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https://github.com/ethereum/solidity
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448 lines
18 KiB
ReStructuredText
###########################
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Frequently Asked Questions
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###########################
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This list was originally compiled by `fivedogit <mailto:fivedogit@gmail.com>`_.
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***************
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Basic Questions
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***************
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What is the transaction "payload"?
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==================================
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This is just the bytecode "data" sent along with the request.
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Create a contract that can be killed and return funds
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=====================================================
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First, a word of warning: Killing contracts sounds like a good idea, because "cleaning up"
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is always good, but as seen above, it does not really clean up. Furthermore,
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if Ether is sent to removed contracts, the Ether will be forever lost.
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If you want to deactivate your contracts, it is preferable to **disable** them by changing some
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internal state which causes all functions to throw. This will make it impossible
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to use the contract and ether sent to the contract will be returned automatically.
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Now to answering the question: Inside a constructor, ``msg.sender`` is the
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creator. Save it. Then ``selfdestruct(creator);`` to kill and return funds.
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`example <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/05_greeter.sol>`_
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Note that if you ``import "mortal"`` at the top of your contracts and declare
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``contract SomeContract is mortal { ...`` and compile with a compiler that already
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has it (which includes `Remix <https://remix.ethereum.org/>`_), then
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``kill()`` is taken care of for you. Once a contract is "mortal", then you can
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``contractname.kill.sendTransaction({from:eth.coinbase})``, just the same as my
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examples.
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Can you return an array or a ``string`` from a solidity function call?
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======================================================================
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Yes. See `array_receiver_and_returner.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/60_array_receiver_and_returner.sol>`_.
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Is it possible to in-line initialize an array like so: ``string[] myarray = ["a", "b"];``
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=========================================================================================
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Yes. However it should be noted that this currently only works with statically sized memory arrays. You can even create an inline memory
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array in the return statement.
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Example::
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pragma solidity >=0.4.16 <0.6.0;
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contract C {
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function f() public pure returns (uint8[5] memory) {
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string[4] memory adaArr = ["This", "is", "an", "array"];
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adaArr[0] = "That";
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return [1, 2, 3, 4, 5];
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}
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}
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Can a contract function return a ``struct``?
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============================================
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Yes, but only in ``internal`` function calls or if ``pragma experimental "ABIEncoderV2";`` is used.
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If I return an ``enum``, I only get integer values in web3.js. How to get the named values?
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===========================================================================================
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Enums are not supported by the ABI, they are just supported by Solidity.
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You have to do the mapping yourself for now, we might provide some help
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later.
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Can state variables be initialized in-line?
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===========================================
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Yes, this is possible for all types (even for structs). However, for arrays it
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should be noted that you must declare them as static memory arrays.
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Examples::
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pragma solidity >=0.4.0 <0.6.0;
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contract C {
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struct S {
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uint a;
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uint b;
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}
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S public x = S(1, 2);
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string name = "Ada";
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string[4] adaArr = ["This", "is", "an", "array"];
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}
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contract D {
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C c = new C();
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}
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How do structs work?
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====================
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See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_.
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How do for loops work?
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======================
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Very similar to JavaScript. Such as the following example:
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``for (uint i = 0; i < a.length; i ++) { a[i] = i; }``
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See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_.
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What are some examples of basic string manipulation (``substring``, ``indexOf``, ``charAt``, etc)?
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==================================================================================================
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There are some string utility functions at `stringUtils.sol <https://github.com/ethereum/dapp-bin/blob/master/library/stringUtils.sol>`_
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which will be extended in the future. In addition, Arachnid has written `solidity-stringutils <https://github.com/Arachnid/solidity-stringutils>`_.
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For now, if you want to modify a string (even when you only want to know its length),
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you should always convert it to a ``bytes`` first::
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pragma solidity >=0.4.0 <0.6.0;
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contract C {
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string s;
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function append(byte c) public {
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bytes(s).push(c);
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}
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function set(uint i, byte c) public {
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bytes(s)[i] = c;
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}
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}
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Can I concatenate two strings?
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==============================
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Yes, you can use ``abi.encodePacked``::
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pragma solidity >=0.4.0 <0.6.0;
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library ConcatHelper {
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function concat(bytes memory a, bytes memory b)
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internal pure returns (bytes memory) {
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return abi.encodePacked(a, b);
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}
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}
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Why is the low-level function ``.call()`` less favorable than instantiating a contract with a variable (``ContractB b;``) and executing its functions (``b.doSomething();``)?
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=============================================================================================================================================================================
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If you use actual functions, the compiler will tell you if the types
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or your arguments do not match, if the function does not exist
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or is not visible and it will do the packing of the
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arguments for you.
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See `ping.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol>`_ and
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`pong.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol>`_.
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When returning a value of say ``uint`` type, is it possible to return an ``undefined`` or "null"-like value?
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============================================================================================================
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This is not possible, because all types use up the full value range.
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You have the option to ``throw`` on error, which will also revert the whole
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transaction, which might be a good idea if you ran into an unexpected
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situation.
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If you do not want to throw, you can return a pair::
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pragma solidity >0.4.23 <0.6.0;
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contract C {
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uint[] counters;
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function getCounter(uint index)
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public
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view
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returns (uint counter, bool error) {
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if (index >= counters.length)
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return (0, true);
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else
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return (counters[index], false);
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}
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function checkCounter(uint index) public view {
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(uint counter, bool error) = getCounter(index);
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if (error) {
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// Handle the error
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} else {
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// Do something with counter.
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require(counter > 7, "Invalid counter value");
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}
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}
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}
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Are comments included with deployed contracts and do they increase deployment gas?
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==================================================================================
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No, everything that is not needed for execution is removed during compilation.
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This includes, among others, comments, variable names and type names.
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What happens if you send ether along with a function call to a contract?
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========================================================================
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It gets added to the total balance of the contract, just like when you send ether when creating a contract.
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You can only send ether along to a function that has the ``payable`` modifier,
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otherwise an exception is thrown.
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Is it possible to get a tx receipt for a transaction executed contract-to-contract?
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===================================================================================
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No, a function call from one contract to another does not create its own transaction,
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you have to look in the overall transaction. This is also the reason why several
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block explorer do not show Ether sent between contracts correctly.
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******************
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Advanced Questions
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******************
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How do you get a random number in a contract? (Implement a self-returning gambling contract.)
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=============================================================================================
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Getting randomness right is often the crucial part in a crypto project and
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most failures result from bad random number generators.
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If you do not want it to be safe, you build something similar to the `coin flipper <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/35_coin_flipper.sol>`_
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but otherwise, rather use a contract that supplies randomness, like the `RANDAO <https://github.com/randao/randao>`_.
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Get return value from non-constant function from another contract
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=================================================================
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The key point is that the calling contract needs to know about the function it intends to call.
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See `ping.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol>`_
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and `pong.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol>`_.
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How do you create 2-dimensional arrays?
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=======================================
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See `2D_array.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/55_2D_array.sol>`_.
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Note that filling a 10x10 square of ``uint8`` + contract creation took more than ``800,000``
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gas at the time of this writing. 17x17 took ``2,000,000`` gas. With the limit at
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3.14 million... well, there’s a pretty low ceiling for what you can create right
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now.
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Note that merely "creating" the array is free, the costs are in filling it.
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Note2: Optimizing storage access can pull the gas costs down considerably, because
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32 ``uint8`` values can be stored in a single slot. The problem is that these optimizations
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currently do not work across loops and also have a problem with bounds checking.
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You might get much better results in the future, though.
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What happens to a ``struct``'s mapping when copying over a ``struct``?
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======================================================================
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This is a very interesting question. Suppose that we have a contract field set up like such::
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struct User {
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mapping(string => string) comments;
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}
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function somefunction public {
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User user1;
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user1.comments["Hello"] = "World";
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User user2 = user1;
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}
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In this case, the mapping of the struct being copied over into ``user2`` is ignored as there is no "list of mapped keys".
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Therefore it is not possible to find out which values should be copied over.
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How do I initialize a contract with only a specific amount of wei?
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==================================================================
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Currently the approach is a little ugly, but there is little that can be done to improve it.
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In the case of a ``contract A`` calling a new instance of ``contract B``, parentheses have to be used around
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``new B`` because ``B.value`` would refer to a member of ``B`` called ``value``.
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You will need to make sure that you have both contracts aware of each other's presence and that ``contract B`` has a ``payable`` constructor.
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In this example::
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pragma solidity >0.4.99 <0.6.0;
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contract B {
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constructor() public payable {}
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}
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contract A {
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B child;
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function test() public {
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child = (new B).value(10)(); //construct a new B with 10 wei
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}
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}
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Can a contract function accept a two-dimensional array?
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=======================================================
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If you want to pass two-dimensional arrays across non-internal functions,
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you most likely need to use ``pragma experimental "ABIEncoderV2";``.
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What is the relationship between ``bytes32`` and ``string``? Why is it that ``bytes32 somevar = "stringliteral";`` works and what does the saved 32-byte hex value mean?
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========================================================================================================================================================================
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The type ``bytes32`` can hold 32 (raw) bytes. In the assignment ``bytes32 samevar = "stringliteral";``,
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the string literal is interpreted in its raw byte form and if you inspect ``somevar`` and
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see a 32-byte hex value, this is just ``"stringliteral"`` in hex.
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The type ``bytes`` is similar, only that it can change its length.
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Finally, ``string`` is basically identical to ``bytes`` only that it is assumed
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to hold the UTF-8 encoding of a real string. Since ``string`` stores the
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data in UTF-8 encoding it is quite expensive to compute the number of
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characters in the string (the encoding of some characters takes more
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than a single byte). Because of that, ``string s; s.length`` is not yet
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supported and not even index access ``s[2]``. But if you want to access
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the low-level byte encoding of the string, you can use
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``bytes(s).length`` and ``bytes(s)[2]`` which will result in the number
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of bytes in the UTF-8 encoding of the string (not the number of
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characters) and the second byte (not character) of the UTF-8 encoded
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string, respectively.
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Can a contract pass an array (static size) or string or ``bytes`` (dynamic size) to another contract?
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=====================================================================================================
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Sure. Take care that if you cross the memory / storage boundary,
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independent copies will be created::
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pragma solidity >=0.4.16 <0.6.0;
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contract C {
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uint[20] x;
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function f() public {
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g(x);
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h(x);
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}
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function g(uint[20] memory y) internal pure {
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y[2] = 3;
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}
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function h(uint[20] storage y) internal {
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y[3] = 4;
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}
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}
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The call to ``g(x)`` will not have an effect on ``x`` because it needs
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to create an independent copy of the storage value in memory.
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On the other hand, ``h(x)`` successfully modifies ``x`` because only
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a reference and not a copy is passed.
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Sometimes, when I try to change the length of an array with ex: ``arrayname.length = 7;`` I get a compiler error ``Value must be an lvalue``. Why?
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==================================================================================================================================================
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You can resize a dynamic array in storage (i.e. an array declared at the
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contract level) with ``arrayname.length = <some new length>;``. If you get the
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"lvalue" error, you are probably doing one of two things wrong.
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1. You might be trying to resize an array in "memory", or
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2. You might be trying to resize a non-dynamic array.
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::
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pragma solidity >=0.4.18 <0.6.0;
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// This will not compile
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contract C {
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int8[] dynamicStorageArray;
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int8[5] fixedStorageArray;
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function f() public {
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int8[] memory memArr; // Case 1
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memArr.length++; // illegal
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int8[5] storage storageArr = fixedStorageArray; // Case 2
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storageArr.length++; // illegal
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int8[] storage storageArr2 = dynamicStorageArray;
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storageArr2.length++; // legal
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}
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}
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**Important note:** In Solidity, array dimensions are declared backwards from the way you
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might be used to declaring them in C or Java, but they are access as in
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C or Java.
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For example, ``int8[][5] somearray;`` are 5 dynamic ``int8`` arrays.
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The reason for this is that ``T[5]`` is always an array of 5 ``T``'s,
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no matter whether ``T`` itself is an array or not (this is not the
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case in C or Java).
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Is it possible to return an array of strings (``string[]``) from a Solidity function?
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=====================================================================================
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Only when ``pragma experimental "ABIEncoderV2";`` is used.
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What does the following strange check do in the Custom Token contract?
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======================================================================
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::
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require((balanceOf[_to] + _value) >= balanceOf[_to]);
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Integers in Solidity (and most other machine-related programming languages) are restricted to a certain range.
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For ``uint256``, this is ``0`` up to ``2**256 - 1``. If the result of some operation on those numbers
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does not fit inside this range, it is truncated. These truncations can have
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`serious consequences <https://en.bitcoin.it/wiki/Value_overflow_incident>`_, so code like the one
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above is necessary to avoid certain attacks.
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Why are explicit conversions between fixed-size bytes types and integer types failing?
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======================================================================================
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Since version 0.5.0 explicit conversions between fixed-size byte arrays and integers are only allowed,
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if both types have the same size. This prevents unexpected behaviour when truncating or padding.
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Such conversions are still possible, but intermediate casts are required that make the desired
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truncation and padding convention explicit. See :ref:`types-conversion-elementary-types` for a full
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explanation and examples.
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Why can number literals not be converted to fixed-size bytes types?
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===================================================================
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Since version 0.5.0 only hexadecimal number literals can be converted to fixed-size bytes
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types and only if the number of hex digits matches the size of the type. See :ref:`types-conversion-literals`
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for a full explanation and examples.
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More Questions?
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===============
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If you have more questions or your question is not answered here, please talk to us on
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`gitter <https://gitter.im/ethereum/solidity>`_ or file an `issue <https://github.com/ethereum/solidity/issues>`_.
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