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Diffstat (limited to 'docs/frequently-asked-questions.rst')
| -rw-r--r-- | docs/frequently-asked-questions.rst | 61 |
1 files changed, 26 insertions, 35 deletions
diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst index 786c3341..d2b7de9c 100644 --- a/docs/frequently-asked-questions.rst +++ b/docs/frequently-asked-questions.rst @@ -62,7 +62,8 @@ Example:: contract C { function f() public pure returns (uint8[5] memory) { string[4] memory adaArr = ["This", "is", "an", "array"]; - return ([1, 2, 3, 4, 5]); + adaArr[0] = "That"; + return [1, 2, 3, 4, 5]; } } @@ -186,9 +187,10 @@ If you do not want to throw, you can return a pair:: function checkCounter(uint index) public view { (uint counter, bool error) = getCounter(index); if (error) { - // ... + // Handle the error } else { - // ... + // Do something with counter. + require(counter > 7, "Invalid counter value"); } } } @@ -236,13 +238,6 @@ The key point is that the calling contract needs to know about the function it i See `ping.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol>`_ and `pong.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol>`_. -Get contract to do something when it is first mined -=================================================== - -Use the constructor. Anything inside it will be executed when the contract is first mined. - -See `replicator.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/50_replicator.sol>`_. - How do you create 2-dimensional arrays? ======================================= @@ -372,15 +367,14 @@ contract level) with ``arrayname.length = <some new length>;``. If you get the :: - // This will not compile - pragma solidity ^0.4.18; + // This will not compile contract C { int8[] dynamicStorageArray; int8[5] fixedStorageArray; - function f() { + function f() public { int8[] memory memArr; // Case 1 memArr.length++; // illegal @@ -409,28 +403,6 @@ Is it possible to return an array of strings (``string[]``) from a Solidity func Not yet, as this requires two levels of dynamic arrays (``string`` is a dynamic array itself). -If you issue a call for an array, it is possible to retrieve the whole array? Or must you write a helper function for that? -=========================================================================================================================== - -The automatic :ref:`getter function<getter-functions>` for a public state variable of array type only returns -individual elements. If you want to return the complete array, you have to -manually write a function to do that. - - -What could have happened if an account has storage value(s) but no code? Example: http://test.ether.camp/account/5f740b3a43fbb99724ce93a879805f4dc89178b5 -========================================================================================================================================================== - -The last thing a constructor does is returning the code of the contract. -The gas costs for this depend on the length of the code and it might be -that the supplied gas is not enough. This situation is the only one -where an "out of gas" exception does not revert changes to the state, -i.e. in this case the initialisation of the state variables. - -https://github.com/ethereum/wiki/wiki/Subtleties - -After a successful CREATE operation's sub-execution, if the operation returns x, 5 * len(x) gas is subtracted from the remaining gas before the contract is created. If the remaining gas is less than 5 * len(x), then no gas is subtracted, the code of the created contract becomes the empty string, but this is not treated as an exceptional condition - no reverts happen. - - What does the following strange check do in the Custom Token contract? ====================================================================== @@ -445,6 +417,25 @@ does not fit inside this range, it is truncated. These truncations can have above is necessary to avoid certain attacks. +Why are explicit conversions between fixed-size bytes types and integer types failing? +====================================================================================== + +Since version 0.5.0 explicit conversions between fixed-size byte arrays and integers are only allowed, +if both types have the same size. This prevents unexpected behaviour when truncating or padding. +Such conversions are still possible, but intermediate casts are required that make the desired +truncation and padding convention explicit. See :ref:`types-conversion-elementary-types` for a full +explanation and examples. + + +Why can number literals not be converted to fixed-size bytes types? +=================================================================== + +Since version 0.5.0 only hexadecimal number literals can be converted to fixed-size bytes +types and only if the number of hex digits matches the size of the type. See :ref:`types-conversion-literals` +for a full explanation and examples. + + + More Questions? =============== |