2019-01-07 10:58:11 +00:00
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.. index:: ! visibility, external, public, private, internal
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.. _visibility-and-getters:
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**********************
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Visibility and Getters
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**********************
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2019-12-12 09:56:06 +00:00
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Solidity knows two kinds of function calls: internal
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2019-01-07 10:58:11 +00:00
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ones that do not create an actual EVM call (also called
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a "message call") and external
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2019-12-12 09:56:06 +00:00
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ones that do. Because of that, there are four types of visibility for
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2019-01-07 10:58:11 +00:00
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functions and state variables.
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Functions have to be specified as being ``external``,
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``public``, ``internal`` or ``private``.
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For state variables, ``external`` is not possible.
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2020-04-20 16:24:31 +00:00
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``external``
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2019-01-07 10:58:11 +00:00
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External functions are part of the contract interface,
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which means they can be called from other contracts and
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via transactions. An external function ``f`` cannot be called
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internally (i.e. ``f()`` does not work, but ``this.f()`` works).
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External functions are sometimes more efficient when
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2019-12-12 09:56:06 +00:00
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they receive large arrays of data, because the data
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is not copied from calldata to memory.
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2019-01-07 10:58:11 +00:00
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2020-04-20 16:24:31 +00:00
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``public``
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2019-01-07 10:58:11 +00:00
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Public functions are part of the contract interface
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and can be either called internally or via
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messages. For public state variables, an automatic getter
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function (see below) is generated.
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2020-04-20 16:24:31 +00:00
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``internal``
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2019-01-07 10:58:11 +00:00
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Those functions and state variables can only be
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accessed internally (i.e. from within the current contract
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or contracts deriving from it), without using ``this``.
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2020-04-20 16:24:31 +00:00
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``private``
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2019-01-07 10:58:11 +00:00
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Private functions and state variables are only
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visible for the contract they are defined in and not in
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derived contracts.
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.. note::
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Everything that is inside a contract is visible to
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all observers external to the blockchain. Making something ``private``
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2019-05-15 22:56:21 +00:00
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only prevents other contracts from reading or modifying
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2019-01-07 10:58:11 +00:00
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the information, but it will still be visible to the
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whole world outside of the blockchain.
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The visibility specifier is given after the type for
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state variables and between parameter list and
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return parameter list for functions.
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2019-03-05 17:10:09 +00:00
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pragma solidity >=0.4.16 <0.7.0;
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2019-01-07 10:58:11 +00:00
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contract C {
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function f(uint a) private pure returns (uint b) { return a + 1; }
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function setData(uint a) internal { data = a; }
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uint public data;
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}
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In the following example, ``D``, can call ``c.getData()`` to retrieve the value of
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``data`` in state storage, but is not able to call ``f``. Contract ``E`` is derived from
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``C`` and, thus, can call ``compute``.
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2020-01-28 15:34:38 +00:00
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pragma solidity >=0.4.16 <0.7.0;
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contract C {
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uint private data;
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function f(uint a) private pure returns(uint b) { return a + 1; }
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function setData(uint a) public { data = a; }
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function getData() public view returns(uint) { return data; }
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function compute(uint a, uint b) internal pure returns (uint) { return a + b; }
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}
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// This will not compile
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contract D {
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function readData() public {
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C c = new C();
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uint local = c.f(7); // error: member `f` is not visible
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c.setData(3);
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local = c.getData();
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local = c.compute(3, 5); // error: member `compute` is not visible
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}
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}
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contract E is C {
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function g() public {
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C c = new C();
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uint val = compute(3, 5); // access to internal member (from derived to parent contract)
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}
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}
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.. index:: ! getter;function, ! function;getter
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.. _getter-functions:
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Getter Functions
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================
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The compiler automatically creates getter functions for
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all **public** state variables. For the contract given below, the compiler will
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generate a function called ``data`` that does not take any
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arguments and returns a ``uint``, the value of the state
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variable ``data``. State variables can be initialized
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when they are declared.
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2020-01-28 15:34:38 +00:00
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pragma solidity >=0.4.16 <0.7.0;
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contract C {
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uint public data = 42;
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}
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contract Caller {
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C c = new C();
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function f() public view returns (uint) {
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return c.data();
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}
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}
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The getter functions have external visibility. If the
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symbol is accessed internally (i.e. without ``this.``),
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it evaluates to a state variable. If it is accessed externally
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(i.e. with ``this.``), it evaluates to a function.
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2019-03-05 17:10:09 +00:00
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pragma solidity >=0.4.0 <0.7.0;
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2019-01-07 10:58:11 +00:00
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contract C {
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uint public data;
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function x() public returns (uint) {
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data = 3; // internal access
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return this.data(); // external access
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}
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}
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If you have a ``public`` state variable of array type, then you can only retrieve
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single elements of the array via the generated getter function. This mechanism
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exists to avoid high gas costs when returning an entire array. You can use
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arguments to specify which individual element to return, for example
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``data(0)``. If you want to return an entire array in one call, then you need
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to write a function, for example:
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2020-01-28 15:34:38 +00:00
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pragma solidity >=0.4.16 <0.7.0;
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2019-10-23 20:13:17 +00:00
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contract arrayExample {
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// public state variable
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uint[] public myArray;
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2019-01-07 10:58:11 +00:00
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2019-10-23 20:13:17 +00:00
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// Getter function generated by the compiler
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/*
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function myArray(uint i) public view returns (uint) {
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return myArray[i];
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}
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*/
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2019-01-07 10:58:11 +00:00
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2019-10-23 20:13:17 +00:00
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// function that returns entire array
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function getArray() public view returns (uint[] memory) {
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return myArray;
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}
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}
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Now you can use ``getArray()`` to retrieve the entire array, instead of
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``myArray(i)``, which returns a single element per call.
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The next example is more complex:
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::
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2020-05-13 15:45:58 +00:00
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// SPDX-License-Identifier: GPL-3.0
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2019-03-05 17:10:09 +00:00
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pragma solidity >=0.4.0 <0.7.0;
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2019-01-07 10:58:11 +00:00
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contract Complex {
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struct Data {
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uint a;
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bytes3 b;
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mapping (uint => uint) map;
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}
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mapping (uint => mapping(bool => Data[])) public data;
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}
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It generates a function of the following form. The mapping in the struct is omitted
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because there is no good way to provide the key for the mapping:
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::
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function data(uint arg1, bool arg2, uint arg3) public returns (uint a, bytes3 b) {
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a = data[arg1][arg2][arg3].a;
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b = data[arg1][arg2][arg3].b;
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}
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