GetWindowText malfunktion - c++

Question

I'm trying to learn how to make a GUI for a C++ using visual studio. but I have encountered some problems while using the GetWindowText() function. It does not change the LPTSTR title to the text in the textbox and I also get an error when debugging saying: “Exception thrown at 0x74F8207D (user32.dll) in Win32Project1.exe: 0xC0000005: Access violation writing location 0x002B8D38.” The code can be seen below what am I doing wrong?
#define ID_BUTTON1 1
#define ID_BUTTON2 2
#define ID_TEXT3 3
static HWND hWndTextbox;
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_CREATE:
{
HMENU hMenubar = CreateMenu();
HMENU hFile = CreateMenu();
HMENU hOptions = CreateMenu();
AppendMenu(hMenubar, MF_POPUP, (UINT_PTR)hFile, L"File");
AppendMenu(hMenubar, MF_POPUP, NULL, L"Edit");
AppendMenu(hMenubar, MF_POPUP, (UINT_PTR)hOptions, L"Options");
AppendMenu(hFile, MF_STRING, NULL, L"Exit");
AppendMenu(hOptions, MF_STRING, NULL, L"option 1");
AppendMenu(hOptions, MF_STRING, NULL, L"option 2");
SetMenu(hWnd, hMenubar);
CreateWindow(TEXT("button"), TEXT("Hello"),
WS_VISIBLE | WS_CHILD,
10, 10, 80, 25,
hWnd, (HMENU) ID_BUTTON1, NULL, NULL);
static HWND hWndTextbox = CreateWindow(TEXT("edit"), TEXT("tekst goes here"),
WS_VISIBLE | WS_CHILD|WS_BORDER | ES_AUTOHSCROLL,
90, 120, 300, 20,
hWnd, (HMENU) ID_TEXT3, NULL, NULL);
CreateWindow(TEXT("button"), TEXT("shiny"),
WS_VISIBLE | WS_CHILD,
50, 50, 80, 50,
hWnd, (HMENU) ID_BUTTON2, NULL, NULL);
}
break;
case WM_COMMAND:
{
if (LOWORD(wParam) == ID_BUTTON1) {
MessageBox(hWnd, L"Button has been clicked", L"title for popup", MB_ICONINFORMATION);
}
if (LOWORD(wParam) == ID_BUTTON2) {
// create some default vars
// int length = GetWindowTextLength(hWndTextbox) + 1;
LPTSTR title = L"test" ;
GetWindowText(hWndTextbox, title, GetWindowTextLength(hWndTextbox) + 1);
MessageBox(hWnd, title, L"Message box", MB_OK);
}
break;
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:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
// TODO: Add any drawing code that uses hdc here...
EndPaint(hWnd, &ps);
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}
the rest of the code was auto generated by visual studio so I have only included the part where I made changes.

Answer 1

The documentation for GetWindowText() states that parameter lpString is supposed to point to a "buffer that will receive the text", which implies a writable buffer.
With the code LPTSTR title = L"test"; you create a pointer to a string literal which usually is located in read-only memory. When GetWindowText() tries to write to that memory, it causes an access violation.
To solve the issue, use a writable buffer like this:
// Allocate buffer including terminating null
std::wstring title( GetWindowTextLength(hWndTextbox) + 1, 0 );
// Address of first character is used to obtain pointer to non-const data
// (as opposed to wstring::c_str()).
int size = GetWindowText( hWndTextbox, &title[0], title.size() );
// Resize buffer to the actual text length
title.resize( size );
// MessageBox only needs pointer to const string, so we can use wstring::c_str() here.
MessageBox(hWnd, title.c_str(), L"Message box", MB_OK);