Re: [boost] [rfc] rcpp

Subject: Re: [boost] [rfc] rcpp
From: Daniel Trebbien (dtrebbien_at_[hidden])
Date: 2010-02-28 18:13:47

> What do you think about the following functions?
>
> rcpp::win::file_handle clone(rcpp::win::file_handle const & f)
> {
>    HANDLE h;
>    BOOL r = ::DuplicateHandle(GetCurrentProcess(),
>                               f.get(),
>                               GetCurrentProcess(),
>                               &h,
>                               0,
>                               FALSE,
>                               DUPLICATE_SAME_ACCESS);
>
>    if (r == FALSE)
>       return rcpp::win::file_handle(); // or throw an exception
>
>    return rcpp::win::file_handle(h);
> }
>
> rcpp::win::kernel_handle clone(rcpp::win::kernel_handle const & f)
> {
>    HANDLE h;
>    BOOL r = ::DuplicateHandle(GetCurrentProcess(),
>                               f.get(),
>                               GetCurrentProcess(),
>                               &h,
>                               0,
>                               FALSE,
>                               DUPLICATE_SAME_ACCESS);
>
>    if (r == FALSE)
>       return rcpp::win::kernel_handle(); // or throw an exception
>
>    return rcpp::win::kernel_handle(h);
> }
>

In `clone(rcpp::win::file_handle const &)`, what if `h` is  
`INVALID_HANDLE_VALUE` and `r` is not `FALSE` (`DuplicateHandle`  
succeeded, but the resulting `HANDLE` was `INVALID_HANDLE_VALUE`)? Then  
the function would return an "empty" `rcpp::win::file_handle` and the  
`INVALID_HANDLE_VALUE` `HANDLE` would never be closed. Similarly, if  
`DuplicateHandle` could set `h` to `NULL` in  
`clone(rcpp::win::kernel_handle const &)`, then the `NULL` `HANDLE` would  
never be closed.

I know, you're thinking that this is picky. Yes, it's really picky, but I  
can't help from thinking about this case, especially because the MSDN  
documentation does not definitively state that this can not happen. I also  
looked at the documentation for `CloseHandle` to see if it can not accept  
`NULL` or `INVALID_HANDLE_VALUE`, but there is nothing on that issue,  
either. (If `CloseHandle` did not accept `NULL`, for example, then  
`DuplicateHandle` could never use `NULL` for the resulting `HANDLE` value.)

I like the solution that you had, which is to use  
`boost::optional<HANDLE>` with invalid value `boost::none`.