Questions tagged [solidity]

Solidity is a contract-oriented, high-level language whose syntax is similar to that of JavaScript and it is designed for writing smart contracts in Ethereum to run on the EVM. Solidity is statically typed, supports inheritance, libraries and complex user-defined types among other features.

Filter by
Sorted by
Tagged with
159 votes
4 answers
39k views

Are there well-solved and simple storage patterns for Solidity?

Simple and appropriate data organization can challenge Solidity newcomers. It wants us to organize everything in ways many of us aren’t accustomed to. Are there well-solved general patterns for ...
user avatar
  • 51.2k
178 votes
13 answers
49k views

Upgradeable smart contracts

If the contract issuer wants to have a way to upgrade the contract code, so that account data and other things carry over, can Ethereum provide this? Also can this be done without changing the ...
user avatar
36 votes
6 answers
12k views

Error while compiling: Stack too deep

When trying to compile, I am getting the following error: "Internal compiler error: Stack too deep, try removing local variables." Is there any way around this? I'm not sure if I'll be able to ...
user avatar
84 votes
2 answers
33k views

What's the difference between 'msg.sender' and 'tx.origin'?

If either are used in the body of a Solidity function that may change the state of a contract (write call), what's the primary difference and/or benefit of using one over the other. For example: ...
user avatar
  • 6,033
68 votes
4 answers
57k views

<address>.send vs <address>.transfer best practice usage?

I noticed that address's offer two member functions that perform the transfer of ether to the specified address. send returns false upon failure and transfer raises an exception. In the Solidity ...
user avatar
  • 905
61 votes
7 answers
29k views

How can you decompile a smart contract?

On the blockchain I can inspect the code of a contract, and see the EVM opcodes. Is there a way to decompile this and convert it back to (Solidity) source code?
user avatar
  • 8,191
41 votes
11 answers
50k views

How to convert a bytes32 to string

How can I convert a bytes32 to a string? Does anyone has a magic function or library which does it?
user avatar
29 votes
3 answers
32k views

How to get return values when a non view function is called?

I am writing below a simple contract which stores all results of questionnaires for each ID. contract answer{ mapping(address => mapping(string => bool)) voters; struct qList { uint ...
user avatar
  • 415
36 votes
4 answers
33k views

How can I represent decimal values in Solidity?

I just started writing some solidity and I noticed that there are no decimal values such as double or float. What do you guys do when you need to return send to a wallet let's say 1.57 ether? Or when ...
user avatar
  • 682
36 votes
2 answers
17k views

Deploy contract from contract in Solidity

It's a common pattern to use a hub/factory contract to create multiple instances of a standard contract. It's also common to require a way to enumerate the addresses of the created contracts. Is ...
user avatar
  • 51.2k
10 votes
1 answer
4k views

What fixed or float point math libraries are available in solidity?

Solidity doesn't have float or double variables. What solutions are there to handle the cases where we really have to represent numbers as percentages or fractions?
user avatar
49 votes
8 answers
29k views

How to quickly test a Solidity function?

There are snippets and functions of Solidity code provided on this Ethereum Stack Exchange and other sites. What are ways to quickly test them, possibly debug a little, and play around?
user avatar
  • 78.3k
96 votes
5 answers
39k views

Difference between require and assert and the difference between revert and throw

I was looking at the docs and I'm looking for clarification on the difference between require and assert and throw and revert. assert(bool condition): abort execution and revert state changes if ...
user avatar
  • 1,481
33 votes
6 answers
26k views

ecrecover from Geth and web3.eth.sign

I have tried to use ecrecover() to verify the signature of a message. I have looked at lots of references here and elsewhere, like: https://lab-blog.ey.com/2016/12/19/ecrecover/ How does one ...
user avatar
33 votes
2 answers
25k views

How can you call a payable function in another contract with arguments and send funds?

I have a contract named Call: contract Call { ... function () payable { // TODO: Call the call function in the main contract // and forward all funds (msg.value) sent to this contract ...
user avatar
  • 1,019
42 votes
1 answer
24k views

What are Solidity events and how they are related to topics and logs?

I know that indexed arguments index the values for those arguments so that filtering will be faster. But what are topics? And what are they used for? I know that signature of an event is a topic. ...
user avatar
74 votes
2 answers
30k views

Solidity: What is the difference between `view` and `constant`?

Solidity 0.4.16 introduced the view and constant function modifiers. The documentation says: constant for functions: Same as view. Does this mean view is just an alias for constant? If so, why do ...
user avatar
  • 8,114
58 votes
7 answers
62k views

How to delete an element at a certain index in an array?

Anyone know how to delete an element in a array? Is there any built in method to do that? If not, does anyone know of how to implement such a method?
user avatar
  • 1,886
18 votes
1 answer
4k views

How much computation can be done in a fallback function?

Simply sending Ether to a contract, will run the contract's code. In Solidity, this code maps to what it calls the fallback function. How much work can be done in a fallback function?
user avatar
  • 78.3k
49 votes
4 answers
25k views

What is the zero, empty or null value of a struct?

If I have a mapping of string => custom defined struct, how do I check if a value is defined in the mapping? The docs state that "every possible key exists and is mapped to a value whose byte-...
user avatar
40 votes
1 answer
18k views

Is it possible to get the transaction hash from within a Solidity function call?

Within a Solidity function, is the transaction hash available as a global variable from within the contract? I would have thought it would be something like tx.hash in the same way you can do tx....
user avatar
  • 1,205
10 votes
2 answers
2k views

Unable to define greeterContract in the Greeter tutorial. Breaking change in Solidity 0.4.9!

I was following the Greeter tutorial on Go Ethereum wiki and got stuck on the "var greeterContract = web3.eth.contract(greeterCompiled.greeter.info.abiDefinition)" phase returning TypeError: Cannot ...
user avatar
  • 103
30 votes
2 answers
15k views

Ethereum ecrecover signature verification and encryption

The question of signing and verifying signatures with Solidity has been asked in these questions, both of which I have tried to study: How can I sign a piece of data with the private key of an ...
user avatar
  • 443
27 votes
6 answers
16k views

How to compile Solidity contracts with Geth v1.6?

I tried to compile the following contract within geth: pragma solidity ^0.4.8; contract Test { uint256 public value; function Test() { value = 123; } } using the following ...
user avatar
9 votes
6 answers
17k views

How to Fix "Stack Too Deep" Error?

I know that this has been covered in other questions here but I'm not sure what's happening in my case. My code is returning a compiler error "Stack too deep, try removing local variables." But I only ...
user avatar
25 votes
1 answer
21k views

What is the receive keyword in solidity?

Solidity has a receive keyword. What is it and how do I use it?
user avatar
21 votes
5 answers
35k views

Conversion of uint to string

I am trying to convert uint to string in solidity. Is it possible to convert? if so how can I convert it? Convertion to base 10 is prefered, but in hexadecimal might also be helpful.
user avatar
13 votes
2 answers
5k views

How to load Solidity source file into geth

Is it possible to load Solidity source file .sol file to go-ethereum. I am using the 'loadScript' method by saving a .js file like the following and loads it. source = "contract test {\n" + " /// ...
user avatar
  • 9,343
7 votes
2 answers
13k views

Approve contract to withdraw funds from users's wallet

I want to achieve the following flow: A user clicks a button allowing my contract to spend funds on the user's behalf A user clicks a second button whereupon funds are transferred from the user's ...
user avatar
  • 360
18 votes
2 answers
4k views

Can a contract access another contract's storage?

I have a contract with numerous complex structs. Is it possible for a second contract to iterate over this data directly? I can't really use public accessor/getter methods as it's not possible to use ...
user avatar
  • 423
51 votes
1 answer
16k views

Are underscores `_` in modifiers code or are they just meant to look cool?

I often see _ in modifiers modifier onlyOwner() { if (msg.sender != owner) throw; _ } Does it execute any code or is it meant to make the code easier to read ?
user avatar
  • 655
43 votes
7 answers
52k views

How to convert a string to bytes32?

How can I convert a string to a bytes32? Does anyone has a magic function or library which does it? For example: This works since as input I gave 32-bits. But it won't work with "> 32-bit" chars. ...
user avatar
  • 7,689
59 votes
4 answers
20k views

Why does uint8 cost more gas than uint256?

contract A { uint8 a = 0; } costs 20150 + 2000 gas during creation. as compared to contract A { uint a = 0; // or uint256 } costing 5050 + 2000 gas during creation It is odd that a variable ...
user avatar
  • 950
71 votes
7 answers
65k views

How to concatenate strings in solidity?

I'm trying to concatenate two strings in solidity. I tried with + or . but neither option works. There is not much documentation on string concatenation yet. Is it even possible? If not, can I ...
user avatar
  • 31.4k
59 votes
6 answers
84k views

payable() function In solidity

I have read many examples of payable functions, but I didn't understand any of them. Also, they talked about modifier in payable. My questions are: What exactly does payable() function do? ...
user avatar
  • 2,019
28 votes
4 answers
28k views

Calling function from deployed contract

I want to know how to instantiate an existing contract deployed on the blockchain using its address. e.g: contract A { function f1() {} } A is deployed on the blockchain, and in contract B I ...
user avatar
  • 970
18 votes
1 answer
2k views

Concealing sensitive data in a public contract

I have some sensitive data I'd like to keep in my contract in privacy. I'd like to make a simple version of Zero-Knowledge Proofs and have an idea in mind. Let's say I have a customer who filled out ...
user avatar
  • 2,038
12 votes
4 answers
5k views

How to Compile and Deploy a Smart Contract without running a full node?

I'm new to solidity and smart contract programming. I have already made and deploy smart contracts successfully, but always using Mist. Now I'm not able to do so, because I can't complete a node ...
user avatar
17 votes
3 answers
8k views

Could we call a constant function without spending any gas inside a transaction function?

I have a sorted linked list that contains around 1000 items. When I receive a new item my goal is to push the item to the list keeping the sorted order. So if I receive very small value, I am required ...
user avatar
  • 7,689
3 votes
1 answer
7k views

Solidity convert hex string to bytes

I try to convert hex string to bytes. function MyContract() public { string memory str = "A76A95918C39eE40d4a43CFAF19C35050E32E271"; array1 = bytes(str); bytes memory str1 = hex"...
user avatar
4 votes
2 answers
4k views

Remix Warning - No visibility specified, Violate Checks-Effects-Interaction pattern, Function state mutability can be restricted to pure - Can ignore?

Two months ago, I wrote a smart contract with Remix.ethereum.org. Line 1 of my code has: pragma solidity ^0.4.11; It compiled fine with no errors and nor warnings. I tested every function in ...
user avatar
  • 307
67 votes
7 answers
88k views

How do you get a json file (ABI) from a known contract address?

I want to watch a Solidity contract using the Mist browser. I know how to do that and it needs the address and the json. I have the address, but I don't have the json. I do have the source code. Can ...
user avatar
28 votes
2 answers
5k views

What is the merit of creating new smart contract languages like Solidity instead of using other languages?

What is the pros and cons of creating new languages like Solidity for smart contracts instead of using other computer languages like Golang or Python?
user avatar
47 votes
3 answers
15k views

Where do contract event logs get stored in the Ethereum architecture?

Background on Events: https://github.com/ethereum/wiki/wiki/Solidity-Features Events allow the convenient usage of the EVM logging facilities. Events are inheritable members of contracts. When they ...
user avatar
  • 1,605
17 votes
6 answers
17k views

How to convert an address to bytes in Solidity?

What is the recommended way to convert an address to bytes in Solidity?
user avatar
  • 78.3k
16 votes
2 answers
7k views

Is transfer() still safe after the Istanbul update?

In other words, is using transfer() safe? function transfer(address contractB) public { contractB.transfer(1000); //balances[msg.sender] -= 1000; } How about using call.gas? function ...
user avatar
9 votes
1 answer
2k views

Does Solidity's constant keyword do anything?

It seems that the purpose of function foo() constant {} is to indicate that a function does not change the contract's state. And yet, as it states here, the compiler doesn't actually enforce this. So ...
user avatar
29 votes
2 answers
6k views

Ethereum smart contract security checklist

Is there a tool, checklist of methodology to help reviewing smart contract security? E.g. for each of function you review Is there re-entry possibility and how it behaves on re-entry How it behaves ...
user avatar
41 votes
2 answers
8k views

What are the steps to compile and deploy a library in Solidity?

I have several pieces of documentation about libraries, but I am not yet able to infer what steps are needed to compile and deploy a library and then a contract that uses it. The behavior of the ...
user avatar
  • 3,420
47 votes
4 answers
39k views

When to use "View" and "Pure" in place of "Constant" [duplicate]

According to the solidity 0.4.17 Release Notes this release finally checks the modifiers view (used to be named constant) and pure on functions. As a rule of thumb, use view if your function ...
user avatar
  • 51.2k

1
2 3 4 5
28