1145258 - Create a version of LogicalSize::.ConvertTo that returns a const reference, to reduce copying

Status: RESOLVED INVALID

(Core :: Layout, defect)

Description

[Daniel Holbert [:dholbert]]

Right now, LogicalSize::ConvertTo always requires a copy. (It returns type "LogicalSize".)

It seems like we should optimize for the common case where the "from" & "to" writing-mode are the same, and no copy is needed. (In that case, we can just return a const reference to "this").

I think we can do this & still return a temporary LogicalSize if we change the return type to "const LogicalSize&". Then, in the uncommon case where the "to" writing-mode is different, the C++ rule about const-reference-prolonging-the-life-of-a-temporary will kick in & make it OK to return a reference to a local variable.

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: attempted fix (causes fatal build warning)

Actually, this might be impossible... I tried to write a trivial fix for this, but I think I was misremembering how const-reference-to-temporary lifetime-extension works.  (I recall that it's weird & slightly-counterintuitive, but it apparently doesn't work how I thought I remembered it working.)

I thought this might do it, but clang gives me this warning (treated as an error in this directory)...
> WritingModes.h:706:12: error: returning reference to local temporary object [-Werror,-Wreturn-stack-address]

...for the "return LogicalPoint(...)" statement.

Comment 2

[Daniel Holbert [:dholbert]]

Resolving as INVALID, but if it turns out this is possible, feel free to reopen & post a working fix/strategy.