Delegate function call to another smart contract implementation

DelegateProxy.sol

pragma solidity ^0.5.0;
contract DelegateProxy {
    address internal implementation;

    function() external payable {
        address addr = implementation;

        assembly {
            calldatacopy(0, 0, calldatasize())
            let result := delegatecall(gas(), addr, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            switch result
            case 0 { revert(0, returndatasize()) }
            default { return(0, returndatasize()) }
        }
    }
}