Cheatcodes

Most of the time, simply testing your smart contracts outputs isn’t enough. To manipulate the state of the blockchain, as well as test for specific reverts and events, Foundry is shipped with a set of cheatcodes.

Cheatcodes allow you to change the block number, your identity, and more. They are invoked by calling specific functions on a specially designated address: 0x7109709ECfa91a80626fF3989D68f67F5b1DD12D.

You can access cheatcodes easily via the vm instance available in Forge Standard Library’s Test contract. Forge Standard Library is explained in greater detail in the following section.

Let’s write a test for a smart contract that is only callable by its owner.

pragma solidity 0.8.10;

import "forge-std/Test.sol";

error Unauthorized();

contract OwnerUpOnly {
    address public immutable owner;
    uint256 public count;

    constructor() {
        owner = msg.sender;
    }

    function increment() external {
        if (msg.sender != owner) {
            revert Unauthorized();
        }
        count++;
    }
}

contract OwnerUpOnlyTest is Test {
    OwnerUpOnly upOnly;

    function setUp() public {
        upOnly = new OwnerUpOnly();
    }

    function test_IncrementAsOwner() public {
        assertEq(upOnly.count(), 0);
        upOnly.increment();
        assertEq(upOnly.count(), 1);
    }
}

If we run forge test now, we will see that the test passes, since OwnerUpOnlyTest is the owner of OwnerUpOnly.

$ forge test
Compiling 24 files with 0.8.10
Solc 0.8.10 finished in 1.13s
Compiler run successful!

Ran 1 test for test/OwnerUpOnly.t.sol:OwnerUpOnlyTest
[PASS] test_IncrementAsOwner() (gas: 29161)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 409.13µs (86.55µs CPU time)

Ran 1 test suite in 2.61ms (409.13µs CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)

Let’s make sure that someone who is definitely not the owner can’t increment the count:

contract OwnerUpOnlyTest is Test {
    OwnerUpOnly upOnly;

    // ...

    function testFail_IncrementAsNotOwner() public {
        vm.prank(address(0));
        upOnly.increment();
    }
}

If we run forge test now, we will see that all the test pass.

$ forge test
No files changed, compilation skipped

Ran 2 tests for test/OwnerUpOnly.t.sol:OwnerUpOnlyTest
[PASS] testFail_IncrementAsNotOwner() (gas: 8314)
[PASS] test_IncrementAsOwner() (gas: 29161)
Suite result: ok. 2 passed; 0 failed; 0 skipped; finished in 475.90µs (156.00µs CPU time)

Ran 1 test suite in 2.78ms (475.90µs CPU time): 2 tests passed, 0 failed, 0 skipped (2 total tests)

The test passed because the prank cheatcode changed our identity to the zero address for the next call (upOnly.increment()). The test case passed since we used the testFail prefix, however, using testFail is considered an anti-pattern since it does not tell us anything about why upOnly.increment() reverted.

If we run the tests again with traces turned on, we can see that we reverted with the correct error message.

$ forge test -vvvv --match-test testFail_IncrementAsNotOwner
No files changed, compilation skipped

Ran 1 test for test/OwnerUpOnly.t.sol:OwnerUpOnlyTest
[PASS] testFail_IncrementAsNotOwner() (gas: 8314)
Traces:
  [8314] OwnerUpOnlyTest::testFail_IncrementAsNotOwner()
    ├─ [0] VM::prank(0x0000000000000000000000000000000000000000)
    │   └─ ← [Return] 
    ├─ [247] OwnerUpOnly::increment()
    │   └─ ← [Revert] Unauthorized()
    └─ ← [Revert] Unauthorized()

Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 350.86µs (39.43µs CPU time)

Ran 1 test suite in 2.82ms (350.86µs CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)

To be sure in the future, let’s make sure that we reverted because we are not the owner using the expectRevert cheatcode:

contract OwnerUpOnlyTest is Test {
    OwnerUpOnly upOnly;

    // ...

    // Notice that we replaced `testFail` with `test`
    function test_RevertWhen_CallerIsNotOwner() public {
        vm.expectRevert(Unauthorized.selector);
        vm.prank(address(0));
        upOnly.increment();
    }
}

If we run forge test one last time, we see that the test still passes, but this time we are sure that it will always fail if we revert for any other reason.

$ forge test
No files changed, compilation skipped

Ran 1 test for test/OwnerUpOnly.t.sol:OwnerUpOnlyTest
[PASS] test_IncrementAsOwner() (gas: 29161)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 342.27µs (57.62µs CPU time)

Ran 1 test suite in 2.61ms (342.27µs CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)

Another cheatcode that is perhaps not so intuitive is the expectEmit function. Before looking at expectEmit, we need to understand what an event is.

Events are inheritable members of contracts. When you emit an event, the arguments are stored on the blockchain. The indexed attribute can be added to a maximum of three parameters of an event to form a data structure known as a “topic.” Topics allow users to search for events on the blockchain.

pragma solidity 0.8.10;

import "forge-std/Test.sol";

contract EmitContractTest is Test {
    event Transfer(address indexed from, address indexed to, uint256 amount);

    function test_ExpectEmit() public {
        ExpectEmit emitter = new ExpectEmit();
        // Check that topic 1, topic 2, and data are the same as the following emitted event.
        // Checking topic 3 here doesn't matter, because `Transfer` only has 2 indexed topics.
        vm.expectEmit(true, true, false, true);
        // The event we expect
        emit Transfer(address(this), address(1337), 1337);
        // The event we get
        emitter.t();
    }

    function test_ExpectEmit_DoNotCheckData() public {
        ExpectEmit emitter = new ExpectEmit();
        // Check topic 1 and topic 2, but do not check data
        vm.expectEmit(true, true, false, false);
        // The event we expect
        emit Transfer(address(this), address(1337), 1338);
        // The event we get
        emitter.t();
    }
}

contract ExpectEmit {
    event Transfer(address indexed from, address indexed to, uint256 amount);

    function t() public {
        emit Transfer(msg.sender, address(1337), 1337);
    }
}

When we call vm.expectEmit(true, true, false, true);, we want to check the 1st and 2nd indexed topic for the next event.

The expected Transfer event in test_ExpectEmit() means we are expecting that from is address(this), and to is address(1337). This is compared against the event emitted from emitter.t().

In other words, we are checking that the first topic from emitter.t() is equal to address(this). The 3rd argument in expectEmit is set to false because there is no need to check the third topic in the Transfer event, since there are only two. It does not matter even if we set to true.

The 4th argument in expectEmit is set to true, which means that we want to check “non-indexed topics”, also known as data.

For example, we want the data from the expected event in test_ExpectEmit - which is amount - to equal to the data in the actual emitted event. In other words, we are asserting that amount emitted by emitter.t() is equal to 1337. If the fourth argument in expectEmit was set to false, we would not check amount.

In other words, test_ExpectEmit_DoNotCheckData is a valid test case, even though the amounts differ, since we do not check the data.


📚 Reference

See the Cheatcodes Reference for a complete overview of all the available cheatcodes.