C / C++ / MFC

void CString::AllocBeforeWrite(int nLen)
{
	if (GetData()->nRefs > 1 || nLen > GetData()->nAllocLength)
	{
               // This release will release all the memory allocated previously, so you wont face any memory leak.
		Release();
		AllocBuffer(nLen);
	}
	ASSERT(GetData()->nRefs <= 1);
}

if (!isChildOpen)
{
    GetWindowRect(hwndDlg, &myRect);
    imgChildHWND = CreateWindowEx(0, "picWnd", myWad.getLumpName(resNum), WS_SYSMENU|WS_CAPTION|WS_POPUP|WS_VISIBLE|WS_CHILD, myRect.right, myRect.top, 300, 300, hwndDlg, NULL, hInst, 0);
    isChildOpen = true;
}
else
    SetWindowText(imgChildHWND, myWad.getLumpName(resNum));

case WM_CLOSE:
     PostMessage(GetParent(hwnd), IDC_CHILDCLOSING, 0, 0);

if (hwnd == g_hWindow)
{
//MessageBox Main Window
}
if (hwnd == g_hChildWindow)
{
//MessageBox Child Window
}

#include <windows.h>

/*  Declare Windows procedure  */
LRESULT CALLBACK childWndProc(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK parentWndProc(HWND, UINT, WPARAM, LPARAM);
bool myRegClass(WNDPROC lpfnWndProc, char *szClassName, HINSTANCE hInst);
char *szParentClassName = "parentClass", *szChildClassName = "childClass";
HINSTANCE hThisInstance;
HWND parentWnd;

#define IDC_FROMPARENT   1000
#define IDC_FROMCHILD   1001


int WINAPI WinMain (HINSTANCE hCurInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR lpszArgument,
                     int nCmdShow)
{
    HWND hwndParent;                          /* This is the handle for our window */
    MSG messages;                             /* Here messages to the application are saved */

    hThisInstance = hCurInstance;

    if (myRegClass(parentWndProc, szParentClassName, hThisInstance) == false)
        return 0;

    if (myRegClass(childWndProc, szChildClassName, hThisInstance) == false)
        return 0;

    /* both the classes are registered, let's create the program*/
    parentWnd = CreateWindowEx(
           0,
           szParentClassName,         /* Classname */
           "Code::Blocks Template Windows App - parent",       /* Title Text */
           WS_OVERLAPPEDWINDOW|WS_VISIBLE, /* default window */
           CW_USEDEFAULT,       /* Windows decides the position */
           CW_USEDEFAULT,       /* where the window ends up on the screen */
           544,                 /* The programs width */
           375,                 /* and height in pixels */
           HWND_DESKTOP,        /* The window is a child-window to desktop */
           NULL,                /* No menu */
           hThisInstance,       /* Program Instance handler */
           NULL                 /* No Window Creation data */
           );

    while (GetMessage (&messages, NULL, 0, 0))
    {
        TranslateMessage(&messages);
        DispatchMessage(&messages);
    }
    return messages.wParam;
}


bool myRegClass(WNDPROC lpfnWndProc, char *szClassName, HINSTANCE hInst)
{
    WNDCLASSEX wincl;        /* Data structure for the windowclass */

    /* The Window structure */
    wincl.hInstance = hInst;
    wincl.lpszClassName = szClassName;
    wincl.lpfnWndProc = lpfnWndProc;      /* This function is called by windows */
    wincl.style = CS_DBLCLKS;                 /* Catch double-clicks */
    wincl.cbSize = sizeof (WNDCLASSEX);

    /* Use default icon and mouse-pointer */
    wincl.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hCursor = LoadCursor (NULL, IDC_ARROW);
    wincl.lpszMenuName = NULL;                 /* No menu */
    wincl.cbClsExtra = 0;                      /* No extra bytes after the window class */
    wincl.cbWndExtra = 0;                      /* structure or the window instance */
    /* Use Windows's default colour as the background of the window */
    wincl.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

    /* Register the window class, and if it fails quit the program */
    if (RegisterClassEx (&wincl))
        return true;
    else
        return false;
}

/*  This function is called by the Windows function DispatchMessage()  */
LRESULT CALLBACK parentWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static HWND childWnd, listBox;
    switch (message)                  /* handle the messages */
    {
        case WM_CREATE:
               childWnd = CreateWindowEx(
               0,
               szChildClassName,         /* Classname */
               "Code::Blocks Template Windows App - child",       /* Title Text */
               WS_OVERLAPPEDWINDOW|WS_VISIBLE, /* default window */
               CW_USEDEFAULT,       /* Windows decides the position */
               CW_USEDEFAULT,       /* where the window ends up on the screen */
               544/2,               /* The programs width */
               375/2,               /* and height in pixels */
               NULL,          /* The window is a child-window to desktop */
               NULL,                /* No menu */
               hThisInstance,       /* Program Instance handler */
               NULL                 /* No Window Creation data */
               );
            CreateWindowEx(0, "BUTTON", "Msg to Child", WS_VISIBLE | WS_CHILD | WS_TABSTOP | 0x00000001, 8, 8, 150, 23, hwnd, (HMENU)IDOK, hThisInstance, 0);
            return 0;
            break;

         case IDC_FROMCHILD:
            MessageBox(NULL, "Child window sent me a message", "Parent", MB_OK);
         break;

        case WM_COMMAND:
                switch(LOWORD(wParam))
                {
                 case IDOK:
                    PostMessage(childWnd, IDC_FROMPARENT, 0, 0);
                 break;
                }
            break;

        case WM_DESTROY:
            PostQuitMessage (0);       /* send a WM_QUIT to the message queue */
            break;
        default:                      /* for messages that we don't deal with */
            return DefWindowProc (hwnd, message, wParam, lParam);
    }
    return 0;
}

/*  This function is called by the Windows function DispatchMessage()  */
LRESULT CALLBACK childWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)                  /* handle the messages */
    {
        case WM_CREATE:
            CreateWindowEx(0, "BUTTON", "Msg to Parent", WS_VISIBLE | WS_CHILD | WS_TABSTOP | 0x00000001, 8, 8, 150, 23, hwnd, (HMENU)IDOK, hThisInstance, 0);
            //parentWnd = GetParent(hwnd);
            return 0;
            break;

        case WM_DESTROY:
            PostQuitMessage (0);       /* send a WM_QUIT to the message queue */
            break;

         case IDC_FROMPARENT:
            MessageBox(NULL, "Parent window sent me a message", "Child", MB_OK);
         break;

        case WM_COMMAND:
                switch(LOWORD(wParam))
                {
                 case IDOK:
                    PostMessage(parentWnd, IDC_FROMCHILD, 0, 0);
                 break;
                }
            break;

        default:                      /* for messages that we don't deal with */
            return DefWindowProc (hwnd, message, wParam, lParam);
    }
    return 0;
}