Issue with Int to LPWSTR function

Question

I am making a win32 program that is a level editing tool to go with the library I am creating for a 2D tile system.

I want to create dialog box displaying the maps properties when the user selects it from the menu. This means a conversion from int to a wchar_t array. I have created a function that I hoped would do this. However currently it just returns a blank string that the return variable is initialized as. This conversion is necessary to work with the SetDlgItemText() function called by the map properties dialog box.

Here is the function I have currently:

LPWSTR IntToLPWSTR(int value)
{
    std::ostringstream  convert;
    std::string         out;

    convert << value;
    out = convert.str();

    const char*             in;
    in = out.c_str();

    LPWSTR ret = L"";
    MultiByteToWideChar(CP_ACP, MB_COMPOSITE, in, strlen(in), ret, wcslen(ret));

    return ret;
}

It is being called from here:

    case WM_INITDIALOG:
        if (mapToEdit)
        {
            SetDlgItemText(hDlg, IDC_TILE_WIDTH_LBL, IntToLPWSTR(mapToEdit->TileWidth()));
            SetDlgItemText(hDlg, IDC_TILE_HEIGHT_LBL, L"");
            SetDlgItemText(hDlg, IDC_MAP_WIDTH_LBL, L"");
            SetDlgItemText(hDlg, IDC_MAP_HEIGHT_LBL, L"");
        }
        else
        {
            EndDialog(hDlg, LOWORD(wParam));
            MessageBox(hWnd, L"You must create a map first", L"Error", 1);
        }

Map to edit is simply a pointer to my own map class that contains the properies I want to display. The bottom three calls to SetDlgItemText() pass L"" as their string, the intention is that they will also use the function when it works.

Answer 1

std::to_wstring is simpler, but to point out the problem in your code, you never created a buffer. LPWSTR ret = L""; makes ret a pointer to an array held in static memory. This array cannot be modified.

Here is one way to fix the code by using std::wstring as the buffer:

std::wstring IntToWstring(int value)
{
    std::ostringstream  convert;
    std::string         out;

    convert << value;
    out = convert.str();
    std::wstring ret;
    // Find proper length
    int length = MultiByteToWideChar(CP_ACP, 0, out.c_str(), out.length(), nullptr, 0);
    ret.resize(length);
    // Probably should also check for errors (got rid of MB_COMPOSITE flag)
    MultiByteToWideChar(CP_ACP, 0, out.c_str(), out.length(), &ret[0], length);

    return ret;
}

If you don't want to use std::wstring you could dynamically allocate a buffer LPWSTR ret = new LPWSTR[length];.

EDIT

Also, keep in mind that you could simplify the code to the following:

std::wstring IntToWstring(int value)
{
    std::wostringstream  convert;

    convert << value;
    convert.str();
}

Answer 2

You don't need to go to a lot of effort to convert an int into a const wchar_t *. Since C++11, you can take a two-step approach to a std::wstring and a const wchar_t * from there:

SetDlgItemText(hDlg, IDC_TILE_WIDTH_LBL, std::to_wstring(mapToEdit->TileWidth()).c_str());

Sure you could put that into a function to make it one step, but keep in mind that you cannot let the std::wstring be destroyed by the time you use the pointer.