mirror of
https://github.com/ethereum/solidity
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118 lines
4.7 KiB
ReStructuredText
118 lines
4.7 KiB
ReStructuredText
.. index:: ! contract;creation, constructor
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******************
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Creating Contracts
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******************
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Contracts can be created "from outside" via Ethereum transactions or from within Solidity contracts.
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IDEs, such as `Remix <https://remix.ethereum.org/>`_, make the creation process seamless using UI elements.
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Creating contracts programmatically on Ethereum is best done via using the JavaScript API `web3.js <https://github.com/ethereum/web3.js>`_.
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It has a function called `web3.eth.Contract <https://web3js.readthedocs.io/en/1.0/web3-eth-contract.html#new-contract>`_
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to facilitate contract creation.
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When a contract is created, its :ref:`constructor <constructor>` (a function declared with the ``constructor`` keyword) is executed once.
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A constructor is optional. Only one constructor is allowed, which means
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overloading is not supported.
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After the constructor has executed, the final code of the contract is deployed to the
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blockchain. This code includes all public and external functions and all functions
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that are reachable from there through function calls. The deployed code does not
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include the constructor code or internal functions only called from the constructor.
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.. index:: constructor;arguments
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Internally, constructor arguments are passed :ref:`ABI encoded <ABI>` after the code of
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the contract itself, but you do not have to care about this if you use ``web3.js``.
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If a contract wants to create another contract, the source code
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(and the binary) of the created contract has to be known to the creator.
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This means that cyclic creation dependencies are impossible.
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::
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pragma solidity >=0.4.22 <0.6.0;
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contract OwnedToken {
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// `TokenCreator` is a contract type that is defined below.
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// It is fine to reference it as long as it is not used
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// to create a new contract.
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TokenCreator creator;
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address owner;
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bytes32 name;
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// This is the constructor which registers the
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// creator and the assigned name.
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constructor(bytes32 _name) public {
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// State variables are accessed via their name
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// and not via e.g. `this.owner`. Functions can
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// be accessed directly or through `this.f`,
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// but the latter provides an external view
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// to the function. Especially in the constructor,
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// you should not access functions externally,
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// because the function does not exist yet.
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// See the next section for details.
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owner = msg.sender;
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// We do an explicit type conversion from `address`
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// to `TokenCreator` and assume that the type of
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// the calling contract is `TokenCreator`, there is
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// no real way to check that.
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creator = TokenCreator(msg.sender);
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name = _name;
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}
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function changeName(bytes32 newName) public {
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// Only the creator can alter the name --
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// the comparison is possible since contracts
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// are explicitly convertible to addresses.
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if (msg.sender == address(creator))
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name = newName;
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}
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function transfer(address newOwner) public {
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// Only the current owner can transfer the token.
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if (msg.sender != owner) return;
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// We ask the creator contract if the transfer
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// should proceed by using a function of the
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// `TokenCreator` contract defined below. If
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// the call fails (e.g. due to out-of-gas),
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// the execution also fails here.
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if (creator.isTokenTransferOK(owner, newOwner))
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owner = newOwner;
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}
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}
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contract TokenCreator {
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function createToken(bytes32 name)
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public
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returns (OwnedToken tokenAddress)
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{
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// Create a new `Token` contract and return its address.
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// From the JavaScript side, the return type is
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// `address`, as this is the closest type available in
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// the ABI.
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return new OwnedToken(name);
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}
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function changeName(OwnedToken tokenAddress, bytes32 name) public {
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// Again, the external type of `tokenAddress` is
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// simply `address`.
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tokenAddress.changeName(name);
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}
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// Perform checks to determine if transferring a token to the
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// `OwnedToken` contract should proceed
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function isTokenTransferOK(address currentOwner, address newOwner)
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public
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pure
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returns (bool ok)
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{
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// Check an arbitrary condition to see if transfer should proceed
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return keccak256(abi.encodePacked(currentOwner, newOwner))[0] == 0x7f;
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}
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}
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