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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.

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174 votes
4 answers

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 ...
Rob Hitchens's user avatar
  • 55.4k
187 votes
13 answers

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 ...
Mikko Ohtamaa's user avatar
43 votes
8 answers

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 ...
Amer Ameen's user avatar
3 votes
2 answers

Missing Eth after sending to contract

I tried following a tutorial on YT and my ETH has been missing. I keep on looking at the tos and froms but I can't seem to find where my ETH has been sent. This is the created contract address ...
RGJ MIA's user avatar
  • 31
16 votes
1 answer

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

Solidity doesn't have fixed-point, floating-point, or double types. What solutions are there to handle the cases where we really have to represent numbers as percentages or fractions?
Paul Razvan Berg's user avatar
108 votes
2 answers

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: ...
dbryson's user avatar
  • 6,393
79 votes
4 answers

<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 ...
Michael's user avatar
  • 1,065
70 votes
7 answers

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?
J-B's user avatar
  • 8,961
46 votes
6 answers

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 ...
sfrj's user avatar
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30 votes
4 answers

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 ...
A. Take's user avatar
  • 424
50 votes
12 answers

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?
arodriguezdonaire's user avatar
41 votes
2 answers

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 ...
Rob Hitchens's user avatar
  • 55.4k
105 votes
5 answers

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 ...
Daniel Kobe's user avatar
  • 1,591
54 votes
8 answers

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?
eth's user avatar
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38 votes
2 answers

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 ...
Pabi's user avatar
  • 1,129
80 votes
2 answers

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 ...
SCBuergel's user avatar
  • 8,774
34 votes
6 answers

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: How does one ...
Daniel Mermelstein's user avatar
67 votes
9 answers

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?
user697's user avatar
  • 2,176
47 votes
1 answer

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. ...
Narayan Prusty's user avatar
36 votes
1 answer

What is the receive keyword in solidity?

Solidity has a receive keyword. What is it and how do I use it?
Shane Fontaine's user avatar
19 votes
1 answer

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?
eth's user avatar
  • 85.9k
77 votes
6 answers

Fallback function in Solidity

I have seen many examples of the fallback function in Solidity, but I didn't understand any of them. Also, I've seen references to the payable modifier. My questions are: What exactly does the ...
Gopal ojha's user avatar
  • 2,249
58 votes
1 answer

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 ?
dor's user avatar
  • 725
54 votes
4 answers

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-...
Thomas Clowes's user avatar
95 votes
7 answers

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 ...
q9f's user avatar
  • 32.9k
42 votes
7 answers

How does a contract find out if another address is a contract?

Is it possible, from within a contract written in Solidity, to check if a contract is placed on a specific address or if this address does not contain any code?
Thorkil Værge's user avatar
35 votes
2 answers

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 ...
Mikko Ohtamaa's user avatar
19 votes
2 answers

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 ...
slothbag's user avatar
  • 433
10 votes
2 answers

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(" phase returning TypeError: Cannot ...
Henrik's user avatar
  • 103
74 votes
5 answers

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 ...
uzyn's user avatar
  • 1,100
63 votes
3 answers

What's the difference between 'address' and 'address payable'?

I saw a Solidity smart contract where some variables were declared with the type address and some were declared with the type address payable. What are the differences between the two? Do they store ...
Jesbus's user avatar
  • 10.5k
45 votes
3 answers

What is the difference between transaction cost and execution cost in remix?

What is the difference between transaction cost and execution cost as seen after contract instantiation in remix? I don't think it matters, but here is my contract: contract DepositCounter { ...
Raine Revere's user avatar
  • 3,610
36 votes
2 answers

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 ...
arik's user avatar
  • 503
27 votes
6 answers

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 ...
BokkyPooBah's user avatar
  • 40.3k
23 votes
5 answers

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.
madhan siva's user avatar
13 votes
2 answers

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" + " /// ...
niksmac's user avatar
  • 9,673
5 votes
2 answers

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"...
fragnatic's user avatar
4 votes
2 answers

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 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 ...
Curt's user avatar
  • 307
53 votes
7 answers

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 will works since as input I gave bytes32 . But it does not work with the ...
alper's user avatar
  • 8,394
44 votes
1 answer

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....
billett's user avatar
  • 1,255
30 votes
4 answers

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 ...
Sig Touri's user avatar
  • 1,090
18 votes
4 answers

How can I 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 ...
alper's user avatar
  • 8,394
18 votes
1 answer

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 ...
Igor Barinov's user avatar
  • 2,138
8 votes
5 answers

invalid opcode: opcode 0x5f not defined

I'm new to Solidity and it's my first try to write a smart contract using Openzeppelin, Pancakeswap and BEP20 libraries basically to make a BEP-20 token. When i try to deploy the smart contract, i get ...
Mohammed's user avatar
75 votes
7 answers

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 ...
Thomas Jay Rush's user avatar
51 votes
3 answers

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

Background on Events: Events allow the convenient usage of the EVM logging facilities. Events are inheritable members of contracts. When they ...
Alan Wunsche's user avatar
  • 1,665
29 votes
2 answers

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?
Satoshi Nakanishi's user avatar
27 votes
4 answers

Can't do any integer division

I'm building a new smart contract but really can't figure out how to perform integer division. I know that fixed point numbers haven't been implemented yet but it should be possible to perform 100/3 = ...
Hugo Moreau's user avatar
24 votes
9 answers

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 ...
Jeffrey Liang's user avatar
21 votes
2 answers

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 ...
Robert Ggg's user avatar

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