Qore Programming Language Reference Manual: References

Qore Programming Language Reference Manual 0.8.11

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References

References allow for addressing other lvalues, including complex lvalue expressions, with an alias called a reference. References to lvalues are defined by placing a `"\"` character in front of an expression that gives an lvalue, such as in the following examples:

my reference $r1 = \$a;
my reference $r2 = \$recs[$rn].order[$on];

References are especially convenient when aliasing an internal part of a complex data structure. Consider the following example (a list of records, where each record is a hash with a list of orders under the `"order"` key - in the following example, we want to set the last record's last order's `"numberofitems"` key to be equal to the number of elements under the order's `"items"` key):

$recs[$recs.size() - 1].order[$recs[$recs.size() - 1].order.size() - 1].numberofitems = $recs[$recs.size() - 1].order[$recs[$recs.size() - 1].order.size() - 1].items.size();

In the above code, using references could greatly simplfy the readability of the code:

my reference $lastrec = \$recs[$recs.size() - 1];
my reference $lastorder = \$lastrec.order[$lastrec.order.size() - 1];
$lastorder.numberofitems = $lastorder.items.size();

Note
  local variables that are referenced are automatically converted to a special type of local variable that is protected by a mutual-exclusion thread lock so that the reference can be safely used in background threads. The lvalue represented by the local variable will exist beyond its local scope; as long as the reference to the lvalue exists, the local variable's lvalue will exist and remain valid.

Since
  Qore 0.8.5 universal lvalue references are supported; previously references could only be used with arguments to a function or method call or the like. As of Qore 0.8.5+, references can be used anywhere in Qore in any expression, even with references to local variables in background expressions and in closure argument lists.