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diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst index acc0c106..43fba332 100644 --- a/docs/frequently-asked-questions.rst +++ b/docs/frequently-asked-questions.rst @@ -9,33 +9,12 @@ This list was originally compiled by `fivedogit <mailto:fivedogit@gmail.com>`_. Basic Questions *************** -What is Solidity? +Example contracts ================= -Solidity is the DEV-created (i.e. Ethereum Foundation-created), -Javascript-inspired language that can be used to create smart contracts -on the Ethereum blockchain. There are other -languages you can use as well (LLL, Serpent, etc). The main points in -favour of Solidity is that it is statically typed and offers many -advanced features like inheritance, libraries, complex -user-defined types and a bytecode optimizer. - -Solidity contracts can be compiled a few different ways (see below) and the -resulting output can be cut/pasted into a geth console to deploy them to the -Ethereum blockchain. - There are some `contract examples <https://github.com/fivedogit/solidity-baby-steps/tree/master/contracts/>`_ by fivedogit and there should be a `test contract <https://github.com/ethereum/solidity/blob/develop/test/libsolidity/SolidityEndToEndTest.cpp>`_ for every single feature of Solidity. -How do I compile contracts? -=========================== - -Probably the fastest way is the `online compiler <https://ethereum.github.io/browser-solidity/>`_. - -You can also use the ``solc`` binary which comes with cpp-ethereum to compile -contracts or an emerging option is to use Mix, the IDE. - - Create and publish the most basic contract possible =================================================== @@ -71,13 +50,6 @@ several blockchain explorers. Contracts on the blockchain should have their original source code published if they are to be used by third parties. -Does ``selfdestruct()`` free up space in the blockchain? -======================================================== - -It removes the contract bytecode and storage from the current block -into the future, but since the blockchain stores every single block (i.e. -all history), this will not actually free up space on full/archive nodes. - Create a contract that can be killed and return funds ===================================================== @@ -113,32 +85,6 @@ Use a non-constant function (req ``sendTransaction``) to increment a variable in See `value_incrementer.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/20_value_incrementer.sol>`_. -Get contract address in Solidity -================================ - -Short answer: The global variable ``this`` is the contract address. - -See `basic_info_getter <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/15_basic_info_getter.sol>`_. - -Long answer: ``this`` is a variable representing the current contract. -Its type is the type of the contract. Since any contract type basically inherits from the -``address`` type, ``this`` is always convertible to ``address`` and in this case contains -its own address. - -What is the difference between a function marked ``constant`` and one that is not? -================================================================================== - -``constant`` functions can perform some action and return a value, but cannot -change state (this is not yet enforced by the compiler). In other words, a -constant function cannot save or update any variables within the contract or wider -blockchain. These functions are called using ``c.someFunction(...)`` from -geth or any other web3.js environment. - -"non-constant" functions (those lacking the ``constant`` specifier) must be called -with ``c.someMethod.sendTransaction({from:eth.accounts[x], gas: 1000000});`` -That is, because they can change state, they have to have a gas -payment sent along to get the work done. - Get a contract to return its funds to you (not using ``selfdestruct(...)``). ============================================================================ @@ -146,52 +92,6 @@ This example demonstrates how to send funds from a contract to an address. See `endowment_retriever <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/30_endowment_retriever.sol>`_. -What is a ``mapping`` and how do we use them? -============================================= - -A mapping is very similar to a K->V hashmap. -If you have a state variable of type ``mapping (string -> uint) x;``, then you can -access the value by ``x["somekeystring"]``. - -How can I get the length of a ``mapping``? -========================================== - -Mappings are a rather low-level data structure. It does not store the keys -and it is not possible to know which or how many values are "set". Actually, -all values to all possible keys are set by default, they are just -initialised with the zero value. - -In this sense, the attribute ``length`` for a mapping does not really apply. - -If you want to have a "sized mapping", you can use the iterable mapping -(see below) or just a dynamically-sized array of structs. - -Are ``mapping``'s iterable? -=========================== - -Mappings themselves are not iterable, but you can use a higher-level -datastructure on top of it, for example the `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_. - -Can I put arrays inside of a ``mapping``? How do I make a ``mapping`` of a ``mapping``? -======================================================================================= - -Mappings are already syntactically similar to arrays as they are, therefore it doesn't make much sense to store an array in them. Rather what you should do is create a mapping of a mapping. - -An example of this would be:: - - contract C { - struct myStruct { - uint someNumber; - string someString; - } - - mapping(uint => mapping(string => myStruct)) myDynamicMapping; - - function storeInMapping() { - myDynamicMapping[1]["Foo"] = myStruct(2, "Bar"); - } - } - Can you return an array or a ``string`` from a solidity function call? ====================================================================== @@ -223,61 +123,6 @@ Example:: } } -What are ``event``'s and why do we need them? -============================================= - -Let us suppose that you need a contract to alert the outside world when -something happens. The contract can fire an event, which can be listened to -from web3 (inside geth or a web application). The main advantage of events -is that they are stored in a special way on the blockchain so that it -is very easy to search for them. - -What are the different function visibilities? -============================================= - -The visibility specifiers do not only change the visibility but also -the way functions can be called. In general, functions in the -same contract can also be called internally (which is cheaper -and allows for memory types to be passed by reference). This -is done if you just use ``f(1,2)``. If you use ``this.f(1,2)`` -or ``otherContract.f(1,2)``, the function is called externally. - -Internal function calls have the advantage that you can use -all Solidity types as parameters, but you have to stick to the -simpler ABI types for external calls. - -* ``external``: all, only externally - -* ``public``: all (this is the default), externally and internally - -* ``internal``: only this contract and contracts deriving from it, only internally - -* ``private``: only this contract, only internally - - -Do contract constructors have to be publicly visible? -===================================================== - -You can use the visibility specifiers, but they do not yet have any effect. -The constructor is removed from the contract code once it is deployed, - -Can a contract have multiple constructors? -========================================== - -No, a contract can have only one constructor. - -More specifically, it can only have one function whose name matches -that of the constructor. - -Having multiple constructors with different number of arguments -or argument types, as it is possible in other languages -is not allowed in Solidity. - -Is a constructor required? -========================== - -No. If there is no constructor, a generic one without arguments and no actions will be used. - Are timestamps (``now,`` ``block.timestamp``) reliable? ======================================================= @@ -363,14 +208,6 @@ Examples:: C c = new C(); } -What is the ``modifier`` keyword? -================================= - -Modifiers are a way to prepend or append code to a function in order -to add guards, initialisation or cleanup functionality in a concise way. - -For examples, see the `features.sol <https://github.com/ethereum/dapp-bin/blob/master/library/features.sol>`_. - How do structs work? ==================== @@ -590,12 +427,6 @@ The correct way to do this is the following:: } } -Can a regular (i.e. non-contract) ethereum account be closed permanently like a contract can? -============================================================================================= - -No. Non-contract accounts "exist" as long as the private key is known by -someone or can be generated in some way. - What is the difference between ``bytes`` and ``byte[]``? ======================================================== @@ -641,16 +472,6 @@ Use the constructor. Anything inside it will be executed when the contract is fi See `replicator.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/50_replicator.sol>`_. -Can a contract create another contract? -======================================= - -Yes, see `replicator.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/50_replicator.sol>`_. - -Note that the full code of the created contract has to be included in the creator contract. -This also means that cyclic creations are not possible (because the contract would have -to contain its own code) - at least not in a general way. - - How do you create 2-dimensional arrays? ======================================= @@ -709,10 +530,12 @@ How do I initialize a contract with only a specific amount of wei? Currently the approach is a little ugly, but there is little that can be done to improve it. In the case of a ``contract A`` calling a new instance of ``contract B``, parentheses have to be used around ``new B`` because ``B.value`` would refer to a member of ``B`` called ``value``. -You will need to make sure that you have both contracts aware of each other's presence. +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:: - contract B {} + contract B { + function B() payable {} + } contract A { |