Anatomy of smart contracts

A smart contract is a program that runs at an address on Ethereum. They're made up of data and functions that can execute upon receiving a transaction. Here's an overview of what makes up a smart contract.

Prerequisites

Make sure you've read about smart contracts first. This document assumes you're already familiar with programming languages such as JavaScript or Python.

Data

Any contract data must be assigned to a location: either to storage or memory. It's costly to modify storage in a smart contract so you need to consider where your data should live.

Storage

Persistent data is referred to as storage and is represented by state variables. These values get stored permanently on the blockchain. You need to declare the type so that the contract can keep track of how much storage on the blockchain it needs when it compiles.

1// Solidity example
2contract SimpleStorage {
3    uint storedData; // State variable
4    // ...
5}
 Kopy

1# Vyper example
2storedData: int128
 Kopy

If you've already programmed object-oriented languages, you'll likely be familiar with most types. However address should be new to you if you're new to Ethereum development.

An address type can hold an Ethereum address which equates to 20 bytes or 160 bits. It returns in hexadecimal notation with a leading 0x.

Other types include:

boolean
integer
fixed point numbers
fixed-size byte arrays
dynamically-sized byte arrays
Rational and integer literals
String literals
Hexadecimal literals
Enums

For more explanation, take a look at the docs:

Memory

Values that are only stored for the lifetime of a contract function's execution are called memory variables. Since these are not stored permanently on the blockchain, they are much cheaper to use.

Learn more about how the EVM stores data (Storage, Memory, and the Stack) in the Solidity docs(opens in a new tab).

Environment variables

In addition to the variables you define on your contract, there are some special global variables. They are primarily used to provide information about the blockchain or current transaction.

Examples:

Prop	State variable	Description
`block.timestamp`	uint256	Current block epoch timestamp
`msg.sender`	address	Sender of the message (current call)

Functions

In the most simplistic terms, functions can get information or set information in response to incoming transactions.

There are two types of function calls:

internal – these don't create an EVM call
- Internal functions and state variables can only be accessed internally (i.e. from within the current contract or contracts deriving from it)
external – these do create an EVM call
- External functions are part of the contract interface, which means they can be called from other contracts and via transactions. An external function f cannot be called internally (i.e. f() does not work, but this.f() works).

They can also be public or private

public functions can be called internally from within the contract or externally via messages
private functions are only visible for the contract they are defined in and not in derived contracts

Both functions and state variables can be made public or private

Here's a function for updating a state variable on a contract:

1// Solidity example
2function update_name(string value) public {
3    dapp_name = value;
4}
 Kopy

The parameter value of type string is passed into the function: update_name
It's declared public, meaning anyone can access it
It's not declared view, so it can modify the contract state

View functions

These functions promise not to modify the state of the contract's data. Common examples are "getter" functions – you might use this to receive a user's balance for example.

1// Solidity example
2function balanceOf(address _owner) public view returns (uint256 _balance) {
3    return ownerPizzaCount[_owner];
4}
 Kopy

1dappName: public(string)
2
3@view
4@public
5def readName() -> string:
6  return dappName
 Kopy

What is considered modifying state:

Writing to state variables.
Emitting events(opens in a new tab).
Creating other contracts(opens in a new tab).
Using selfdestruct.
Sending ether via calls.
Calling any function not marked view or pure.
Using low-level calls.
Using inline assembly that contains certain opcodes.

Constructor functions

constructor functions are only executed once when the contract is first deployed. Like constructor in many class-based programming languages, these functions often initialize state variables to their specified values.

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}
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 Kopy

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
 Kopy

Built-in functions

In addition to the variables and functions you define on your contract, there are some special built-in functions. The most obvious example is:

address.send() – Solidity
send(address) – Vyper

These allow contracts to send ETH to other accounts.

Writing functions

Your function needs:

parameter variable and type (if it accepts parameters)
declaration of internal/external
declaration of pure/view/payable
returns type (if it returns a value)

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}
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 Kopy

A complete contract might look something like this. Here the constructor function provides an initial value for the dapp_name variable.

Events and logs

Events enable your smart contract to communicate with your frontend or other subscribing applications. Once a transaction is validated and added to a block, smart contracts can emit events and log information, which the frontend can then process and utilize.

Annotated examples

These are some examples written in Solidity. If you'd like to play with the code, you can interact with them in Remix(opens in a new tab).

Hello world

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}
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 Kopy

Token

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}
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Unique digital asset

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
65        // note that address(0) is the zero address,
66        // indicating that pizza[id] is not yet allocated to a particular user.
67
68        assert(pizzaToOwner[id] == address(0));
69
70        // Maps the Pizza to the owner
71        pizzaToOwner[id] = msg.sender;
72        ownerPizzaCount[msg.sender] = SafeMath.add(
73            ownerPizzaCount[msg.sender],
74            1
75        );
76    }
77
78    // Creates a random Pizza from string (name)
79    function createRandomPizza(string memory _name) public {
80        uint256 randDna = generateRandomDna(_name, msg.sender);
81        _createPizza(_name, randDna);
82    }
83
84    // Generates random DNA from string (name) and address of the owner (creator)
85    function generateRandomDna(string memory _str, address _owner)
86        public
87        // Functions marked as `pure` promise not to read from or modify the state
88        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#pure-functions
89        pure
90        returns (uint256)
91    {
92        // Generates random uint from string (name) + address (owner)
93        uint256 rand = uint256(keccak256(abi.encodePacked(_str))) +
94            uint256(_owner);
95        rand = rand % dnaModulus;
96        return rand;
97    }
98
99    // Returns array of Pizzas found by owner
100    function getPizzasByOwner(address _owner)
101        public
102        // Functions marked as `view` promise not to modify state
103        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#view-functions
104        view
105        returns (uint256[] memory)
106    {
107        // Uses the `memory` storage location to store values only for the
108        // lifecycle of this function call.
109        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/introduction-to-smart-contracts.html#storage-memory-and-the-stack
110        uint256[] memory result = new uint256[](ownerPizzaCount[_owner]);
111        uint256 counter = 0;
112        for (uint256 i = 0; i < pizzas.length; i++) {
113            if (pizzaToOwner[i] == _owner) {
114                result[counter] = i;
115                counter++;
116            }
117        }
118        return result;
119    }
120
121    // Transfers Pizza and ownership to other address
122    function transferFrom(address _from, address _to, uint256 _pizzaId) public {
123        require(_from != address(0) && _to != address(0), "Invalid address.");
124        require(_exists(_pizzaId), "Pizza does not exist.");
125        require(_from != _to, "Cannot transfer to the same address.");
126        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
127
128        ownerPizzaCount[_to] = SafeMath.add(ownerPizzaCount[_to], 1);
129        ownerPizzaCount[_from] = SafeMath.sub(ownerPizzaCount[_from], 1);
130        pizzaToOwner[_pizzaId] = _to;
131
132        // Emits event defined in the imported IERC721 contract
133        emit Transfer(_from, _to, _pizzaId);
134        _clearApproval(_to, _pizzaId);
135    }
136
137    /**
138     * Safely transfers the ownership of a given token ID to another address
139     * If the target address is a contract, it must implement `onERC721Received`,
140     * which is called upon a safe transfer, and return the magic value
141     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
142     * otherwise, the transfer is reverted.
143    */
144    function safeTransferFrom(address from, address to, uint256 pizzaId)
145        public
146    {
147        // solium-disable-next-line arg-overflow
148        this.safeTransferFrom(from, to, pizzaId, "");
149    }
150
151    /**
152     * Safely transfers the ownership of a given token ID to another address
153     * If the target address is a contract, it must implement `onERC721Received`,
154     * which is called upon a safe transfer, and return the magic value
155     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
156     * otherwise, the transfer is reverted.
157     */
158    function safeTransferFrom(
159        address from,
160        address to,
161        uint256 pizzaId,
162        bytes memory _data
163    ) public {
164        this.transferFrom(from, to, pizzaId);
165        require(_checkOnERC721Received(from, to, pizzaId, _data), "Must implement onERC721Received.");
166    }
167
168    /**
169     * Internal function to invoke `onERC721Received` on a target address
170     * The call is not executed if the target address is not a contract
171     */
172    function _checkOnERC721Received(
173        address from,
174        address to,
175        uint256 pizzaId,
176        bytes memory _data
177    ) internal returns (bool) {
178        if (!isContract(to)) {
179            return true;
180        }
181
182        bytes4 retval = IERC721Receiver(to).onERC721Received(
183            msg.sender,
184            from,
185            pizzaId,
186            _data
187        );
188        return (retval == _ERC721_RECEIVED);
189    }
190
191    // Burns a Pizza - destroys Token completely
192    // The `external` function modifier means this function is
193    // part of the contract interface and other contracts can call it
194    function burn(uint256 _pizzaId) external {
195        require(msg.sender != address(0), "Invalid address.");
196        require(_exists(_pizzaId), "Pizza does not exist.");
197        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
198
199        ownerPizzaCount[msg.sender] = SafeMath.sub(
200            ownerPizzaCount[msg.sender],
201            1
202        );
203        pizzaToOwner[_pizzaId] = address(0);
204    }
205
206    // Returns count of Pizzas by address
207    function balanceOf(address _owner) public view returns (uint256 _balance) {
208        return ownerPizzaCount[_owner];
209    }
210
211    // Returns owner of the Pizza found by id
212    function ownerOf(uint256 _pizzaId) public view returns (address _owner) {
213        address owner = pizzaToOwner[_pizzaId];
214        require(owner != address(0), "Invalid Pizza ID.");
215        return owner;
216    }
217
218    // Approves other address to transfer ownership of Pizza
219    function approve(address _to, uint256 _pizzaId) public {
220        require(msg.sender == pizzaToOwner[_pizzaId], "Must be the Pizza owner.");
221        pizzaApprovals[_pizzaId] = _to;
222        emit Approval(msg.sender, _to, _pizzaId);
223    }
224
225    // Returns approved address for specific Pizza
226    function getApproved(uint256 _pizzaId)
227        public
228        view
229        returns (address operator)
230    {
231        require(_exists(_pizzaId), "Pizza does not exist.");
232        return pizzaApprovals[_pizzaId];
233    }
234
235    /**
236     * Private function to clear current approval of a given token ID
237     * Reverts if the given address is not indeed the owner of the token
238     */
239    function _clearApproval(address owner, uint256 _pizzaId) private {
240        require(pizzaToOwner[_pizzaId] == owner, "Must be pizza owner.");
241        require(_exists(_pizzaId), "Pizza does not exist.");
242        if (pizzaApprovals[_pizzaId] != address(0)) {
243            pizzaApprovals[_pizzaId] = address(0);
244        }
245    }
246
247    /*
248     * Sets or unsets the approval of a given operator
249     * An operator is allowed to transfer all tokens of the sender on their behalf
250     */
251    function setApprovalForAll(address to, bool approved) public {
252        require(to != msg.sender, "Cannot approve own address");
253        operatorApprovals[msg.sender][to] = approved;
254        emit ApprovalForAll(msg.sender, to, approved);
255    }
256
257    // Tells whether an operator is approved by a given owner
258    function isApprovedForAll(address owner, address operator)
259        public
260        view
261        returns (bool)
262    {
263        return operatorApprovals[owner][operator];
264    }
265
266    // Takes ownership of Pizza - only for approved users
267    function takeOwnership(uint256 _pizzaId) public {
268        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
269        address owner = this.ownerOf(_pizzaId);
270        this.transferFrom(owner, msg.sender, _pizzaId);
271    }
272
273    // Checks if Pizza exists
274    function _exists(uint256 pizzaId) internal view returns (bool) {
275        address owner = pizzaToOwner[pizzaId];
276        return owner != address(0);
277    }
278
279    // Checks if address is owner or is approved to transfer Pizza
280    function _isApprovedOrOwner(address spender, uint256 pizzaId)
281        internal
282        view
283        returns (bool)
284    {
285        address owner = pizzaToOwner[pizzaId];
286        // Disable solium check because of
287        // https://github.com/duaraghav8/Solium/issues/175
288        // solium-disable-next-line operator-whitespace
289        return (spender == owner ||
290            this.getApproved(pizzaId) == spender ||
291            this.isApprovedForAll(owner, spender));
292    }
293
294    // Check if Pizza is unique and doesn't exist yet
295    modifier isUnique(string memory _name, uint256 _dna) {
296        bool result = true;
297        for (uint256 i = 0; i < pizzas.length; i++) {
298            if (
299                keccak256(abi.encodePacked(pizzas[i].name)) ==
300                keccak256(abi.encodePacked(_name)) &&
301                pizzas[i].dna == _dna
302            ) {
303                result = false;
304            }
305        }
306        require(result, "Pizza with such name already exists.");
307        _;
308    }
309
310    // Returns whether the target address is a contract
311    function isContract(address account) internal view returns (bool) {
312        uint256 size;
313        // Currently there is no better way to check if there is a contract in an address
314        // than to check the size of the code at that address.
315        // See https://ethereum.stackexchange.com/a/14016/36603
316        // for more details about how this works.
317        // TODO Check this again before the Serenity release, because all addresses will be
318        // contracts then.
319        // solium-disable-next-line security/no-inline-assembly
320        assembly {
321            size := extcodesize(account)
322        }
323        return size > 0;
324    }
325}
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Anatomy of smart contracts

Prerequisites

Data

Storage

Memory

Environment variables

Functions

View functions

Constructor functions

Built-in functions

Writing functions

Events and logs

Annotated examples

Hello world

Token

Unique digital asset

Further reading

Shey dis artikol dey helep?

Anatomy of smart contracts

Prerequisites

Data

Storage

Memory

Environment variables

Functions

View functions

Constructor functions

Built-in functions

Writing functions

Events and logs

Annotated examples

Hello world

Token

Unique digital asset

Further reading

Related topics

Related tutorials

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