On 3/11/14, 8:04 PM, Manu wrote:
I'm really trying to keep my lid on here...
Yep, here we go again :o).
I'll just remind that in regard to this particular point which sounds reasonable, it's easy to forgot that *all library code where the author didn't care* is now unusable by anybody who does. The converse would not be true if the situation was reversed.
There's an asymmetry introduced by the fact there's today code in use.
virtual-by-default is incompatible with optimisation, and it's reliable to assume that anybody who doesn't explicitly care about this will stick with the default, which means many potentially useful libraries may be eliminated for use by many customers.
Virtual by default is, however, compatible with customization and flexibility.
Unstated assumption: "many potential useful libraries" assumes many libraries use traditional OO design in their core components.
Unstated assumption: "many customers".
Also, as discussed at length, revoking virtual from a function is a breaking change, adding virtual is not.
Changing the default is a breaking change.
Which means that, instead of making a controlled breaking change with a clear migration path here and now, we are committing every single instance of any user's intent to 'optimise' their libraries (by finalising unnecessarily virtuals) to breaking changes in their ABI - which *will* occur, since virtual is the default.
Unstated assumption: "every single instance" assumes again that people interested in writing fast libraries have virtual calls as a major bottleneck, and furthermore they didn't care about speed to start with, to wake up later. This pictures library designers as quite incompetent people.
According to semantic versioning, this requires bumping the major version number... that's horrible!
Appeal to emotion.
What's better; implementing a controlled deprecation path now, or leaving it up to any project that ever uses the 'class' keyword to eventually confront breaking changes in their API when they encounter a performance oriented customer?
It's better to leave things be. All I see is the same anecdote gets vividly told again whenever the topic comes up.
Andrei
