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Recently, I was studying the ERC721 smart contract and came across something which I coult not understand.

Look at the following code -

    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        _requireMinted(tokenId);

        string memory baseURI = _baseURI();
        return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : "";
    }

It depicts the tokenURI() method implementation in the ERC721 token.

Also, look at the _baseURI implementation written inside ERC721-

    function _baseURI() internal view virtual returns (string memory) {
        return "";
    }

Now, in the tokenURI() method, the _baseURI() returns "" and hence setting the variable baseURI to "".
If we do bytes(baseURI).length for baseURI="", we get 0 and hence the condition bytes(baseURI).length > 0 always evaluates to false and in turn tokenURI(tokenId) for any tokenId always returns ""

Also, check out this - trial

Now my doubt is how the tokenURI which is the concatenation of baseURI and tokenId is set and returned.

Please let me where am I understanding it wrong. Thanks in advance!

1 Answer 1

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That's because the _baseURI() function is virtual and is supposed to be overridden from a derived contract and actually return a string value that the developer that overrides it considers.

For example:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.16;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol";

contract MyERC721 is ERC721("Abc", "ABC") {

    function doSomething(uint256 tokenId) public view returns(string memory) {
        return tokenURI(tokenId);
    }

    function _baseURI() internal view override returns (string memory) {
        return "myBaseUrl";
    }

    // more logic

}

Notice how I inherited from ERC721 contract.

I overrode the _baseURI() function. Then, on the doSomething function I call tokenURI, which in turn will call _baseURI(), but not the _baseURI() with the empty string in it, but the _baseURI() function that I overrode with the "myBaseUrl" string.

So, the ERC721 is providing you with a base functionality so you override any virtual function that you need to add your desired logic/data.

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    Thanks for the answer! Understood it well! Sep 21, 2022 at 6:04

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