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improve documentation for 'var T return values'; refs #7373
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,20 @@ | ||
Memory safety for returning by ``var T`` is ensured by a simple borrowing | ||
rule: If ``result`` does not refer to a location pointing to the heap | ||
(that is in ``result = X`` the ``X`` involves a ``ptr`` or ``ref`` access) | ||
then it has to be deviated by the routine's first parameter: | ||
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||
.. code-block:: nim | ||
proc forward[T](x: var T): var T = | ||
result = x # ok, deviated from the first parameter. | ||
proc p(param: var int): var int = | ||
var x: int | ||
# we know 'forward' provides a view into the location deviated by | ||
# its first argument 'x'. | ||
result = forward(x) # Error: location is derived from ``x`` | ||
# which is not p's first parameter and lives | ||
# on the stack. | ||
In other words, the lifetime of what ``result`` points to is attached to the | ||
lifetime of the first parameter and that is enough knowledge to verify | ||
memory safety at the callsite. |
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