I think these seem like valuable additions to nscore.h. It would be
helpful to extend these facilities that would allow more code to use the
Result-based programming model.
(I'm not too much of a fan of the NS_TRY name, but can't think of a
better name myself... :/ )
On 05/07/2017 03:34 PM, Kris Maglione wrote:
I've been trying to write most of my new code to use Result.h as much
as possible. When I have to mix Result-based code with nsresult-based
code, though, things tend to get ugly, and I wind up writing helpers.
At this point I've wound up writing the same helpers in 3 or 4
different places, so it may be time to try to standardize on something.
The helpers I've been using look something like:
template <>
class MOZ_MUST_USE_TYPE GenericErrorResult<nsresult>
{
nsresult mErrorValue;
template<typename V, typename E2> friend class Result;
public:
explicit GenericErrorResult(nsresult aErrorValue) :
mErrorValue(aErrorValue) {}
operator nsresult() { return mErrorValue; }
};
static inline Result<Ok, nsresult>
WrapNSResult(PRStatus aRv)
{
if (aRv != PR_SUCCESS) {
return Err(NS_ERROR_FAILURE);
}
return Ok();
}
static inline Result<Ok, nsresult>
WrapNSResult(nsresult aRv)
{
if (NS_FAILED(aRv)) {
return Err(aRv);
}
return Ok();
}
#define NS_TRY(expr) MOZ_TRY(WrapNSResult(expr))
And their use looks something like:
Result<nsCOMPtr<nsIFile>, nsresult>
GetFile(nsIFile* aBase, const char* aChild)
{
nsCOMPtr<nsIFile> file;
NS_TRY(aBase->Clone(getter_AddRefs(file)));
NS_TRY(aBase->AppendNative(aChild));
return Move(file);
}
nsresult
ReadFile(const char* aPath)
{
nsCOMPtr<nsIFile> file;
MOZ_TRY_VAR(file, GetFile(mBase, aPath));
PRFileDesc* fd;
NS_TRY(file->OpenNSPRFileDesc(PR_RDONLY, 0, &fd));
...
return NS_OK;
}
Where NS_TRY converts a nsresult or PRStatus to an appropriate Result
or GenericErrorResult, and the GenericErrorResult<nsresult>
specialization automatically converts an nsresult when used in
nsresult-based code.
Does this seem like the kind of thing we should implement in some
standard header, or would a different approach be better?
-Kris
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