On Sat, Jun 23, 2012 at 2:13 AM, Nathan Ridge <zeratul976@hotmail.com>wrote:
By the way, my first thought when I encountered this problem is that C++11 ought to have specified the range-based for loop in such a way that the lifetime of temporaries in the range expression is extended for the duration of the loop.
Does anyone share this opinion? Would there be any downsides to this?
I, too, was thinking along the lines of Dave: "Well of course there would. Extending the lifetime of a non-trivial temporary beyond what's needed creates inefficiencies (memory pressure, register pressure, worse locality of reference...)" So, I'm not sure, either.
It is also interesting to note that boost::for_each together with a lambda - a from to which any range-based for loop can easily be converted - does not suffer from this problem. I think this corroborates the fact that the range-base for loop is suboptimally specified in the standard - after all the range- based for loop was meant to replace things like boost::for_each.
Maybe, like Dave suggested, there should be a language construct to preserve temporaries within an expression "beyond the semicolon" within a code block: some-block-keyword( x = expr ) { /*...no dangling references within x...*/ } - Jeff