Are there gotchas using varargs with reference parameters

AnsiString working(AnsiString format,...)
{
    va_list argptr;
    AnsiString buff;

    va_start(argptr, format);
    buff.vprintf(format.c_str(), argptr);

    va_end(argptr);
    return buff;
}

AnsiString broken(const AnsiString &format,...)
{
... the rest, totally identical ...
}

AnsiString s1, s2;
    s1 = working("Hello %s", "World");
    s2 = broken("Hello %s", "World");

va_start(argptr, format);

argptr = (va_list) (&format+1);

AnsiString working_ptr(const AnsiString *format,...)
{
    ASSERT(format != NULL);
    va_list argptr;
    AnsiString buff;

    va_start(argptr, format);
    buff.vprintf(format->c_str(), argptr);

    va_end(argptr);
    return buff;
}

...

AnsiString format = "Hello %s";
s1 = working_ptr(&format, "World");

AnsiString working_dummy(const AnsiString &format, int dummy, ...)
{
    va_list argptr;
    AnsiString buff;

    va_start(argptr, dummy);
    buff.vprintf(format.c_str(), argptr);

    va_end(argptr);
    return buff;
}

...

s1 = working_dummy("Hello %s", 0, "World");

alloca(sizeof(class));
memcpy(stack, &instance, sizeof(class);