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<!doctype html>
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<title>CCC: Cryptocurrency Course slide set</title>
<meta name="description" content="A set of slides for a course on Cryptocurrency">
<meta name="author" content="Aaron Bloomfield">
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<section data-markdown><textarea>
# CS 4501
### Cryptocurrency
<p class='titlep'> </p>
<div class="titlesmall"><p>
<a href="http://www.cs.virginia.edu/~asb">Aaron Bloomfield</a> (aaron@virginia.edu)<br>
<a href="http://github.com/aaronbloomfield/ccc">@github</a> | <a href="index.html">↑</a> | <a href="?print-pdf"><img class="print" width="20" src="../slides/images/print-icon.png" style="top:0px;vertical-align:middle"></a>
</p></div>
<p class='titlep'> </p>
## Solidity
</textarea></section>
<section data-markdown><textarea>
# Contents
[Types](#/types)
[Concepts](#/concepts)
[Poll Example](#/poll)
[Debtor's Example](#/debtor)
[Testing & Debugging](#/debugging)
</textarea></section>
<!-- ============================================================ -->
<section>
<section id="types" data-markdown class="center"><textarea>
# Types
</textarea></section>
<section data-markdown data-separator="^\n\n\n"><textarea>
## Sources
- The main reference for this slide set is the [Solidity language documentation](https://docs.soliditylang.org/en/latest/index.html)
- URL: https://docs.soliditylang.org/en/latest/index.html
- This section is the [Types section therein](https://docs.soliditylang.org/en/latest/types.html)
- https://docs.soliditylang.org/en/latest/types.html
- Many of the examples herein are from there
- Also some code examples from the [Poll.sol](../hws/dappintro/Poll.sol.html) contract from the [dApp introduction](../hws/dappintro/index.html) assignment
## A note about these slides
- These slides are meant as a explanation
- You are not expected to remember everything!
- Since you all know OOP, it's just showing what is different in Solidity
- You can come back to use this as a reference later
## Value types: integers
- Signed and unsigned ints from 8 bits (1 byte) to 256 bits (32 bytes) in size
- And in all byte sizes between
- Unsigned: `uint8`, `uint16`, `uint24`, ... `uint256`
- `uint` is an alias for `uint256`
- Signed: `int8`, `int16`, `int24`, ..., `int256`
- `int` is an alias for `int256`
- Standard operations (arithmetic, comparisons, bit operators, shift operators)
- Exponentiation is $\ast \ast$
- `type(int256).max` and `type(uint160).min` for min/max
## Value types: integers
- Most used: `uint` (aka `uint256`) and `uint8`
- Warning: a subtraction into a `uint` that is negative will cause a reversion
- A revert is basically an abort -- more on this later
- And it will not give a useful error message indicating so!
## Integer literals
- Scientific notation: `1e12` equals $10^{12}$
- `3.4e8` equals $3.4 \ast 10^8$
- Underscores separate digits for readability, but do not affect its value
- 0x12345678 == 0x1234_5678
## Other value types
- Booleans: type is `bool`, values are `true` and `false`
- All comparisons evaluate to a Boolean
- Fixed point numbers: not yet fully supported
- Types are `fixed` and `ufixed`
- But don't use them!
- No floating point numbers!
- Note that all variables have an initial value of 0
## Addresses
- Types are `address` and `address payable`
- It's just a number: a 160-bit (20 byte) Ethereum address
- Allows standard comparison operators
- Used for:
- Keeping track of who is the contract initiator
- Addresses of other contracts to call
- Can cast between `address` and: `uint160` or `bytes20`
```
address a = address(0);
uint160 u = uint160(a);
bytes20 b = bytes20(u);
```
- The address of a caller of a function is in `msg.sender`
## Address examples
- You can convert to a payable address:
```
address a;
// set a = something...
address payable b = payable(a);
```
- Conversions:
```
uint160 b = uint160(msg.sender);
bytes20 c = bytes20(msg.sender);
```
- More examples:
```
if ( a == address(0) ) { ... }
address initiator = msg.sender;
```
## Address fields
- Although a primitive type, it has fields and methods:
- `balance`: the balance, in wei, for that address
```
address payable x = payable(0x123);
address myAddress = address(this);
if (x.balance < 10 && myAddress.balance >= 10) {...}
```
- Other methods we won't see in this course:
- `transfer()` to send wei (requires a `receive()` function on the receiving contract)
- `send()`: low-level and not safe version of `transfer()`
- `call()`, `delegatecall()`, and `staticcall()`: calling a contract with an unknown ABI
- `code()` and `codehash()` to get the bytecode of the contract
## Fixed-sized Byte Arrays
- Fixed sized array of bytes: `bytes1`, `bytes2`, up to `bytes32`
- The size is the integer in the type name; max of 32 bytes
- `.length` to get the length
- Indexing with `[]`
- Operators:
- Comparisons, bit operators, shift operators
- Treats the byte array like a similarly sized `uint` when performing the operation
## Dynamically-sized arrays
- `bytes`: dynamically sized array of bytes
- `string`: dynamically sized array of UTF-8 characters
- We'll see string functions in a bit...
## String literals
- Strings can be enclosed in single quotes or double quotes
- Can use the backslash for escape characters
- Two strings next to each other is concatenation:
```
string s = "CS" '4501'; // equals "CS4501"
```
## More general arrays
- We can create arrays of any type:
```
uint8[6] memory p = [ 1, 2, 3, 4, 5, 6 ];
```
- We'll see the `memory` keyword shortly...
## Enums
- Enumerated type, which is mapped to a `uint`
```
enum ActionChoices { GoLeft, GoRight,
GoStraight, SitStill }
//
ActionChoices choice;
ActionChoices constant default = ActionChoices.GoLeft;
//
function setGoStraight() public {
choice = ActionChoices.GoStraight;
}
```
## Mappings
- An associative array (like a hash table) that holds a key-value pair
```
mapping (address => bool) public override voted;
mapping (uint => Choice) internal _choices;
```
- We'll talk about `public`, `internal`, and `override` later
- To access a mapping value:
```
if ( voted[msg.sender] ) { ... }
```
- To write to a mapping:
```
voted[msg.sender] = true;
```
## Mappings of mappings
- You can have mappings to mappings:
```
mapping (uint => mapping(string => address) )
public twodmap;
```
- It's just like a 2-dimensional array:
```
twodmap[3]["foo"]
```
## Mapping keys
- The keys are actually the *hash* of the key you provide
- If you provide the character `1` as the key, the actual key is the Keccak256 hash of that
- Or 0xc89efdaa54c0f20c7adf612882df0950f5a951637e0307cd cb4c672f298b8bc6
- In every mapping, *all* $2^{256}$ keys exist
- Anything not explicitly set returns 0 / false / all zero's
- All mappings thus have size $2^{256} \approx 1.15 \ast 10^{77}$
- You can delete an element from a mapping:
- `delete map[1];`
- This really just sets the value to 0
- And it *restores* gas (since it's shrinking the EVM state)
## Is an element in a mapping?
- Consider:
```
mapping (address => bool) public override voted;
// this next one was not in the original Poll.sol:
mapping (address => uint) public how_voted;
//
// or:
//
mapping (uint => Choice) internal _choices;
uint public override num_choices;
```
- The `voters` tells if an address has voted
- If they haven't, then that key's value will be 0 (false)
- The `how_voted` will tell what they voted for
- For the Choices, the uint tells how many keys there are
- Our code has keys 0 to num_choies-1
## Structs
- Like a class without methods
```
struct Choice {
uint id;
string name;
uint votes;
}
```
- Structs can contain mappings as well
- They are often kept in a mapping:
```
mapping (uint => Choice) internal _choices;
```
- If you put the same `Choice` struct into different mappings, you *might* now have *two* copies of that struct!
## Time
- All time is kept as a UNIX timestamp
- The number of seconds since January 1st, 1970
- This is how it's kept in the blockchain as well
- Only time available in a contract is the timestamp of the block
- Via `block.timestamp`
- Can use `seconds`, `minutes`, `hours`, `days`, and `weeks` as time units
- They are always pluralized
- But the number before each must be a literal, not a variable!
- Example on the next slide
## Time units example
```
uint a = 1;
uint b = 5;
uint c = 2;
// valid:
uint endTime = block.timestamp +
1 seconds + 5 minutes + 2 days;
// not valid:
uint endTime = block.timestamp +
a seconds + b minutes + c days;
// valid:
uint endTime = block.timestamp +
a * 1 seconds + b * 1 minutes + c * 1 days;
```
## Ether units
- Can convert ether units as well
- All convert to wei (recall that 1 ether is $10^{18}$ wei)
```
assert(1 wei == 1);
assert(1 gwei == 1e9);
assert(1 ether == 1e18);
```
## Reference types
- Reference types are: strings, arrays, mappings, and structs
- Like Java and Python, the language often hides that it's a reference
## Memory locations
- All reference types must specify a memory location
- `storage`: for all state variables, it is stored on the blockchain
- statically allocated, like the stack on x86
- two sub-types:
- local storage (local subroutine variable)
- state storage (state/class variable on the blockchain)
- `memory`: a local variable, it only exists while the subroutine is executing
- dynamically allocated, like the heap in x86
- `calldata`: read-only, used for function parameters
## Assignments of references
- If you assign one reference type to another, what happens depends on where the data is stored:
| To (lhs) | From (rhs) | Result |
|--|--|--|
| memory | memory | reference aliasing |
| memory | any storage | full copy |
| memory | calldata | full copy |
| any storage | memory | full copy |
| any storage | calldata | full copy |
| local storage | any storage | reference aliasing |
| state storage | any storage | full copy |
| calldata | (any) | not allowed |
## Assignments of references
- How to remember:
- You can't ever write to `calldata`
- Anything assigned between *different* types of memory is always a copy, as references can't point from one memory type to another
- State storage, which is on the blockchain, never stores references -- so anything copied to state storage is always a full copy
- Reference aliasing works in the 2 cases when the memory types are the same and it's not going into state storage: memory to memory, and any storage to local storage
## Type names
- For certain types, you have to give a qualifier as to where it is stored
- `bytes` and `string`
- The qualifier is `memory`, `storage`, or `calldata`
- The type name is *two words*
- `string memory`
- `bytes storage`
## String functions
- No (direct) built-in string comparison!
- Reasons:
- It is difficult / expensive to compare across memory locations
- Instead, compare their hashes:
```
if ( keccak256(x) == keccak256(y) ) { ... }
```
- Note that `keccak256()` uses a lot of gas
- Can concatenate two strings:
```
string memory s = string.concat(s1, s2);
```
- Only returns a `string memory`
- There are 3rd party string function libraries
- Which contain comparisons, among other functions
<h2>Code Example</h2>
<pre class="code-wrapper"><code class="hljs d small" style="height:100%;">// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.9.0;
contract C {
// The data location of x is storage; this is the
// only place where the data location can be omitted
uint[] x;
// The data location of memoryArray is memory.
function f(uint[] memory memoryArray) public {
x = memoryArray; // works, copies the whole array to storage
uint[] storage y = x; // works, assigns a pointer, y's location is storage
y[7]; // fine, returns the 8th element
y.pop(); // fine, modifies x through y
delete x; // fine, clears the array, also modifies y
// The following doesn't work; it would need to create a new temporary /
// unnamed array in storage, but storage is "statically" allocated:
// y = memoryArray;
// This does not work either, since it would "reset" the pointer,
// but there is no sensible location it could point to.
// delete y;
g(x); // calls g, handing over a reference to x
h(x); // calls h and creates an independent, temporary copy in memory
}
function g(uint[] storage) internal pure {}
function h(uint[] memory) public pure {}
}
</code></pre>
## Even more...
- There are a lot of types we are skipping
- See them all in the [Solidity types reference](https://docs.soliditylang.org/en/latest/types.html)
- https://docs.soliditylang.org/en/latest/types.html
</textarea></section>
</section>
<!-- ============================================================ -->
<section>
<section id="concepts" data-markdown class="center"><textarea>
# Concepts
</textarea></section>
<section data-markdown data-separator="^\n\n\n"><textarea>
## Learning Solidity
- The easy part: learning the language
- It's just an OO language with familiar syntax
- The hard part is understanding:
- The restrictions that blockchain programs have
- No time or random numbers
- No print statements!
- How to best interact with the blockchain
- What the heck do you do with it?
- The easy part will be done in the next 20 minutes
- The hard part will be the rest of this course
## Sources
- The main reference is the [Solidity language documentation](https://docs.soliditylang.org/en/latest/index.html)
- URL: https://docs.soliditylang.org/en/latest/index.html
- [Ethereum developer resources](https://ethereum.org/en/developers/)
- URL: https://ethereum.org/en/developers/
- [Solidity reference](https://docs.soliditylang.org)
- URL: https://docs.soliditylang.org
- [Remix documentation](https://remix-ide.readthedocs.io/en/latest/) (the Solidity IDE)
- URL: https://remix-ide.readthedocs.io/en/latest/
## Solidity Overview
- An object-oriented language with C++/Java like syntax
- Same set of control structures with the same syntax (for, if/else, while, etc.)
- A class is called a "contract"
- Methods are called functions
## License identifier
- All Solidity smart contracts must have a license line as the first line
- SPDX is the [Software Package Development Exchange](https://spdx.dev/)
- Examples:
```
// SPDX-License-Identifier: MIT
```
```
// SPDX-License-Identifier: CC BY-SA
```
```
// SPDX-License-Identifier: GPL
```
- A compiler will issue a warning if it's not there
- The actual license type is not checked, so you can also use:
```
// SPDX-License-Identifier: Unlicensed
```
## Pragma
- We'll only use the following Solidity version pragma:
```
pragma solidity ^0.8.17;
```
- The three numbers are: major version, minor version, bugfix version
- By default, it forces a compiler error if the wrong compiler version is being used
- Some platforms, such as Truffle or Remix, will try to find the right compiler version
- This will only compile with version 0.8.17 or later
- But will NOT allow compilation with version 0.9.0 or later
- Note that breaking changes (very few!) are only (intentionally) introduced on major versions
## Pragma
- More complicated examples are possible:
```
pragma solidity >=0.4.0 <0.6.0;
```
- One to avoid: note there is no carat before the version number
- Reason: a later bugfix version, 0.8.12, will not compile this program
```
pragma solidity 0.8.11;
```
- In this course, we'll use ONLY the following:
- As this will compile on all of the platforms we will be using
```
pragma solidity ^0.8.17;
```
## Comments
- C++ and Java style comments
```
// this is a comment
/* this is a
multi-line
comment
*/
/* this is a
* multi-line
* comment with better
* formatting
*/
```
## Import
- Default import statement:
```
import "./filename.sol";
```
- Note that this will pollute the namespace
- We can also use:
```
import * as symbolName from "./filename.sol";
```
- All names from the file can be accessed via `symbolName.thing`
- An equivalent version of this is:
```
import "./filename.sol" as symbolName;
```
- Naming collision? Then rename it upon import:
```
import {symbol1 as alias, symbol2} from "./filename.sol";
```
## High-level "things"
- `contract`
- Essentially an OO class
- Can also be `abstract`
- `interface`
- Just like interfaces in Java, or pure abstract classes in C++
- By convention, their names always start with a capital I (India)
- `library`
- Can contain ONLY functions that do not access state
- Meaning no contract field access
- Specifically, they can only contain `pure` functions (we'll see `pure` shortly)
- Unless you have a reason otherwise, have everything inside have `internal` visibility
- We'll see visibilities shortly...
## Inheritance
- Contracts can inherit from interfaces and other contracts
- Interfaces can inherit from (only) other interfaces
- Via the `is` keyword
- The super-class (or super-interface) must be defined above or `import`'ed
```
interface Foo is Bar {
// ...
}
```
```
contract Student is Person, Learner {
// ...
}
```
- Any contract that inherits from an interface, but does not implement all the methods, must be declared `abstract`
</textarea></section>
<section data-markdown data-separator="^\n\n\n" id='multioverride'><textarea>
## Inheritance
- `virtual`: if a *thing* (function, field, etc.) in a contract can be overridden
- All function prototypes in interfaces are assumed to be `virtual`
- `override`: when a sub-class is replacing an inherited function
- If the function is multiply inherited, you may have to specify which (or all) that it overrides:
```
function foo() public override(IERC165,ERC721) {
```
- This will be needed in the Tokens assignment
- The requirement to specify `override` varies by compiler and version
</textarea></section>
<section data-markdown data-separator="^\n\n\n" id='abscon'><textarea>
## Abstract Contracts
- A `contract` must have all the methods implemented
- An `interface` must have no methods implemented
- An `abstract contract` can have some methods implemented and some not
- Part interface, part contract
- Example: if you inherit from an interface, but only implement some of the methods
```
abstract contract Foo is Bar {
// ...
}
```
</textarea></section>
<section data-markdown data-separator="^\n\n\n"><textarea>
## What we have so far
```
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
interface IERC165 {
// ...
}
interface IERC721 is IERC165 {
// ...
}
contract ERC721 is IERC721 {
// ...
}
library Address {
// ...
}
```
## Visibilities
- `public`: like any other OO language, it can be called by anybody or anyone
- For fields, this is only the *readability* of the field; only contract functions can *write* to the field
- `private`: like any other OO language, it can be called only by code in that class
- `internal`: like `protected` in other OO languages: it can be called by that class or it's sub-classes
- `external`: like `public`, but can ONLY be called by code *outside* the contract; code in the contract can not call an `external` function
- Functions in interfaces must be declared `external`
- The implementing contract can then "raise" the visibility to `public`
</textarea></section>
<section data-markdown data-separator="^\n\n\n" id="getters"><textarea>
## Visibilities
- The stated visibility in an interface can be expanded (only) in the contract
- Example: if an interface specifies a function as `external`...
- Then the implementing contract can specify it either as `external` or `public`
- As `public` is *more* visible than `external`
- Anything field defined as `public` has a getter function defined for it -- see the next slide
- If you have `uint public k`, then you can call the `k()` getter function to access its value
- The default, if unspecified, is `public`
- But without a getter function
- Note that all data is still visible on the blockchain, even for `private` fields!
</textarea></section>
<section data-markdown data-separator="^\n\n\n"><textarea>
## Getter functions
- The following code:
```
uint public num_assignments;
mapping (address => string) public aliases;
mapping (uint => mapping(string => address) )
public twodmap;
```
- Creates the following getter functions:
```
function num_assignments() public view returns (uint);
function aliases(address a) public view
returns (string memory);
function twodmap(uint id, string s) public view
returns (address);
```
## Function Qualifiers
- `view`: this function does not *modify* the state of the contract
- It can read state variables, but not write to any state variables
- It can write to local variables, of course
- Like `const` in C++
- `pure`: this function does not *read or write* the state of the contract
- Any function in a `library` must be `pure`
- `view` and `pure` are mutually exclusive
- Using these qualifiers allows calls without having to make a transaction
- And *may* allow optimizations which reduce contract size and gas costs
## Declarations for fields
- Format:
```
<type> <visibility> <override?> <name> [= <value>];
```
- Examples:
```
uint public duration = 86400;
uint public override k;
address public override initiator;
mapping (address => uint) internal values;
```
- Local variables are similar
- But without the visibility and `override` qualifiers
## Declarations for functions
- Format:
```
function <name> ([<parameter_list>]) <visibility>
<qualifiers> [returns (<list>)] {
```
- Examples:
```
function addChoice (string memory _name) public
{ /* ... */ }
function unnecessaryFunction() public view
returns (string memory) { /* ... */ }
function overridePureFunction() external pure
override returns (uint,bool) { /* ... */ }
```
## Require and revert
- A reversion means the transaction method fails, and all state is rolled back to before it started
- Still costs gas fees!
- `revert()` will do this
- Also:
- `require(a>0);` will revert if $a$ is not greater than zero
- Better to use the two-parameter version:
- `require(a>0,"a must be greater than zero");`
- Now, if/when it reverts, it will tell you why
- Use this all the time!!!
## Receiving ether
- The `payable` qualifier means the function can accept ether as part of the call
- The amount of ether received is in `msg.value`
- And is added to the contract's balance
- ... assuming it doesn't revert
## Constructors
- Syntax:
```
constructor() {
// ...
}
```
- No `function` keyword, no return type
- Default, if not specified, is an empty body
- Can take parameters:
```
constructor(uint foo) {
// ...
}
```
- Only one constructor is allowed per class!
- They may eventually allow multiple, differentiated by the parameter list
## Destructor
- `selfdesctruct()`
- Only if enabled (declared)
- Disables the contract permanently
- Caller claims the ether balance
## Contract elements
- A contract can have:
- Fields
- Functions
- A constructor and a `selfdestruct()` method
- A few other special-purpose methods we won't see here
- Modifiers
## Modifiers
- A modifier changes how a function works
- You put in conditions (or whatever) and then indicate where the function body goes
- That indication is with the underscore
- Example:
```
modifier onlyOwner() {
require(msg.sender == owner, "Not owner");
_;
}
```
## Modifiers
- This code adapted from [solidity-by-example.org](https://solidity-by-example.org/function-modifier/):
<pre class="code-wrapper"><code class="hljs d small" style="height:100%;"
>contract Modifiers {
address public owner;
constructor() {
owner = msg.sender;
}
modifier onlyOwner() {
require(msg.sender == owner, "Not owner");
_;
}
modifier validAddress(address _addr) {
require(_addr != address(0), "Not valid address");
_;
}
function changeOwner(address _newOwner) public onlyOwner
validAddress(_newOwner) {
owner = _newOwner;
}
}
</code></pre>
## Events & emit
- An *event* is when something happens
- Allows for logging of what has happened to a contract
- They are declared in contracts or interfaces:
```
event votedEvent (uint indexed _id);
event choiceAddedEvent (uint indexed _id);
```
- The `indexed` keyword allows for searching the event log by that value
- Events are called via `emit`:
```
emit choiceAddedEvent(num_choices);
emit votedEvent(_id);
```
- Events can be "watched" for (or "subscribed to")
- We'll see this later this semester
## Try-catch
- Try-catch:
```
try <something> {
// what to do if it succeeds
} catch {
// what to do if it fails
}
```
- The "something" can be:
- Calling a function
- Creating a new contract
- Or anything else
- If the *something* reverts, the catch clause is executed
</textarea></section>
<section data-markdown data-separator="^\n\n\n" id='payeth'><textarea>
## Transferring ether in Solidity
- To transfer ether from a Solidity contract:
```
(bool success, ) = payable(a).call{value: v}("");
require(success, "Failed to transfer ETH");
```
- The amount, in `v`, is in wei
- The address to pay it to is in `a`
- This reverts if the contract does not have sufficient balance
- Or if it fails for any other reason, such as an invalid address
</textarea></section>
<section data-markdown data-separator="^\n\n\n"><textarea>
## Contracts creating contracts
- We can have a contract `Foo` deploy another contract `Bar`
- Given a field in Foo such as either:
```
Bar public b;
address public b;
```
- Then we can initialize it via either (respectively):
```
b = new Bar();
b = address(new Bar());
```
## Most useful global variables
From [here](https://docs.soliditylang.org/en/v0.8.12/units-and-global-variables.html)
- block.timestamp (uint)
- current block timestamp as seconds since unix epoch
- block.difficulty (uint)
- current block difficulty
- msg.sender (address)
- sender of the message (current call)
- msg.value (uint)
- number of wei sent with the message
- this
- the current contract (example: address(this).balance)
## Other global variables
- block.basefee (uint): current block's base fee (EIP-3198 and EIP-1559)