Locale aware edit control subclassing for decimal numbers ( format [sign] [xxx...] [decimal separator] [yy...] )

Question 1

Taking into consideration locale-specific settings

You certainly can do everything yourself, however you have an option to use VarI4FromStr or similar API which does dirty stuff for you. You put string in, you get LONG out. Locale aware.

"Should accept only"

You don't specify how the control should enforce this exactly. What if the input string is not valid? Control should be still accepting it because, for example, string is just not yet valid and user is still typing. If you are validating the input in external handler, such as when OK button is pressed, then you don't even need to subclass. If you want to check input every time it changes, you don't need to subclass either since you have EN_CHANGE notifications on parent. You might want to subclass for other reasons though.

It is user-friendly to accept any input and then indicate validity somehow (such as underlining with red if invalid) either on text change or on input validation.

Question 2

After taking into account code from Advice required to insert one string into another once obtaining text from clipboard, I was able to put up a subclassing procedure that meets the requirement.

The point of my solution is to simulate edit control's behavior as mentioned in that post and then validate the resulting text.

When handling VK_DELETE, selected text is deleted and the result is then parsed to check if valid decimal format is left. If everything is OK message is passed to the default procedure else is discarded. The same method is performed for WM_CUT, WM_CLEAR, and for backspace in the WM_CHAR handler ( here we must protect ourself from crashing the app by accessing the element of the string with the ordinal -1, that is the reason why I have added the line if ( start > 0 ) ).

When handling WM_PASTE we merge the edit control's text with the clipboard text and then we parse the resulting string to check its validity. Again, if all is OK we pass the message else we discard it.

The same thing applies for WM_CHAR except we here insert character in the selected part of the edit control's text and then we perform the validity check.

Since the inputted text will be always correct this way, we do not have to handle WM_UNDO.

Finaly, here is the code:

LRESULT CALLBACK Decimalni( HWND hwnd, UINT message, 
    WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData )
{

    switch (message)
    {
    case WM_KEYDOWN:
        {
            if( wParam == VK_DELETE )
            {
                DWORD start, end;

                int len = GetWindowTextLength(hwnd);

                std::wstring buffer( len, 0 );

                // get current window text

                if( len > 0 )
                   GetWindowText( hwnd, &buffer[0], len + 1 );

                // get current selection
                SendMessage( hwnd, EM_GETSEL, (WPARAM)&start, (LPARAM)&end );

                if( end > start )
                    buffer.erase( start, end - start );
                else
                    buffer.erase( start, 1 );

                if( buffer.empty() )
                    return ::DefSubclassProc( hwnd, message, wParam, lParam);

                bool IsTextValid = true; // indicates validity of inputed text

                // TODO: parse buffer

                if( IsTextValid )
                     return ::DefSubclassProc( hwnd, message, wParam, lParam);
                else
                {
                     // TODO: indicate error
                     return FALSE;
                }
            }
        }
        return ::DefSubclassProc( hwnd, message, wParam, lParam);;
        break;
    case WM_CLEAR:
    case WM_CUT:
        {
            DWORD start, end;

            int len = GetWindowTextLength(hwnd);

            std::wstring buffer( len, 0 );

            // get current window text

           if( len > 0 )
               GetWindowText( hwnd, &buffer[0], len + 1 );

            // get current selection
            SendMessage( hwnd, EM_GETSEL, (WPARAM)&start, (LPARAM)&end );

            if( end > start )
                buffer.erase( start, end - start );

            if( buffer.empty() )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);

            // TODO: parse buffer 
            bool IsTextValid = true;

            if( IsTextValid )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);
            else
            {
                // TODO: Indicate error
                return FALSE;
            }
        }
        break;
    case WM_PASTE:
        {
            int len = GetWindowTextLength(hwnd);

            std::wstring clipboard, wndtxt( len, 0 );

            if( len > 0 )
                GetWindowText( hwnd, &wndtxt[0], len + 1 );

            if( !OpenClipboard(hwnd) )
                return FALSE;

            HANDLE hClipboardData;

            if( hClipboardData = GetClipboardData(CF_UNICODETEXT) )
            {
                 clipboard = (wchar_t*)GlobalLock(hClipboardData);
                 GlobalUnlock(hClipboardData);  

            }

            CloseClipboard();

            if( clipboard.empty() )
                return FALSE;

            DWORD start, end;
            SendMessage( hwnd, EM_GETSEL, (WPARAM)&start, (LPARAM)&end );

            // merge strings into one
            if( end > start )
               wndtxt.replace( start, end - start, clipboard );
            else
                wndtxt.insert( start, clipboard );

            // TODO: parse the text
            bool ITextValid = true;

            // process the result
            if( IsTextValid )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);
            else
            {
                // TODO: indicate error
                return FALSE;
            }

        }
        break;
    case WM_CHAR:
        {
            DWORD start, end;

            int len = GetWindowTextLength(hwnd);

            std::wstring buffer( len, 0 );

            // get current window text

            if( len > 0 )
                GetWindowText( hwnd, &buffer[0], len + 1 );

            // get current selection
            SendMessage( hwnd, EM_GETSEL, (WPARAM)&start, (LPARAM)&end );

            // allow copy/paste but leave backspace for special handler
            if( ( wParam < 0x020 ) && ( wParam != 0x08 ) )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);}

            // process backspace
            if( wParam == 0x08 ) 
            {
                if( end > start )
                    buffer.erase( start, end - start );
                else
                    if( start > 0 )    // it is safe to move back one place
                        buffer.erase( start - 1, 1 );
                    else  // start-1 < 0 , can't access buffer[-1] !!
                        return FALSE;

                if( buffer.empty() )
                    return ::DefSubclassProc( hwnd, message, wParam, lParam);

                // TODO: parse buffer

                // process the result
                if( IsTextValid )
                     return ::DefSubclassProc( hwnd, message, wParam, lParam);
                else
                {
                     //TODO: indicate error
                     return FALSE;
                }
            }

            // insert character and parse text

            if( end > start )
                buffer.replace( start, end - start, 1, (wchar_t)wParam );
            else
                buffer.insert( start, 1, (wchar_t)wParam );

            // TODO: parse text

            // process the result
            if( IsTextValid )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);
            else
            {
                //TODO: indicate error
                return FALSE;
            }
        }
        break;
    case WM_NCDESTROY:
        ::RemoveWindowSubclass( hwnd, Decimalni, 0 );
        break;
    }
    return ::DefSubclassProc( hwnd, message, wParam, lParam);
}

Locale aware edit control subclassing for decimal numbers ( format [sign] [xxx...] [decimal separator] [yy...] )

INTRODUCTION AND RELEVANT INFORMATION:

MY EFFORTS TO SOLVE THIS:

QUESTION:

APPENDIX:

Taking into consideration locale-specific settings

"Should accept only"