Win32 - Appending text to an Edit Control

Question

Trying to append text to an edit control inside a dialog box. I can\'t get _tcscat_s to append correctly. It crashes and says something about the buffer being too small or s

Answer 1

http://support.microsoft.com/kb/109550

Is your answer:

   string buffer = "append this!"
   HWND hEdit = GetDlgItem (hDlg, ID_EDIT);
   int index = GetWindowTextLength (hEdit);
   SetFocus (hEdit); // set focus
   SendMessageA(hEdit, EM_SETSEL, (WPARAM)index, (LPARAM)index); // set selection - end of text
   SendMessageA(hEdit, EM_REPLACESEL, 0, (LPARAM)buffer.c_str()); // append!

Answer 2

GetWindowTextLength() returns the number of TCHAR elements in the text, but GlobalAlloc() expects a byte count instead. If you are compiling for Unicode, TCHAR is 2 bytes, not 1 byte, but you are not taking that into account. You are also not allocating the buffer large enough to hold both the existing text and the new text being appended. You are also leaking the memory that you allocate.

Try this:

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get new length to determine buffer size
    int outLength = GetWindowTextLength( hwndOutput ) + lstrlen(newText) + 1;

    // create buffer to hold current and new text
    TCHAR * buf = ( TCHAR * ) GlobalAlloc( GPTR, outLength * sizeof(TCHAR) );
    if (!buf) return;

    // get existing text from edit control and put into buffer
    GetWindowText( hwndOutput, buf, outLength );

    // append the newText to the buffer
    _tcscat_s( buf, outLength, newText );

    // Set the text in the edit control
    SetWindowText( hwndOutput, buf );

    // free the buffer
    GlobalFree( buf );
}

Alternatively:

#include <vector>

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get new length to determine buffer size
    int outLength = GetWindowTextLength( hwndOutput ) + lstrlen(newText) + 1;

    // create buffer to hold current and new text
    std::vector<TCHAR> buf( outLength );
    TCHAR *pbuf = &buf[0];

    // get existing text from edit control and put into buffer
    GetWindowText( hwndOutput, pbuf, outLength );

    // append the newText to the buffer
    _tcscat_s( pbuf, outLength, newText );

    // Set the text in the edit control
    SetWindowText( hwndOutput, pbuf );
}

With that said, getting the window's current text into memory, appending to it, and then replacing the window's text is a very inefficient way to append text to an edit control. Use the EM_REPLACESEL message instead:

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get the current selection
    DWORD StartPos, EndPos;
    SendMessage( hwndOutput, EM_GETSEL, reinterpret_cast<WPARAM>(&StartPos), reinterpret_cast<WPARAM>(&EndPos) );

    // move the caret to the end of the text
    int outLength = GetWindowTextLength( hwndOutput );
    SendMessage( hwndOutput, EM_SETSEL, outLength, outLength );

    // insert the text at the new caret position
    SendMessage( hwndOutput, EM_REPLACESEL, TRUE, reinterpret_cast<LPARAM>(newText) );

    // restore the previous selection
    SendMessage( hwndOutput, EM_SETSEL, StartPos, EndPos );
}