWINDOW | WS_MAXIMIZEBOX | WS_MINIMIZEBOX | WS_SIZEBOX | WS_HSCROLL | WS_VSCROLL, // window style
xPos, // initial x position
yPos, // initial y position
xSize, // initial x size
ySize, // initial y size
NULL, // parent window handle
NULL, // window menu handle
hInstance, // program instance handle
NULL); // creation parameters
//ShowWindow(handle, SW_SHOWMAXIMIZED);
UpdateWindow(handle);
return handle;
}
WPARAM form::updateForm()
{
ShowWindow(mainWindowHandle, 10);
UpdateWindow(mainWindowHandle);
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
form::form()
{
}
form::form(HINSTANCE hInstanceForm)
{
mainWindowHandle = Createform(mainWindowHandle, hInstanceForm);
}
form::~form()
{
}
helper.cpp
#pragma once
#include "helper.h"
#include <strsafe.h>
void helper::getLastError()
{
wchar_t buf[256];
FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM, NULL, GetLastError(),
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), buf, 256, NULL);
MessageBox(NULL, buf, TEXT("error"), MB_OK);
}
source.cpp
#pragma once
#include <Windows.h>
#include "Form.h"
#include "FormButton.h"
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
form myform(hInstance);
Formbutton btn(true);
myform.updateForm();
}

Answer 1

You are creating two instances of form: one in your WinMain() and one in your Formbutton constructor. You want to add your button to the former, but you are really adding it to the latter, and the latter ceases to exist once the constructor returns.
I'm not sure how you want to structure your program, but my suggestion is to pass the correct parent form as a form * parameter to the Formbutton constructor.