[WinApi/C++] konwersja typów char/string na int/float i na odwrót

#include <windows.h>
#include<string>
#include<sstream>

#define ID_OBLICZANIE 501

/*  Declare Windows procedure  */
LRESULT CALLBACK WindowProcedure( HWND, UINT, WPARAM, LPARAM );

/*  Make the class name into a global variable  */
char szClassName[] = "CodeBlocksWindowsApp";

HWND hwnd, a, b, c, obliczanie;

int WINAPI WinMain( HINSTANCE hThisInstance,
HINSTANCE hPrevInstance,
LPSTR lpszArgument,
int nCmdShow )
{
    //standardowy kod do inicjalizacji okna z dodaniem:
    a = CreateWindowEx( WS_EX_CLIENTEDGE, "EDIT", "a", WS_CHILD | WS_VISIBLE | WS_BORDER, 20, 60, 50, 30, hwnd, NULL, hThisInstance, NULL );
    b = CreateWindowEx( WS_EX_CLIENTEDGE, "EDIT", "b", WS_CHILD | WS_VISIBLE | WS_BORDER, 80, 60, 50, 30, hwnd, NULL, hThisInstance, NULL );
    c = CreateWindowEx( WS_EX_CLIENTEDGE, "EDIT", "c", WS_CHILD | WS_VISIBLE | WS_BORDER, 140, 60, 50, 30, hwnd, NULL, hThisInstance, NULL );
    obliczanie = CreateWindowEx( WS_EX_CLIENTEDGE, "BUTTON", "Obliczanie", WS_CHILD | WS_VISIBLE | WS_BORDER, 20, 100, 150, 30, hwnd,( HMENU ) ID_OBLICZANIE, hThisInstance, NULL );
   
}

LRESULT CALLBACK WindowProcedure( HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam )
{
    switch( message ) /* handle the messages */
    {
    case WM_DESTROY:
        PostQuitMessage( 0 ); /* send a WM_QUIT to the message queue */
        break;
       
    case WM_COMMAND:
        switch( wParam )
        {
           
        case ID_OBLICZANIE:
           
            INT liczba;
           
            CHAR skladnik = SendMessage( a, CB_GETCURSEL, 0, 0 );
           
            liczba = skladnik - '0';
           
            liczba = liczba + 12;
           
            std::ostringstream ss;
            ss << liczba;
            LPCSTR wynik = ss.str().c_str();
            MessageBox( hwnd, wynik, "Wynik", MB_ICONINFORMATION );
           
            break;
        }
        break;
        default: /* for messages that we don't deal with */
        return DefWindowProc( hwnd, message, wParam, lParam );
    }
   
    return 0;
}