This is ERC721URIStorage contract:
abstract contract ERC721URIStorage is ERC721 {
using Strings for uint256;
// Optional mapping for token URIs
mapping(uint256 => string) private _tokenURIs;
function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
require(_exists(tokenId), "ERC721URIStorage: URI query for nonexistent token");
string memory _tokenURI = _tokenURIs[tokenId];
string memory base = _baseURI();
// If there is no base URI, return the token URI.
if (bytes(base).length == 0) {
return _tokenURI;
}
if (bytes(_tokenURI).length > 0) {
return string(abi.encodePacked(base, _tokenURI));
}
return super.tokenURI(tokenId);
}
function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
require(_exists(tokenId), "ERC721URIStorage: URI set of nonexistent token");
_tokenURIs[tokenId] = _tokenURI;
}
function _burn(uint256 tokenId) internal virtual override {
super._burn(tokenId);
if (bytes(_tokenURIs[tokenId]).length != 0) {
delete _tokenURIs[tokenId];
}
}
}
As far as I know, an abstract contract can have both implemented and unimplemented functions but there must be at least one of its functions that lacks an implementation. But as you see, all the functions inside the contract are implemented.
