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https://github.com/ethereum/solidity
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216 lines
8.7 KiB
ReStructuredText
216 lines
8.7 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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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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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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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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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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******************
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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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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 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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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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