This question is essentially a follow-up to this question. I have been trying to follow Alchemy documentation to create an NFT contract but encountered the need (as described in the linked question) to import ERC721URIStorage. However, I now get a number of compilation problems that do not make clear sense to me.
In response to the first error (see below), I have tried adding in the import statement for ERC721: import "#openzeppelin/contracts/token/ERC721/ERC721.sol"; This did not change anything in the set of compilation errors.
It should be kinda like this :>
import "#openzeppelin/contracts/token/ERC721/ERC721.sol";
import "#openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "#openzeppelin/contracts/access/Ownable.sol";
import "#openzeppelin/contracts/utils/Counters.sol";
contract LeafNFT is ERC721, ERC721URIStorage, Ownable {
using Counters for Counters.Counter;
Counters.Counter private _tokenIdCounter;
constructor() ERC721("LeafNFT", "NFL") {}
function safeMint(address to, string memory uri) public onlyOwner {
uint256 tokenId = _tokenIdCounter.current();
_tokenIdCounter.increment();
_safeMint(to, tokenId);
_setTokenURI(tokenId, uri);
}
I was able to get the code to work with a slight variation. The relevant points of confusion seem to be:
Whether zero, one, or two of {ERC721, ERC721URIStorage} should be imported. The answer appears to be to import only ERC721URIStorage.
Whether the contract is defined as ERC721URIStorage, ERC721, Ownable, or as ERC721URIStorage, Ownable. The answer appears to be the latter.
Whether the constructor uses ERC721 or ERC721URIStorage. The answer seems to be ERC721.
I think the first two of these points are intuitive (they are the first thing I tried and appear in the code shown in the question). This is because ERC721URIStorage inherits from ERC721, so there is no need to explicitly import ERC721 or define the contract as such.
The third point was less expected, but in hindsight, I think the constructor must cite ERC721 because ERC721URIStorage does not have a constructor of its own and because there is ambiguity about where a constructor can be inherited from (either from a parent of ERC721URIStorage or from Ownable).
Below is the working contract:
Related
Hey i am unable to implement _totalSupply and _balances in Remix but able to use remaining functions like name() and decimal() and symbol()
this is my code
pragma solidity ^0.8.0;
import "#openzeppelin/contracts/token/ERC20/ERC20.sol";
contract METoken is ERC20 {
constructor(uint256 initialSupply) ERC20 ("MAstering ther","MET")
{
_totalSupply=initialSupply;
_balances[msg.sender] = initialSupply;
emit Transfer(address(0), msg.sender, initialSupply);
}
}
i am using ERC20 and i have seen from ERC20.sol file from Github that there are _totalSupply and other variables to use the totalSupply() method but i am not able to use them plz help!
In the OpenZeppelin ERC20 implementation, that you're importing to your contract, properties _totalSupply and _balances have the private visibility.
From the Solidity docs page:
Private state variables are like internal ones but they are not visible in derived contracts.
So these specific properties (with private visibility) are not visible in your contract that derives from the contract that declares them.
Solution: The parent contract defines the _mint() function that effectively increases the balance of a selected address, the total supply, as well as emits the Transfer event.
constructor(uint256 initialSupply) ERC20 ("MAstering ther","MET")
{
// increases `msg.sender`'s balance, total supply, and emits the `Transfer` event
_mint(msg.sender, initialSupply);
}
I am trying to learn solidity on the online ide. I am trying to use Provable within the ide and I have activated the plugin. I thought maybe that could just inherit 'usingProvable' off the bat to my contract but the ide could not find the identifier. I tried importing the plugin from github but still is not able to find it. If anything it creates a bigger problem because the ide does not find any of the openzepplin files when I try to import the package. How would I accomplish this on the online ide?
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
import "#openzeppelin/contracts#4.5.0/token/ERC721/ERC721.sol";
import "#openzeppelin/contracts#4.5.0/token/ERC721/extensions/ERC721Enumerable.sol";
import "#openzeppelin/contracts#4.5.0/access/Ownable.sol";
import "#openzeppelin/contracts#4.5.0/utils/Counters.sol";
import "github.com/provable-things/ethereum-api/provableAPI.sol";
contract Neuron is ERC721, ERC721Enumerable, Ownable {
using Counters for Counters.Counter;
Counters.Counter private _tokenIdCounter;
constructor() ERC721("Neuron", "Neuron") {}
// function make_call() public {
// provable_query("URL","https://api.kraken.com/0/public/Ticker?pair=ETHXBT");
// }
function _baseURI() internal pure override returns (string memory) {
return "http://api.Neuron.com/tokens/";
}
function safeMint(address to) public onlyOwner {
uint256 tokenId = _tokenIdCounter.current();
_tokenIdCounter.increment();
_safeMint(to, tokenId);
}
// The following functions are overrides required by Solidity.
function _beforeTokenTransfer(address from, address to, uint256 tokenId)
internal
override(ERC721, ERC721Enumerable)
{
super._beforeTokenTransfer(from, to, tokenId);
}
function supportsInterface(bytes4 interfaceId)
public
view
override(ERC721, ERC721Enumerable)
returns (bool)
{
return super.supportsInterface(interfaceId);
}
}
this is a simple premade contract on remix and had not changed anything yet. I had commented out the function for testing a query call.
provable_api is a bit outdated. The file you import is a symlink to provableAPI_0.4.25.sol which allows compiler versions >= 0.4.22 < 0.5, while your imported OpenZeppelin contracts require ^0.8.0.
You can find a matching version
You can copy the provable_api contract locally in Remix IDE and use a relative path and try using a higher compiler version
PS. I think Remix IDE is having trouble with symbolic links. You can see the problem easier if you import the library like this:
import "github.com/provable-things/ethereum-api/provableAPI_0.5.sol";
Hi all I am building a blockchain-based game around an NFT project and am looking to understand if it's possible to implement the following.
Have a method on the NFT contract that when called can locks the transfer of all minted NFT's for a period. A bit like a game of tag than when your tag the contract all the (NFT / players) cant (move /transfer)
I assume I would need overide the transfer method then do a boolean check.
Something like
function safeTransferFrom(
address from,
address to,
uint256 tokenId
) public virtual override {
if(!isLocked){
safeTransferFrom(from, to, tokenId, "");
}
}
Will this work as I expect and is there any issues with this and would override the transfer method especially around security etc.
Sorry for such a broad question
Thanks
From the context, it seems like you're using the OpenZeppelin implementation. If that's the case, you can override their _beforeTokenTransfer() function, which is effectively called from all functions performing token transfer (as there is multiple of them).
pragma solidity ^0.8;
import "#openzeppelin/contracts/token/ERC721/ERC721.sol";
contract MyCollection is ERC721 {
constructor() ERC721("MyCollection", "MyC") {
_mint(msg.sender, 1);
}
// needs to be unlocked for the `_mint()` function in constructor
bool locked = false;
function setLocked(bool _locked) external {
locked = _locked;
}
function _beforeTokenTransfer(
address from,
address to,
uint256 tokenId
) internal override {
require(!locked, "Cannot transfer - currently locked");
}
}
If you're using other or custom implementation, or chose to only override the publicly visible functions, make sure that there is no other public or external function allowing the transfer without the lock mechanism.
Also few notes to your current code:
You probably won't need the virtual keyword. It allows the function to be further overridden. Unless you're expecting to override this function as well, you can safely remove the keyword.
You're calling the parent function with the same name but different argument set. If you wanted to call the parent function with the same name and the same argument set (assuming it's defined), you'd need to use the super keyword (same as in Java): super.safeTransferFrom(from, to, tokenId);
I have just started Solidity. For this question, I think it's useful if I first state my understanding of inheritance: If Contract B inherits from Contract A (ie. in contractB.sol we have
contract B is A {...
}
then Contract B will have access to functions from contract A.
Also, from my understanding, if I want to use some functions from another contract by someone else, I would have the following in my code:
contract someoneElsesInterface {
function someFunction() returns(something) }
contract myContract {
someoneElsesInterface someoneElsesContract = someonElsesInterface(address);
value = someoneElsesContract.someFunction();
}
My confusion arises when trying to implement the ERC721 standard. First, I must save the erc721.sol file in my directory; the file contains
contract ERC721 {
event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);
function balanceOf(address _owner) external view returns (uint256);
function ownerOf(uint256 _tokenId) external view returns (address);
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
function approve(address _approved, uint256 _tokenId) external payable;
}
And then in a separate file, I must inherit from the ERC721 contract and then define the content of the four functions balanceOf, ownerOf, transferFrom, approve; as well as emitting the Transfer and Approve events. So the following:
contract myContract is ERC721 {
function balanceOf...
function ownerOf...
function transferrFrom...
...
}
This is what I don't understand: Why is ERC721 not inheriting from myContract, since we are defining functions in myContract and just stating the function name and returns in ERC721 like my example above? What even is the point of the ERC721 contract and having myContract inherit from ERC721, when we already defined all the function content in myContract? When working from on the front end, do I call the functions from myContract or from ERC721?
I hope my question is clear, if not I can clarify in comments. Thank you in advance for the replies.
First of all, you don't inherit an interface, but you implement it. In an interface contract, you don't define, you declare. You declare functionality, inputs, and outputs.
An interface can't inherit from a concrete (actual) contract. Only interfaces can inherit each other. If you think about it, concrete contracts contain more information than interfaces, how can an interface inherit from a concrete contract?
Generally speaking interfaces are contracts between user and program, and this is also true for Solidity. They guarantee a certain functionality to users. By implementing ERC721, you are declaring that your contract is ERC721 compatible. Because of this, the user does not need to know about your contract's implementation details, instead, they can call your contract using the ERC721 interface.
Consider following:
ContractA { ... }
vs
ContractB is ERC721 { ... }
Let's say both contracts are implementing ERC721 functionality.
To be able to call ContractA for ERC721 interaction, I have to import its ABI and verify that it is compatible. For example, I would call it like:
ContractA(ADDRESS_OF_CONTRACT_A).safeTransferFrom(...)
On the other hand to call ContractB, only thing I need is ERC721 ABI, I don't need to know about ContractB. So I can do:
ERC721(ADDRESS_OF_CONTRACT_B).safeTransferFrom(...)
Before I end, I would like to emphasize that your question is not actually about Solidity or smart contracts, but it is about object-oriented programming, inheritance, and interfaces. I recommend you to read about it more to have a better understanding of the concept.
parsing error in smartcontract constructor, can some one please help me out and show me what I am doing wring and why this error is even thought everytrhing is looking fine from outside.
Constructor ERC721 need two string arguments(name and symbol) so you need to write like this
ERC721("Cryptopost","post")
so replace line 16 by this :
constructor(string memory token, string memory symbol)ERC721("Cryptopost","post")
Are you importing the erc721 openzeppelin file before creating the smart contract? For example:
//Import goes here
import "#openzeppelin/contracts/token/ERC721/ERC721.sol";
//this is the constructor function
constructor() public ERC721("Name", "symbol") {}