As of the landing of bug 1366511, it's now possible to use MOZ_TRY with
and from functions that return nsresult values. This means that, among
other things, it's now easy to use mozilla::Result from XPCOM methods,
and vice versa, as long as the Result error type is nsresult:
#include "mozilla/ResultExtensions.h"
Result<nsCOMPtr<nsIFile>, nsresult>
OpenFile(const nsACtring& aPath)
{
nsCOMPtr<nsIFile> file;
MOZ_TRY(NS_NewLocalFile(aPath, false, getter_AddRefs(file)));
return Move(file);
}
nsresult
Thing::DoStuff()
{
nsCOMPtr<nsIFile> file;
MOZ_TRY_VAR(file, OpenFile(mPath));
MOZ_TRY(file->GetNSPRFileDescriptor(...));
...
return NS_OK;
}
This works thanks to a mozilla::ToResult() helper, which MOZ_TRY calls
to convert arbitrary types to Result<T, E> (which incidentally also
supports PRStatus by default), and GenericErrorResult<nsresult> type
that auto-converts to nsresult.
As a side note, there's a particular optimization for the type
Result<Ok, nsresult> so that it occupies the same amount of space as a
bare nsresult, and code like this:
Result<Ok, nsresult> MaybeThing();
Result<Ok, nsresult>
Thing()
{
MOZ_TRY(MaybeThing());
return Ok();
}
compiles to essentially the same machine code as:
nsresult MaybeThing();
nsresult
Thing()
{
nsresult rv = MaybeThing();
if (rv != NS_OK) {
return rv;
}
return NS_OK;
}
With the difference that the only success value MaybeThing() is allowed
to return is NS_OK.
_______________________________________________
dev-platform mailing list
dev-platform@lists.mozilla.org
https://lists.mozilla.org/listinfo/dev-platform
