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Merge pull request #5797 from ethereum/faq-array-pass

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[DOCS] Remove copy between contracts FAQ item
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Chris Chinchilla 2019-01-21 12:57:01 +02:00 committed by GitHub
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25
docs/control-structures.rst
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@ -264,6 +264,31 @@ 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.
In the example below the call to ``g(x)`` has no effect on ``x`` because it needs
to create an independent copy of the storage value in memory. However ``h(x)`` modifies ``x`` because a reference and
not a copy is passed.
::
pragma solidity >=0.4.16 <0.6.0;
contract C {
uint[20] x;
function f() public {
g(x);
h(x);
}
function g(uint[20] memory y) internal pure {
y[2] = 3;
}
function h(uint[20] storage y) internal {
y[3] = 4;
}
}
.. index:: ! scoping, declarations, default value
.. _default-value:

30
docs/frequently-asked-questions.rst
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@ -96,36 +96,6 @@ In this example::
}
}
Can a contract pass an array (static size) or string or ``bytes`` (dynamic size) to another contract?
=====================================================================================================
Sure. Take care that if you cross the memory / storage boundary,
independent copies will be created::
pragma solidity >=0.4.16 <0.6.0;
contract C {
uint[20] x;
function f() public {
g(x);
h(x);
}
function g(uint[20] memory y) internal pure {
y[2] = 3;
}
function h(uint[20] storage y) internal {
y[3] = 4;
}
}
The call to ``g(x)`` will not have an effect on ``x`` because it needs
to create an independent copy of the storage value in memory.
On the other hand, ``h(x)`` successfully modifies ``x`` because only
a reference and not a copy is passed.
What does the following strange check do in the Custom Token contract?
======================================================================