127
votes
Accepted
Difference between require and assert and the difference between revert and throw
edit 2022-12-08: As of Solidity 0.8.0 this answer is no longer accurate with respect to opcode and gas usage.
Furthermore, internal errors like division by zero, assertion failure and others do not ...
- 3,211
29
votes
Accepted
Is it a good practice to log an event every time I throw in Solidity?
With an exception, as with throw, all effects (including events) of a transaction are reverted, except for the payment to the miner. So there would be no benefit in firing an event before a throw.
...
eth♦
- 85.7k
26
votes
How to map EVM trace to contract source?
I went down this rabbit hole and got a proof of concept to work at the end. I can not recommend the journey. There's impedance mismatches on many levels, requiring lots of format conversions. In the ...
- 413
25
votes
Difference between require and assert and the difference between revert and throw
I'm using require for input validation as it's a little more efficient that if/throw.
function foo(uint amount) {
require(amount < totalAmount);
...
}
Where as assert should be used more ...
- 4,320
23
votes
Accepted
Why does a Solidity throw consume all gas?
All gas is consumed because the EVM essentially only has 1 exception: Out of Gas.
To see this clearer, take a look at the difference between a "pure" exception, and an error due to bad/buggy/invalid ...
eth♦
- 85.7k
20
votes
Accepted
Does an entire transaction revert when throw occurs?
In Solidity, by default, yes.
On the EVM-level, a throw (bad jump, out-of-gas, or any other exception) only reverts the call it is inside. Solidity helpfully continues the exception down the stack ...
- 7,300
19
votes
Difference between require and assert and the difference between revert and throw
I think none of the answers is correct.
assert is reserved for conditions in which it is expected that static code analysis tools (maybe Solidity compiler in future versions) will be able to detect ...
- 606
15
votes
Does an entire transaction revert when throw occurs?
To complement @Matthew's answer, it depends on how the call is made in Solidity.
If C calls D.foo(), and foo does a throw, then yes the entire transaction is reverted.
If C does a "lower-level raw ...
eth♦
- 85.7k
15
votes
Accepted
Why do `throw` and `revert()` create different bytecodes?
I'll leave the precise interpretation of the OpCodes to someone else and just point out that they are different instructions so different implementations are to be expected.
According to this: http:/...
- 55.2k
11
votes
Why does a Solidity throw consume all gas?
In part I believe this was a security decision to ensure spamming the network with bogus transactions cost the sender gas. For instance, if gas was refunded after a throw, a malicious actor could ...
- 111
11
votes
Why does a Solidity throw consume all gas?
The Solidity throw instruction gets compiled to an invalid JUMP instruction (i.e. a jump to an invalid location).
The section 9.4.2 of the Yellow Paper explains how exceptional halting works, in ...
- 2,569
10
votes
Throw vs. Return
All the considerations in the question are helpful.
throw is safer since it ensures that there are no side effects that remain, but another consideration is error reporting. With throw there's ...
eth♦
- 85.7k
7
votes
Why does a Solidity throw consume all gas?
The throw instruction is a EVM hack that reverts all changes made by current contract execution. It exploits exceptional termination of EVM which also consumes all gas.
- 1,355
6
votes
Accepted
Does `throw` refund the ether value?
The ether value that was sent along with the transaction (or call if the exception is caught) is returned to the sender if you use throw in Solidity.
- 3,392
6
votes
Is there an advantage rollbacking the transactions with 'throw' over returning an error code?
For example with web3js you would have to guestimate if the full consumtion of gas means that a throw happend or you ran out of gas
Guesstimation is unnecessary, you can get the estimated gas cost ...
- 129
6
votes
Accepted
Exception When Calling Function In a Library that Returns an Array
I've never worked with libraries before, however I believe the issue might be that you are trying to have a dynamically sized array in memory.
This should be
uint8[] memory arr = new uint8[](arraySize)...
- 1,585
5
votes
Accepted
Custom Exceptions in Solidity
Currently there are no custom exceptions in Ethereum Virtual Machine code. All failure conditions are "out of gas" as exception is modelled as consuming all the remaining gas.
eth.debug....
- 22.3k
5
votes
Does `throw` refund the ether value?
Yes, all funds are returned to the sender, except the gas. https://github.com/ethereum/wiki/wiki/Solidity-Features#throw
- 2,103
5
votes
Running contract CrowdFunder in Mix and function modifier causes - Solidity exception (bad jump)
"bad jump" is just the way "throw" is modeled, so it is expected behaviour.
- 3,392
5
votes
Would it be better to use `throw` instead of `return false`?
Just thought I would chime in, in case this Q&A is useful to others. For brevity, just imagine I prefixed everything with in my opinion.
It's a very general question. I would segregate the ...
- 55.2k
5
votes
Why do `throw` and `revert()` create different bytecodes?
The revert is often referred to as cheap throw as it refunds unused gas to the sender.
If you are interested in the detailed design of this feature please look at the original EIP-140 discussion.
- 6,409
4
votes
Accepted
Buy function not working with solidity 0.4.2
From Solidity 0.4 onwards:
Functions that want to receive Ether have to specify the new payable
modifier (otherwise they throw).
A throw consumes all gas, so function buy() payable returns (uint ...
eth♦
- 85.7k
4
votes
Accepted
Throw not generated when accessing mapping with invalid key
In soldidity mappings exist as if all keys were present with their values set to 0x0. For structs this means that all of the fields will be in their 0x0 state.
Since voters is a mapping, you can ...
- 3,592
4
votes
Throw vs. Return
You may also want to consider why not to use throw / return:
Why not use throw
All gas is consumed, everything reverted
Impossible to tell callers why the call failed
Errors cannot be caught, so ...
- 140
4
votes
Accepted
Not enough components (4) in value to assign all variables (6)
It turns out that in solidity, you can't go passing around strings from contract to contract. When I removed the two string values my code compiled without a hitch.
- 201
4
votes
Accepted
Contract created by another contract and exceptions
1. Yes
B will not be created if the transaction containing the creation of B has an exception. Example:
pragma solidity ^0.4.8;
contract A {
B public b;
function foo() {
b = new B()...
eth♦
- 85.7k
4
votes
simulate transaction without sending to blockchain
I am a software engineer at Tenderly and our Transaction Simulator is pretty much the solution for the problem you described above and our most used feature. You can:
Test out bug fixes and ...
- 141
3
votes
Accepted
Logging errors in the fallback function
Since I have only 2300 gas available I can't call another contract that will make a log. Is it correct?
The basic cost for calling another contract is 700 gas (cost of CALL opcode). So it's ...
eth♦
- 85.7k
3
votes
Accepted
Running contract CrowdFunder in Mix and function modifier causes - Solidity exception (bad jump)
The modifier inState(State _state) is basically an assertion insuring that before the actual function runs (represented by the _ in the modifier's code), the current state of the contract is in the ...
- 9,943
3
votes
Is there an advantage rollbacking the transactions with 'throw' over returning an error code?
Throwing an exception makes it easier to reason about the resulting state on error. It's too easy to accidentally not revert all changes that were made on error if you return a value.
Future versions ...
- 33
Only top scored, non community-wiki answers of a minimum length are eligible