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Merge pull request #5025 from ethereum/refTypes

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[DOCS] Update reference types.
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chriseth 2018-09-24 13:54:19 +02:00 committed by GitHub
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@ -648,21 +648,27 @@ Another example that uses external function types::
.. index:: ! type;reference, ! reference type, storage, memory, location, array, struct
Reference Types
==================
===============
Complex types, i.e. types which do not always fit into 256 bits have to be handled
more carefully than the value-types we have already seen. Since copying
them can be quite expensive, we have to think about whether we want them to be
stored in **memory** (which is not persisting) or **storage** (where the state
variables are held).
Values of reference type can be modified through multiple different names.
Contrast this with value types where you get an independent copy whenever
a variable of value type is used. Because of that, reference types have to be handled
more carefully than value types. Currently, reference types comprise structs,
arrays and mappings. If you use a reference type, you always have to explicitly
provide the data area where the type is stored: ``memory`` (whose lifetime is limited
to a function call), ``storage`` (the location where the state variables are stored)
or ``calldata`` (special data location that contains the function arguments,
only available for external function call parameters).
An assignment or type conversion that changes the data location will always incur an automatic copy operation,
while assignments inside the same data location only copy in some cases for storage types.
.. _data-location:
Data location
-------------
Every complex type, i.e. *arrays* and *structs*, has an additional
Every reference type, i.e. *arrays* and *structs*, has an additional
annotation, the "data location", about where it is stored. There are three data locations:
``memory``, ``storage`` and ``calldata``. Calldata is only valid for parameters of external contract
functions and is required for this type of parameter. Calldata is a non-modifiable,