On Fri, Jun 5, 2015 at 3:11 AM, Steven D'Aprano <st...@pearwood.info> wrote: > You need at least one more test to prove pass by value: you need to > demonstrate that the value bound to y is copied when passed to the > function. E.g. pass a mutable value (say, a list) and mutate it inside the > function. If the list in the outer scope is *not* mutated, then and only > then can you say it is pass by value.
Minor quibble: This discussion is about semantics, not timing. A language can use pass-by-value but with optimizations for the read-only case. $ cat array_argument.php <?php function readonly($arr) {if ($arr[1] != 1) echo "WRONG!\n";} function readwrite($arr) {$arr[1] = 1;} $array = array(); //Populate a big array for ($i=0; $i<1000; ++$i) $array[$i] = $i; //Now pass it repeatedly as an argument $start = microtime(1); for ($i=0; $i<100000000; ++$i) readonly($array); $i/=1000000; $t = (microtime(1)-$start); echo "Read-only: $i million iterations in $t seconds\n"; $start = microtime(1); for ($i=0; $i<100000; ++$i) readwrite($array); $i/=1000; $t = (microtime(1)-$start); echo "Read-write: $i thousand iterations in $t seconds\n"; $ time php array_argument.php Read-only: 100 million iterations in 11.251608133316 seconds Read-write: 100 thousand iterations in 2.6384630203247 seconds real 0m13.914s user 0m13.780s sys 0m0.000s PHP passes arrays by value, but optimizes away the copy in the read-only case - which means that array mutations can trigger unexpected copying. But it's still pass-by-value, even though you can appear to have the performance of pass-by-object/pass-by-reference. Conceivably, a language might implement pass-by-value by having a "shadow" that knows that it's "the same as that array only this one got changed", although I can't imagine it'd be efficient for real-world work. It'd still be legitimate pass-by-value though. ChrisA -- https://mail.python.org/mailman/listinfo/python-list