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Merge pull request #5085 from ethereum/updatefaq

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[DOCS] Update FAQ
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Alex Beregszaszi 2018-09-26 18:43:16 +01:00 committed by GitHub
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docs/frequently-asked-questions.rst
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@ -43,17 +43,11 @@ Can you return an array or a ``string`` from a solidity function call?
Yes. See `array_receiver_and_returner.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/60_array_receiver_and_returner.sol>`_. Yes. See `array_receiver_and_returner.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/60_array_receiver_and_returner.sol>`_.
What is problematic, though, is returning any variably-sized data (e.g. a
variably-sized array like ``uint[]``) from a function **called from within Solidity**.
This is a limitation of the EVM and will be solved with the next protocol update.
Returning variably-sized data as part of an external transaction or call is fine.
Is it possible to in-line initialize an array like so: ``string[] myarray = ["a", "b"];`` Is it possible to in-line initialize an array like so: ``string[] myarray = ["a", "b"];``
========================================================================================= =========================================================================================
Yes. However it should be noted that this currently only works with statically sized memory arrays. You can even create an inline memory Yes. However it should be noted that this currently only works with statically sized memory arrays. You can even create an inline memory
array in the return statement. Pretty cool, huh? array in the return statement.
Example:: Example::
@ -70,7 +64,7 @@ Example::
Can a contract function return a ``struct``? Can a contract function return a ``struct``?
============================================ ============================================
Yes, but only in ``internal`` function calls. Yes, but only in ``internal`` function calls or if ``pragma experimental "ABIEncoderV2";`` is used.
If I return an ``enum``, I only get integer values in web3.js. How to get the named values? If I return an ``enum``, I only get integer values in web3.js. How to get the named values?
=========================================================================================== ===========================================================================================
@ -145,7 +139,17 @@ you should always convert it to a ``bytes`` first::
Can I concatenate two strings? Can I concatenate two strings?
============================== ==============================
You have to do it manually for now. Yes, you can use ``abi.encodePacked``::
pragma solidity >=0.4.0 <0.6.0;
library ConcatHelper {
function concat(bytes memory a, bytes memory b)
internal pure returns (bytes memory) {
return abi.encodePacked(a, b);
}
}
Why is the low-level function ``.call()`` less favorable than instantiating a contract with a variable (``ContractB b;``) and executing its functions (``b.doSomething();``)? Why is the low-level function ``.call()`` less favorable than instantiating a contract with a variable (``ContractB b;``) and executing its functions (``b.doSomething();``)?
============================================================================================================================================================================= =============================================================================================================================================================================
@ -299,8 +303,8 @@ In this example::
Can a contract function accept a two-dimensional array? Can a contract function accept a two-dimensional array?
======================================================= =======================================================
This is not yet implemented for external calls and dynamic arrays - If you want to pass two-dimensional arrays across non-internal functions,
you can only use one level of dynamic arrays. you most likely need to use ``pragma experimental "ABIEncoderV2";``.
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? 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?
======================================================================================================================================================================== ========================================================================================================================================================================
@ -401,7 +405,7 @@ case in C or Java).
Is it possible to return an array of strings (``string[]``) from a Solidity function? Is it possible to return an array of strings (``string[]``) from a Solidity function?
===================================================================================== =====================================================================================
Not yet, as this requires two levels of dynamic arrays (``string`` is a dynamic array itself). Only when ``pragma experimental "ABIEncoderV2";`` is used.
What does the following strange check do in the Custom Token contract? What does the following strange check do in the Custom Token contract?
====================================================================== ======================================================================