diff options
Diffstat (limited to 'docs/frequently-asked-questions.rst')
| -rw-r--r-- | docs/frequently-asked-questions.rst | 121 |
1 files changed, 1 insertions, 120 deletions
diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst index d263e0c6..00d9e043 100644 --- a/docs/frequently-asked-questions.rst +++ b/docs/frequently-asked-questions.rst @@ -16,11 +16,6 @@ Enums are not supported by the ABI, they are just supported by Solidity. You have to do the mapping yourself for now, we might provide some help later. -How do structs work? -==================== - -See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_. - What are some examples of basic string manipulation (``substring``, ``indexOf``, ``charAt``, etc)? ================================================================================================== @@ -59,44 +54,10 @@ Yes, you can use ``abi.encodePacked``:: } } - -Why is the low-level function ``.call()`` less favorable than instantiating a contract with a variable (``ContractB b;``) and executing its functions (``b.doSomething();``)? -============================================================================================================================================================================= - -If you use actual functions, the compiler will tell you if the types -or your arguments do not match, if the function does not exist -or is not visible and it will do the packing of the -arguments for you. - -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>`_. - -Are comments included with deployed contracts and do they increase deployment gas? -================================================================================== - -No, everything that is not needed for execution is removed during compilation. -This includes, among others, comments, variable names and type names. - -What happens if you send ether along with a function call to a contract? -======================================================================== - -It gets added to the total balance of the contract, just like when you send ether when creating a contract. -You can only send ether along to a function that has the ``payable`` modifier, -otherwise an exception is thrown. - ****************** Advanced Questions ****************** -How do you get a random number in a contract? (Implement a self-returning gambling contract.) -============================================================================================= - -Getting randomness right is often the crucial part in a crypto project and -most failures result from bad random number generators. - -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>`_ -but otherwise, rather use a contract that supplies randomness, like the `RANDAO <https://github.com/randao/randao>`_. - Get return value from non-constant function from another contract ================================================================= @@ -105,23 +66,6 @@ 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>`_. -How do you create 2-dimensional arrays? -======================================= - -See `2D_array.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/55_2D_array.sol>`_. - -Note that filling a 10x10 square of ``uint8`` + contract creation took more than ``800,000`` -gas at the time of this writing. 17x17 took ``2,000,000`` gas. With the limit at -3.14 million... well, there’s a pretty low ceiling for what you can create right -now. - -Note that merely "creating" the array is free, the costs are in filling it. - -Note2: Optimizing storage access can pull the gas costs down considerably, because -32 ``uint8`` values can be stored in a single slot. The problem is that these optimizations -currently do not work across loops and also have a problem with bounds checking. -You might get much better results in the future, though. - How do I initialize a contract with only a specific amount of wei? ================================================================== @@ -131,7 +75,7 @@ In the case of a ``contract A`` calling a new instance of ``contract B``, parent 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. In this example:: - pragma solidity >0.4.99 <0.6.0; + pragma solidity ^0.5.0; contract B { constructor() public payable {} @@ -145,69 +89,6 @@ In this example:: } } -Can a contract pass an array (static size) or string or ``bytes`` (dynamic size) to another contract? -===================================================================================================== - -Sure. Take care that if you cross the memory / storage boundary, -independent copies will be created:: - - pragma solidity >=0.4.16 <0.6.0; - - contract C { - uint[20] x; - - function f() public { - g(x); - h(x); - } - - function g(uint[20] memory y) internal pure { - y[2] = 3; - } - - function h(uint[20] storage y) internal { - y[3] = 4; - } - } - -The call to ``g(x)`` will not have an effect on ``x`` because it needs -to create an independent copy of the storage value in memory. -On the other hand, ``h(x)`` successfully modifies ``x`` because only -a reference and not a copy is passed. - -What does the following strange check do in the Custom Token contract? -====================================================================== - -:: - - require((balanceOf[_to] + _value) >= balanceOf[_to]); - -Integers in Solidity (and most other machine-related programming languages) are restricted to a certain range. -For ``uint256``, this is ``0`` up to ``2**256 - 1``. If the result of some operation on those numbers -does not fit inside this range, it is truncated. These truncations can have -`serious consequences <https://en.bitcoin.it/wiki/Value_overflow_incident>`_, so code like the one -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? =============== |