Definition of base has to precede definition of derived contract (ERC721 implementation)

Question

The top SO or ETH Stack Exchange answers don't seem to apply to my case (I could be wrong of course)

I'm getting the error describer in the title in this file:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./ERC721Metadata.sol";
import "./ERC721.sol";

contract ERC721Connector is ERC721Metadata, ERC721 {
                                         // ^^^^^^^ (Definition of base has to precede definition of derived contract)
    constructor(string memory name, string memory symbol) ERC721Metadata(name, symbol) {}
}

Here's what ERC721Metadadata looks like:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract ERC721Metadata {
    string private _name;
    string private _symbol;

    constructor(string memory named, string memory symbolified) {
        _name = named;
        _symbol = symbolified;
    }

    function name() external view returns (string memory) {
        return _name;
    }

    function symbol() external view returns (string memory) {
        return _symbol;
    }
}

And here is what ERC721 looks like:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./ERC721Metadata.sol";
import "./ERC721Connector.sol";

contract ERC721 {
    event Transfer(
        address indexed from,
        address indexed to,
        uint256 indexed tokenId
    );

    mapping(uint256 => address) private _tokenOwner;
    mapping(address => uint256) private _OwnedTokensCount;

    function _exists(uint256 tokenId) internal view returns (bool) {
        address owner = _tokenOwner[tokenId];
        return owner != address(0);
    }

    function _mint(address to, uint256 tokenId) internal {
        require(to != address(0), "ERC721: minting to the zero address");
        require(
            !_exists(tokenId),
            "ERC721: minting a token that already exists (been minted)"
        );
        _tokenOwner[tokenId] = to;
        _OwnedTokensCount[to] += 1;

        emit Transfer(address(0), to, tokenId);
    }
}

Answer 1

Do you need to import ERC721Connector contract in your ERC721 contract? If not, you can remove

import "./ERC721Connector.sol"; // line 4, ERC721.sol

from your file and it should work fine. Your imports are causing the issue,

ERC721Connector tries to import ERC721, but ERC721 also needs ERC721Connector so the compiler says

Definition of base has to precede definition of derived contract

Because the base contract doesn't precede the defined contract.