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Forge Standard Library • 
Forge Standard Library is a collection of helpful contracts and libraries for use with Forge and Foundry. It leverages Forge's cheatcodes to make writing tests easier and faster, while improving the UX of cheatcodes.
Learn how to use Forge-Std with the 📖 Foundry Book (Forge-Std Guide).
Install
forge install foundry-rs/forge-std
Contracts
stdError
This is a helper contract for errors and reverts. In Forge, this contract is particularly helpful for the expectRevert cheatcode, as it provides all compiler builtin errors.
See the contract itself for all error codes.
Example usage
import "forge-std/Test.sol";
contract TestContract is Test {
ErrorsTest test;
function setUp() public {
test = new ErrorsTest();
}
function testExpectArithmetic() public {
vm.expectRevert(stdError.arithmeticError);
test.arithmeticError(10);
}
}
contract ErrorsTest {
function arithmeticError(uint256 a) public {
uint256 a = a - 100;
}
}
stdStorage
This is a rather large contract due to all of the overloading to make the UX decent. Primarily, it is a wrapper around the record and accesses cheatcodes. It can always find and write the storage slot(s) associated with a particular variable without knowing the storage layout. The one major caveat to this is while a slot can be found for packed storage variables, we can't write to that variable safely. If a user tries to write to a packed slot, the execution throws an error, unless it is uninitialized (bytes32(0)).
This works by recording all SLOADs and SSTOREs during a function call. If there is a single slot read or written to, it immediately returns the slot. Otherwise, behind the scenes, we iterate through and check each one (assuming the user passed in a depth parameter). If the variable is a struct, you can pass in a depth parameter which is basically the field depth.
I.e.:
struct T {
// depth 0
uint256 a;
// depth 1
uint256 b;
}
Example usage
import "forge-std/Test.sol";
contract TestContract is Test {
using stdStorage for StdStorage;
Storage test;
function setUp() public {
test = new Storage();
}
function testFindExists() public {
// Lets say we want to find the slot for the public
// variable `exists`. We just pass in the function selector
// to the `find` command
uint256 slot = stdstore.target(address(test)).sig("exists()").find();
assertEq(slot, 0);
}
function testWriteExists() public {
// Lets say we want to write to the slot for the public
// variable `exists`. We just pass in the function selector
// to the `checked_write` command
stdstore.target(address(test)).sig("exists()").checked_write(100);
assertEq(test.exists(), 100);
}
// It supports arbitrary storage layouts, like assembly based storage locations
function testFindHidden() public {
// `hidden` is a random hash of a bytes, iteration through slots would
// not find it. Our mechanism does
// Also, you can use the selector instead of a string
uint256 slot = stdstore.target(address(test)).sig(test.hidden.selector).find();
assertEq(slot, uint256(keccak256("my.random.var")));
}
// If targeting a mapping, you have to pass in the keys necessary to perform the find
// i.e.:
function testFindMapping() public {
uint256 slot = stdstore
.target(address(test))
.sig(test.map_addr.selector)
.with_key(address(this))
.find();
// in the `Storage` constructor, we wrote that this address' value was 1 in the map
// so when we load the slot, we expect it to be 1
assertEq(uint(vm.load(address(test), bytes32(slot))), 1);
}
// If the target is a struct, you can specify the field depth:
function testFindStruct() public {
// NOTE: see the depth parameter - 0 means 0th field, 1 means 1st field, etc.
uint256 slot_for_a_field = stdstore
.target(address(test))
.sig(test.basicStruct.selector)
.depth(0)
.find();
uint256 slot_for_b_field = stdstore
.target(address(test))
.sig(test.basicStruct.selector)
.depth(1)
.find();
assertEq(uint(vm.load(address(test), bytes32(slot_for_a_field))), 1);
assertEq(uint(vm.load(address(test), bytes32(slot_for_b_field))), 2);
}
}
// A complex storage contract
contract Storage {
struct UnpackedStruct {
uint256 a;
uint256 b;
}
constructor() {
map_addr[msg.sender] = 1;
}
uint256 public exists = 1;
mapping(address => uint256) public map_addr;
// mapping(address => Packed) public map_packed;
mapping(address => UnpackedStruct) public map_struct;
mapping(address => mapping(address => uint256)) public deep_map;
mapping(address => mapping(address => UnpackedStruct)) public deep_map_struct;
UnpackedStruct public basicStruct = UnpackedStruct({
a: 1,
b: 2
});
function hidden() public view returns (bytes32 t) {
// an extremely hidden storage slot
bytes32 slot = keccak256("my.random.var");
assembly {
t := sload(slot)
}
}
}
stdCheats
This is a wrapper over miscellaneous cheatcodes that need wrappers to be more dev friendly. Currently there are only functions related to prank. In general, users may expect ETH to be put into an address on prank, but this is not the case for safety reasons. Explicitly this hoax function should only be used for addresses that have expected balances as it will get overwritten. If an address already has ETH, you should just use prank. If you want to change that balance explicitly, just use deal. If you want to do both, hoax is also right for you.
Example usage:
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "forge-std/Test.sol";
// Inherit the stdCheats
contract StdCheatsTest is Test {
Bar test;
function setUp() public {
test = new Bar();
}
function testHoax() public {
// we call `hoax`, which gives the target address
// eth and then calls `prank`
hoax(address(1337));
test.bar{value: 100}(address(1337));
// overloaded to allow you to specify how much eth to
// initialize the address with
hoax(address(1337), 1);
test.bar{value: 1}(address(1337));
}
function testStartHoax() public {
// we call `startHoax`, which gives the target address
// eth and then calls `startPrank`
//
// it is also overloaded so that you can specify an eth amount
startHoax(address(1337));
test.bar{value: 100}(address(1337));
test.bar{value: 100}(address(1337));
vm.stopPrank();
test.bar(address(this));
}
}
contract Bar {
function bar(address expectedSender) public payable {
require(msg.sender == expectedSender, "!prank");
}
}
Std Assertions
Contains various assertions.
console.log
Usage follows the same format as Hardhat.
It's recommended to use console2.sol as shown below, as this will show the decoded logs in Forge traces.
// import it indirectly via Test.sol
import "forge-std/Test.sol";
// or directly import it
import "forge-std/console2.sol";
...
console2.log(someValue);
If you need compatibility with Hardhat, you must use the standard console.sol instead.
Due to a bug in console.sol, logs that use uint256 or int256 types will not be properly decoded in Forge traces.
// import it indirectly via Test.sol
import "forge-std/Test.sol";
// or directly import it
import "forge-std/console.sol";
...
console.log(someValue);
Contributing
See our contributing guidelines.
Getting Help
First, see if the answer to your question can be found in book.
If the answer is not there:
- Join the support Telegram to get help, or
- Open a discussion with your question, or
- Open an issue with the bug
If you want to contribute, or follow along with contributor discussion, you can use our main telegram to chat with us about the development of Foundry!
License
Forge Standard Library is offered under either MIT or Apache 2.0 license.