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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`external` vs `public` best practices
Apart from the public modifier Ethereum introduces the external one. Both may be called outside of the contract and inside (the later one by this.f() pattern). Moreover, according to the docs:
...
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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 ...
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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 ...
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When should I use calldata and when should I use memory?
I have seen people use both memory and calldata keywords when writing Solidity. Specifically, they are used when declaring function parameters that take dynamic types like 'structs' or 'arrays'.
When ...
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What does the indexed keyword do?
What does the "indexed" keyword do in the below line of code? I'm guessing it just tells the event object that the following input should be logged? Can we use it other places ie outside of events?
...
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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
...
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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:
...
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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 ...
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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 ...
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<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 ...
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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 ...
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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 ...
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Is the block.timestamp value in Solidity seconds or milliseconds?
I'm looking to use block.timestamp in one of my contracts and I want to know whether the block.timestamp is seconds like in Python or milliseconds like in Java or some other value?
Also, is it safe ...
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How to get a contract's balance in Solidity?
How do I get the balance of a contract with solidity?
I know geth has web3.eth.getBalance(), but that's to get the contract balance after it has been deployed.
Is there a standard way of doing this, ...
user9402
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How to Initialize a Struct?
I have a struct like so :
struct fooStruct {
uint foo;
uint figther;
}
I would like to initialize that struct but it won't be stored in a mapping but inside an array.
Is there a way to ...
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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 ...
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How do you refer to a contract own address inside it?
I am attempting to use 2 contracts together, and I need my 2nd contract to pass his own address to the first contract.
Is there an easy way to do this? Like this.address?
Or should I update my ...
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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?
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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?
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How can I verify that a contract on the blockchain matches the source code?
Given the (Solidity) source code of a smart contract, is there a way to deterministically compile the code and compare it with the code on the blockchain? I want to verify that the contract does what ...
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Hardhat - How to interact with a deployed contract?
I can deploy a contract using ethers and Hardhat with the following:
const myContract = await hre.ethers.getContractFactory("SomeContract");
const deployedContract = await myContract.deploy()...
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Use string type or bytes32?
What are the benefits to using string datatypes over bytes32 to represent text data. It seems my contract runs into an out of gas exception when I switch the data structures from bytes32 to string.
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How can I figure out if a certain key exists in a mapping, Struct defined inside Library (in Solidity)?
pragma solidity ^0.4.8;
library Library {
struct data {
unit val;
}
}
contract Array{
using Library for Library.data;
mapping(address => Library.data) clusterContract;
...
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What is the difference between an internal/external and public/private function in solidity?
Currently reading solidity documentation: https://solidity.readthedocs.io/en/develop/types.html#function-types
By default, function types are internal, so the internal keyword can
be omitted. In ...
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votes
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answers
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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 ...
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votes
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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 ?
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votes
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Who is msg.sender when calling a contract from a contract
When I call a function in my contract which calls another function in another contract. In the second called contract function who is msg.sender? the contract calling the second contract or my account?...
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What does the keyword "memory" do exactly?
I've been looking through the code of Etherdice and noticed that some variables are declared like
ParserResult memory result;
and I haven't found the keyword "memory" in any documentation.
...
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What is an Event?
What is an Event?
Are they only used in Solidity?
How are events defined?
How are events initiated?
Is there a standard term for an event's invocation (do you say an event "happened", "was called",...
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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♦
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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-...
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What are the virtual and override keywords in Solidity?
Solidity 0.6.0 introduced the keywords virtual and override. What do they do?
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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 ...
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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 ...
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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 ...
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What are truffle migrations?
As written in Truffle docs:
Migrations are Javascript files that help you deploy contracts to the
Ethereum network. These files are responsible for staging your
deployment tasks, and they're ...
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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?
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How to write an optimized (gas-cost) smart contract?
As we all know, there are many factors that determine a good smart contract, such as:
Security: it has minimal/zero vulnerability so they cannot be exploited by an adversary. Immune to Attacks.
Cost:...
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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. ...
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How to initialize an empty array and push items into it?
Here's my function:
function Test() constant returns (uint[]) {
var myArray = uint[];
myArray.push(123);
return myArray;
}
Here's an error I get in the solidity online compiler:
...
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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 {
...
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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 ...
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How to loop through mapping in solidity?
I have a mapping in my contract like below:
mapping (address => uint) public voterCount;
To calculate the total votes, I would like to loop through the mapping and sum the uint values. How do I do ...
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Truffle Migrate doesn't work: Error: No network specified. Cannot determine current network
When I try to run my migration with truffle migrate the following error message appears:
Error: No network specified. Cannot determine current network.
at Object.detect (/usr/local/lib/...
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Solidity, create contract from contract
Based on documentation it should be possible to create new contract from contract
contractAddress = new OwnedToken(name);
When I invoke this method from contract, the address I receive seems to be ...
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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....
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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 ...
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What's the Solidity statement to print data to the console?
Sometimes it would be really helpful to be able to see string representations of variable values in the Geth console, like of argument values in functions. Can I somehow print them to the Geth console?...
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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 ...
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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?
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