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

Question

INTRODUCTION AND RELEVANT INFORMATION:

I have an edit control that should accept only signed decimal numbers-something like -123.456. Also, it should be locale aware, since decimal separator is not the same for every country-in US dot is used, while in Europe it is comma and so on.

MY EFFORTS TO SOLVE THIS:

So far I have used subclassing to implement this. Here is my logic for implementing the subclassing, expressed through pseudo code:

if ( ( character is not a [ digit,separator, or CTRL/Shift... ] OR
     ( char is separator and we already have one ) )
{
    discard the character;
}

First I have made a helper function that determines if the char array already has a decimal separator, like this:

bool HasDecimalSeparator( wchar_t *test )
{
    // get the decimal separator
    wchar_t szBuffer[5];

    GetLocaleInfo ( LOCALE_USER_DEFAULT, 
                    LOCALE_SDECIMAL, 
                    szBuffer, 
                    sizeof(szBuffer) / sizeof(szBuffer[0] ) );

    bool p = false; // text already has decimal separator?
    size_t i = 0;   // needed for while loop-iterator

    // go through entire array and calculate the value of the p

    while( !( p = ( test[i] == szBuffer[0] ) ) && ( i++ < wcslen(test) ) );

    return p;
}

And here is the subclassing procedure-I haven't taken minus sign into account:

LRESULT CALLBACK Decimalni( HWND hwnd, UINT message, 
    WPARAM wParam, LPARAM lParam, 
    UINT_PTR uIdSubclass, 
    DWORD_PTR dwRefData )
{
    switch (message)
    {
    case WM_CHAR:
        {
            // get decimal separator
            wchar_t szBuffer[5];

            GetLocaleInfo ( LOCALE_USER_DEFAULT, 
                LOCALE_SDECIMAL, 
                szBuffer, 
                sizeof(szBuffer) / sizeof(szBuffer[0] ) );

                wchar_t t[50];  // here we store edit control's current text
                memset( &t, L'\0', sizeof(t) );

                // get edit control's current text
                GetWindowText( hwnd, t, 50 );

                // if ( ( is Not a ( digit,separator, or CTRL/Shift... )
                // || ( char is separator and we already have one ) )
                // discard the character

                if( ( !( isdigit(wParam) || ( wParam == szBuffer[0] ) ) 
                    && ( wParam >= L' ' ) )     // digit/separator/... ?
                    || ( HasDecimalSeparator(t)        // has separator?    
                    && ( wParam == szBuffer[0] ) ) )
                {
                    return 0;
                }
            }
            break;
    }
    return DefSubclassProc( hwnd, message, wParam, lParam);
}

One important note: I am able to load current user locale settings in my application, thanks to the answers to this question.

QUESTION:

Is there a better way to implement an edit control that accepts signed decimal numbers only, and is locale aware?

If subclassing is the only way, can my code be further improved/optimized ?

Thank you for your time and help.

Best regards.

APPENDIX:

To help you even further, here is a small demo application that creates an edit control and subclasses it to accept only decimal numbers-again, I haven't implemented the part for the minus sign:

#include <windows.h>
#include <commctrl.h>
#include <stdlib.h>
#include <locale.h>

#pragma comment( lib, "comctl32.lib")

const wchar_t g_szClassName[] = L"myWindowClass";

bool HasDecimalSeparator( wchar_t *test )
{
    // get the decimal separator
    wchar_t szBuffer[5];

    GetLocaleInfo ( LOCALE_USER_DEFAULT, 
                    LOCALE_SDECIMAL, 
                    szBuffer, 
                    sizeof(szBuffer) / sizeof(szBuffer[0] ) );

    bool p = false; // text already has decimal separator?
    size_t i = 0;   // needed for while loop-iterator

    // go through entire array and calculate the value of the p

    while( !( p = ( test[i] == szBuffer[0] ) ) && ( i++ < wcslen(test) ) );

    return p;
}

LRESULT CALLBACK Decimalni( HWND hwnd, UINT message, 
    WPARAM wParam, LPARAM lParam, 
    UINT_PTR uIdSubclass, 
    DWORD_PTR dwRefData )
{
    switch (message)
    {
    case WM_CHAR:
        {
            // get decimal separator
            wchar_t szBuffer[5];

            GetLocaleInfo ( LOCALE_USER_DEFAULT, 
                LOCALE_SDECIMAL, 
                szBuffer, 
                sizeof(szBuffer) / sizeof(szBuffer[0] ) );

                wchar_t t[50];  // here we store edit control's current text
                memset( &t, L'\0', sizeof(t) );

                // get edit control's current text
                GetWindowText( hwnd, t, 50 );

                // if ( ( is Not a ( digit,separator, or CTRL/Shift... )
                // || ( char is separator and we already have one ) )
                // discard the character

                if( ( !( isdigit(wParam) || ( wParam == szBuffer[0] ) ) 
                    && ( wParam >= L' ' ) )     // digit/separator/... ?
                    || ( HasDecimalSeparator(t)        // has separator?    
                    && ( wParam == szBuffer[0] ) ) )
                {
                    return 0;
                }
            }
            break;
    }
    return DefSubclassProc( hwnd, message, wParam, lParam);
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch(msg)
    {
    case WM_CREATE:
        {
            /************* load current locale settings *************/

            // max. len: language, country, code page

            wchar_t lpszLocale[64+64+16+3] = L""; 
            wchar_t lpszVal[128];

            LCID nLCID = ::GetUserDefaultLCID(); // current LCID for user
            if ( ::GetLocaleInfo( nLCID, LOCALE_SENGLANGUAGE, lpszVal, 128 ) )
            {
                wcscat_s( lpszLocale, 147, lpszVal ); // language
                if ( ::GetLocaleInfo( nLCID, LOCALE_SENGCOUNTRY, lpszVal, 128 ) )
                {
                    wcscat_s( lpszLocale, 147, L"_" ); // append country/region
                    wcscat_s( lpszLocale, 147, lpszVal );

                    if ( ::GetLocaleInfo( nLCID, 
                        LOCALE_IDEFAULTANSICODEPAGE, lpszVal, 128 ) )
                    { 
                        // missing code page or page number 0 is no error 
                        // (e.g. with Unicode)

                        int nCPNum = _wtoi(lpszVal);
                        if (nCPNum >= 10)
                        {
                            wcscat_s( lpszLocale, 147, L"." ); // append code page
                            wcscat_s( lpszLocale, 147, lpszVal );
                        }
                    }
                }
            }
            // set locale and LCID
            _wsetlocale( LC_ALL, lpszLocale );
            ::SetThreadLocale(nLCID);

            /*************************************************/

            HWND hEdit1;

            hEdit1 = CreateWindowEx(0, L"EDIT", L"", 
                WS_BORDER | WS_CHILD | WS_VISIBLE | ES_AUTOVSCROLL | ES_AUTOHSCROLL, 
                50, 100, 100, 20, 
                hwnd, (HMENU)8001, GetModuleHandle(NULL), NULL);

            SetWindowSubclass( hEdit1, Decimalni, 0, 0);

        }
        break;

    case WM_SETTINGCHANGE:
        if( !wParam && !wcscmp( (wchar_t*)lParam, L"intl" ) )
        {
            // max. len: language, country, code page
            wchar_t lpszLocale[64+64+16+3] = L""; 
            wchar_t lpszVal[128];

            LCID nLCID = ::GetUserDefaultLCID(); // current LCID for user
            if ( ::GetLocaleInfo( nLCID, LOCALE_SENGLANGUAGE, lpszVal, 128 ) )
            {
                wcscat_s( lpszLocale, 147, lpszVal ); // language
                if ( ::GetLocaleInfo( nLCID, LOCALE_SENGCOUNTRY, lpszVal, 128 ) )
                {
                    wcscat_s( lpszLocale, 147, L"_" ); // append country/region
                    wcscat_s( lpszLocale, 147, lpszVal );
                    if ( ::GetLocaleInfo( nLCID, 
                        LOCALE_IDEFAULTANSICODEPAGE, lpszVal, 128 ) )
                    { 
                        // missing code page or page number 0 is no error
                        // (e.g. with Unicode)
                        int nCPNum = _wtoi(lpszVal);
                        if (nCPNum >= 10)
                        {
                             wcscat_s( lpszLocale, 147, L"." ); // append code page
                             wcscat_s( lpszLocale, 147, lpszVal );
                        }
                    }
                 }
             }
             // set locale and LCID
             _wsetlocale( LC_ALL, lpszLocale );
             ::SetThreadLocale(nLCID);

             return 0L;
         }
         else
             break;

    case WM_CLOSE:
        DestroyWindow(hwnd);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hwnd, msg, wParam, lParam);
    }
    return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wc;
    HWND hwnd;
    MSG Msg;

    wc.cbSize        = sizeof(WNDCLASSEX);
    wc.style         = 0;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = g_szClassName;
    wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);

    if(!RegisterClassEx(&wc))
    {
        MessageBox(NULL, L"Window Registration Failed!", L"Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    hwnd = CreateWindowEx(
        0,
        g_szClassName,
        L"theForger's Tutorial Application",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 480, 320,
        NULL, NULL, hInstance, NULL);

    if(hwnd == NULL)
    {
        MessageBox(NULL, L"Window Creation Failed!", L"Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

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

Answer 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.