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https://github.com/ethereum/solidity
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110 lines
3.9 KiB
ReStructuredText
110 lines
3.9 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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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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******************
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Advanced Questions
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******************
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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 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 ^0.5.0;
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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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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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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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