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Merge pull request #8089 from ethereum/array-copy

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[DOCS] Clarify array copying semantics
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chriseth 2020-02-20 16:15:17 +01:00 committed by GitHub
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docs/control-structures.rst
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@ -343,18 +343,8 @@ i.e. the following is not valid: ``(x, uint y) = (1, 2);``
Complications for Arrays and Structs
------------------------------------
The semantics of assignments are a bit more complicated for
non-value types like arrays and structs.
Assigning *to* a state variable always creates an independent
copy. On the other hand, assigning to a local variable creates
an independent copy only for elementary types, i.e. static
types that fit into 32 bytes. If structs or arrays (including
``bytes`` and ``string``) are assigned from a state variable
to a local variable, the local variable holds a reference to
the original state variable. A second assignment to the local
variable does not modify the state but only changes the
reference. Assignments to members (or elements) of the local
variable *do* change the state.
The semantics of assignments are more complicated for non-value types like arrays and structs,
including ``bytes`` and ``string``, see :ref:`Data location and assignment behaviour <data-location-assignment>` for details.
In the example below the call to ``g(x)`` has no effect on ``x`` because it creates
an independent copy of the storage value in memory. However, ``h(x)`` successfully modifies ``x``