Anatomy of smart contracts

1// Solidity example
2contract SimpleStorage {
3    uint storedData; // State variable
4    // ...
5}
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1# Vyper example
2storedData: int128
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1// Solidity example
2function update_name(string value) public {
3    dapp_name = value;
4}
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1// Solidity example
2function balanceOf(address _owner) public view returns (uint256 _balance) {
3    return ownerPizzaCount[_owner];
4}
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1dappName: public(string)
2
3@view
4@public
5def readName() -> string:
6  return dappName
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1// Solidity example
2// Initializes the contract's data, setting the `owner`
3// to the address of the contract creator.
4constructor() public {
5    // All smart contracts rely on external transactions to trigger its functions.
6    // `msg` is a global variable that includes relevant data on the given transaction,
7    // such as the address of the sender and the ETH value included in the transaction.
8    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
9    owner = msg.sender;
10}
11
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1# Vyper example
2
3@external
4def __init__(_beneficiary: address, _bidding_time: uint256):
5    self.beneficiary = _beneficiary
6    self.auctionStart = block.timestamp
7    self.auctionEnd = self.auctionStart + _bidding_time
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1pragma solidity >=0.4.0 <=0.6.0;
2
3contract ExampleDapp {
4    string dapp_name; // state variable
5
6    // Called when the contract is deployed and initializes the value
7    constructor() public {
8        dapp_name = "My Example dapp";
9    }
10
11    // Get Function
12    function read_name() public view returns(string) {
13        return dapp_name;
14    }
15
16    // Set Function
17    function update_name(string value) public {
18        dapp_name = value;
19    }
20}
21
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1// Specifies the version of Solidity, using semantic versioning.
2// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma
3pragma solidity ^0.5.10;
4
5// Defines a contract named `HelloWorld`.
6// A contract is a collection of functions and data (its state).
7// Once deployed, a contract resides at a specific address on the Ethereum blockchain.
8// Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html
9contract HelloWorld {
10
11    // Declares a state variable `message` of type `string`.
12    // State variables are variables whose values are permanently stored in contract storage.
13    // The keyword `public` makes variables accessible from outside a contract
14    // and creates a function that other contracts or clients can call to access the value.
15    string public message;
16
17    // Similar to many class-based object-oriented languages, a constructor is
18    // a special function that is only executed upon contract creation.
19    // Constructors are used to initialize the contract's data.
20    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constructors
21    constructor(string memory initMessage) public {
22        // Accepts a string argument `initMessage` and sets the value
23        // into the contract's `message` storage variable).
24        message = initMessage;
25    }
26
27    // A public function that accepts a string argument
28    // and updates the `message` storage variable.
29    function update(string memory newMessage) public {
30        message = newMessage;
31    }
32}
33
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1pragma solidity ^0.5.10;
2
3contract Token {
4    // An `address` is comparable to an email address - it's used to identify an account on Ethereum.
5    // Addresses can represent a smart contract or an external (user) accounts.
6    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#address
7    address public owner;
8
9    // A `mapping` is essentially a hash table data structure.
10    // This `mapping` assigns an unsigned integer (the token balance) to an address (the token holder).
11    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#mapping-types
12    mapping (address => uint) public balances;
13
14    // Events allow for logging of activity on the blockchain.
15    // Ethereum clients can listen for events in order to react to contract state changes.
16    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#events
17    event Transfer(address from, address to, uint amount);
18
19    // Initializes the contract's data, setting the `owner`
20    // to the address of the contract creator.
21    constructor() public {
22        // All smart contracts rely on external transactions to trigger its functions.
23        // `msg` is a global variable that includes relevant data on the given transaction,
24        // such as the address of the sender and the ETH value included in the transaction.
25        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
26        owner = msg.sender;
27    }
28
29    // Creates an amount of new tokens and sends them to an address.
30    function mint(address receiver, uint amount) public {
31        // `require` is a control structure used to enforce certain conditions.
32        // If a `require` statement evaluates to `false`, an exception is triggered,
33        // which reverts all changes made to the state during the current call.
34        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions
35
36        // Only the contract owner can call this function
37        require(msg.sender == owner, "You are not the owner.");
38
39        // Enforces a maximum amount of tokens
40        require(amount < 1e60, "Maximum issuance exceeded");
41
42        // Increases the balance of `receiver` by `amount`
43        balances[receiver] += amount;
44    }
45
46    // Sends an amount of existing tokens from any caller to an address.
47    function transfer(address receiver, uint amount) public {
48        // The sender must have enough tokens to send
49        require(amount <= balances[msg.sender], "Insufficient balance.");
50
51        // Adjusts token balances of the two addresses
52        balances[msg.sender] -= amount;
53        balances[receiver] += amount;
54
55        // Emits the event defined earlier
56        emit Transfer(msg.sender, receiver, amount);
57    }
58}
59
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1pragma solidity ^0.5.10;
2
3// Imports symbols from other files into the current contract.
4// In this case, a series of helper contracts from OpenZeppelin.
5// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#importing-other-source-files
6
7import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721.sol";
8import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
9import "../node_modules/@openzeppelin/contracts/introspection/ERC165.sol";
10import "../node_modules/@openzeppelin/contracts/math/SafeMath.sol";
11
12// The `is` keyword is used to inherit functions and keywords from external contracts.
13// In this case, `CryptoPizza` inherits from the `IERC721` and `ERC165` contracts.
14// Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#inheritance
15contract CryptoPizza is IERC721, ERC165 {
16    // Uses OpenZeppelin's SafeMath library to perform arithmetic operations safely.
17    // Learn more: https://docs.openzeppelin.com/contracts/2.x/api/math#SafeMath
18    using SafeMath for uint256;
19
20    // Constant state variables in Solidity are similar to other languages
21    // but you must assign from an expression which is constant at compile time.
22    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constant-state-variables
23    uint256 constant dnaDigits = 10;
24    uint256 constant dnaModulus = 10 ** dnaDigits;
25    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;
26
27    // Struct types let you define your own type
28    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#structs
29    struct Pizza {
30        string name;
31        uint256 dna;
32    }
33
34    // Creates an empty array of Pizza structs
35    Pizza[] public pizzas;
36
37    // Mapping from pizza ID to its owner's address
38    mapping(uint256 => address) public pizzaToOwner;
39
40    // Mapping from owner's address to number of owned token
41    mapping(address => uint256) public ownerPizzaCount;
42
43    // Mapping from token ID to approved address
44    mapping(uint256 => address) pizzaApprovals;
45
46    // You can nest mappings, this example maps owner to operator approvals
47    mapping(address => mapping(address => bool)) private operatorApprovals;
48
49    // Internal function to create a random Pizza from string (name) and DNA
50    function _createPizza(string memory _name, uint256 _dna)
51        // The `internal` keyword means this function is only visible
52        // within this contract and contracts that derive this contract
53        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#visibility-and-getters
54        internal
55        // `isUnique` is a function modifier that checks if the pizza already exists
56        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html#function-modifiers
57        isUnique(_name, _dna)
58    {
59        // Adds Pizza to array of Pizzas and get id
60        uint256 id = SafeMath.sub(pizzas.push(Pizza(_name, _dna)), 1);
61
62        // Checks that Pizza owner is the same as current user
63        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions
64        assert(pizzaToOwner[id] == address(0));
65
66        // Maps the Pizza to the owner
67        pizzaToOwner[id] = msg.sender;
68        ownerPizzaCount[msg.sender] = SafeMath.add(
69            ownerPizzaCount[msg.sender],
70            1
71        );
72    }
73
74    // Creates a random Pizza from string (name)
75    function createRandomPizza(string memory _name) public {
76        uint256 randDna = generateRandomDna(_name, msg.sender);
77        _createPizza(_name, randDna);
78    }
79
80    // Generates random DNA from string (name) and address of the owner (creator)
81    function generateRandomDna(string memory _str, address _owner)
82        public
83        // Functions marked as `pure` promise not to read from or modify the state
84        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#pure-functions
85        pure
86        returns (uint256)
87    {
88        // Generates random uint from string (name) + address (owner)
89        uint256 rand = uint256(keccak256(abi.encodePacked(_str))) +
90            uint256(_owner);
91        rand = rand % dnaModulus;
92        return rand;
93    }
94
95    // Returns array of Pizzas found by owner
96    function getPizzasByOwner(address _owner)
97        public
98        // Functions marked as `view` promise not to modify state
99        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#view-functions
100        view
101        returns (uint256[] memory)
102    {
103        // Uses the `memory` storage location to store values only for the
104        // lifecycle of this function call.
105        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/introduction-to-smart-contracts.html#storage-memory-and-the-stack
106        uint256[] memory result = new uint256[](ownerPizzaCount[_owner]);
107        uint256 counter = 0;
108        for (uint256 i = 0; i < pizzas.length; i++) {
109            if (pizzaToOwner[i] == _owner) {
110                result[counter] = i;
111                counter++;
112            }
113        }
114        return result;
115    }
116
117    // Transfers Pizza and ownership to other address
118    function transferFrom(address _from, address _to, uint256 _pizzaId) public {
119        require(_from != address(0) && _to != address(0), "Invalid address.");
120        require(_exists(_pizzaId), "Pizza does not exist.");
121        require(_from != _to, "Cannot transfer to the same address.");
122        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
123
124        ownerPizzaCount[_to] = SafeMath.add(ownerPizzaCount[_to], 1);
125        ownerPizzaCount[_from] = SafeMath.sub(ownerPizzaCount[_from], 1);
126        pizzaToOwner[_pizzaId] = _to;
127
128        // Emits event defined in the imported IERC721 contract
129        emit Transfer(_from, _to, _pizzaId);
130        _clearApproval(_to, _pizzaId);
131    }
132
133    /**
134     * Safely transfers the ownership of a given token ID to another address
135     * If the target address is a contract, it must implement `onERC721Received`,
136     * which is called upon a safe transfer, and return the magic value
137     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
138     * otherwise, the transfer is reverted.
139    */
140    function safeTransferFrom(address from, address to, uint256 pizzaId)
141        public
142    {
143        // solium-disable-next-line arg-overflow
144        this.safeTransferFrom(from, to, pizzaId, "");
145    }
146
147    /**
148     * Safely transfers the ownership of a given token ID to another address
149     * If the target address is a contract, it must implement `onERC721Received`,
150     * which is called upon a safe transfer, and return the magic value
151     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
152     * otherwise, the transfer is reverted.
153     */
154    function safeTransferFrom(
155        address from,
156        address to,
157        uint256 pizzaId,
158        bytes memory _data
159    ) public {
160        this.transferFrom(from, to, pizzaId);
161        require(_checkOnERC721Received(from, to, pizzaId, _data), "Must implmement onERC721Received.");
162    }
163
164    /**
165     * Internal function to invoke `onERC721Received` on a target address
166     * The call is not executed if the target address is not a contract
167     */
168    function _checkOnERC721Received(
169        address from,
170        address to,
171        uint256 pizzaId,
172        bytes memory _data
173    ) internal returns (bool) {
174        if (!isContract(to)) {
175            return true;
176        }
177
178        bytes4 retval = IERC721Receiver(to).onERC721Received(
179            msg.sender,
180            from,
181            pizzaId,
182            _data
183        );
184        return (retval == _ERC721_RECEIVED);
185    }
186
187    // Burns a Pizza - destroys Token completely
188    // The `external` function modifier means this function is
189    // part of the contract interface and other contracts can call it
190    function burn(uint256 _pizzaId) external {
191        require(msg.sender != address(0), "Invalid address.");
192        require(_exists(_pizzaId), "Pizza does not exist.");
193        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
194
195        ownerPizzaCount[msg.sender] = SafeMath.sub(
196            ownerPizzaCount[msg.sender],
197            1
198        );
199        pizzaToOwner[_pizzaId] = address(0);
200    }
201
202    // Returns count of Pizzas by address
203    function balanceOf(address _owner) public view returns (uint256 _balance) {
204        return ownerPizzaCount[_owner];
205    }
206
207    // Returns owner of the Pizza found by id
208    function ownerOf(uint256 _pizzaId) public view returns (address _owner) {
209        address owner = pizzaToOwner[_pizzaId];
210        require(owner != address(0), "Invalid Pizza ID.");
211        return owner;
212    }
213
214    // Approves other address to transfer ownership of Pizza
215    function approve(address _to, uint256 _pizzaId) public {
216        require(msg.sender == pizzaToOwner[_pizzaId], "Must be the Pizza owner.");
217        pizzaApprovals[_pizzaId] = _to;
218        emit Approval(msg.sender, _to, _pizzaId);
219    }
220
221    // Returns approved address for specific Pizza
222    function getApproved(uint256 _pizzaId)
223        public
224        view
225        returns (address operator)
226    {
227        require(_exists(_pizzaId), "Pizza does not exist.");
228        return pizzaApprovals[_pizzaId];
229    }
230
231    /**
232     * Private function to clear current approval of a given token ID
233     * Reverts if the given address is not indeed the owner of the token
234     */
235    function _clearApproval(address owner, uint256 _pizzaId) private {
236        require(pizzaToOwner[_pizzaId] == owner, "Must be pizza owner.");
237        require(_exists(_pizzaId), "Pizza does not exist.");
238        if (pizzaApprovals[_pizzaId] != address(0)) {
239            pizzaApprovals[_pizzaId] = address(0);
240        }
241    }
242
243    /*
244     * Sets or unsets the approval of a given operator
245     * An operator is allowed to transfer all tokens of the sender on their behalf
246     */
247    function setApprovalForAll(address to, bool approved) public {
248        require(to != msg.sender, "Cannot approve own address");
249        operatorApprovals[msg.sender][to] = approved;
250        emit ApprovalForAll(msg.sender, to, approved);
251    }
252
253    // Tells whether an operator is approved by a given owner
254    function isApprovedForAll(address owner, address operator)
255        public
256        view
257        returns (bool)
258    {
259        return operatorApprovals[owner][operator];
260    }
261
262    // Takes ownership of Pizza - only for approved users
263    function takeOwnership(uint256 _pizzaId) public {
264        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
265        address owner = this.ownerOf(_pizzaId);
266        this.transferFrom(owner, msg.sender, _pizzaId);
267    }
268
269    // Checks if Pizza exists
270    function _exists(uint256 pizzaId) internal view returns (bool) {
271        address owner = pizzaToOwner[pizzaId];
272        return owner != address(0);
273    }
274
275    // Checks if address is owner or is approved to transfer Pizza
276    function _isApprovedOrOwner(address spender, uint256 pizzaId)
277        internal
278        view
279        returns (bool)
280    {
281        address owner = pizzaToOwner[pizzaId];
282        // Disable solium check because of
283        // https://github.com/duaraghav8/Solium/issues/175
284        // solium-disable-next-line operator-whitespace
285        return (spender == owner ||
286            this.getApproved(pizzaId) == spender ||
287            this.isApprovedForAll(owner, spender));
288    }
289
290    // Check if Pizza is unique and doesn't exist yet
291    modifier isUnique(string memory _name, uint256 _dna) {
292        bool result = true;
293        for (uint256 i = 0; i < pizzas.length; i++) {
294            if (
295                keccak256(abi.encodePacked(pizzas[i].name)) ==
296                keccak256(abi.encodePacked(_name)) &&
297                pizzas[i].dna == _dna
298            ) {
299                result = false;
300            }
301        }
302        require(result, "Pizza with such name already exists.");
303        _;
304    }
305
306    // Returns whether the target address is a contract
307    function isContract(address account) internal view returns (bool) {
308        uint256 size;
309        // Currently there is no better way to check if there is a contract in an address
310        // than to check the size of the code at that address.
311        // See https://ethereum.stackexchange.com/a/14016/36603
312        // for more details about how this works.
313        // TODO Check this again before the Serenity release, because all addresses will be
314        // contracts then.
315        // solium-disable-next-line security/no-inline-assembly
316        assembly {
317            size := extcodesize(account)
318        }
319        return size > 0;
320    }
321}
322
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