There's 2 contracts A and B, A imports B, where B is a library that can be upgraded. To achieve this, I tried replacing B with a proxy contract and a delegate contract.

We now have contracts Foo, which imports proxy contract Bar, pointing to delegate contract ZeroDelegate.


pragma solidity ^0.4.18;

import './Bar.sol';

contract Foo {
    uint storageData;
    Bar bar;
    address barContractAddress;

    constructor(address _barContractAddress) public {
        barContractAddress = _barContractAddress;

    function set(uint x) public {
        storageData = x;

    function get() view public returns (uint) {
        return storageData;

    function baz() public returns (uint) {
        bar = Bar(barContractAddress);
        storageData = bar.baz(storageData);



pragma solidity ^0.4.18;

import './Proxy.sol';

contract Bar is Proxy {
    function baz(uint x) public returns (uint) {
        return  x * x;


pragma solidity ^0.4.18;

contract ZeroDelegate {
    function baz(uint x) public returns (uint) {
        return x * 0;


pragma solidity ^0.4.18;

import "zeppelin-solidity/contracts/ownership/Ownable.sol";

contract Proxy is Ownable {

    event Upgraded(address indexed implementation);

    address internal _implementation;

    function implementation() public view returns (address) {
        return _implementation;

    function upgradeTo(address impl) public onlyOwner {
        require(_implementation != impl);
        _implementation = impl;
        emit Upgraded(impl);

    function () payable public {
        address _impl = implementation();
        require(_impl != address(0));
        bytes memory data = msg.data;

        assembly {
            let result := delegatecall(gas, _impl, add(data, 0x20), mload(data), 0, 0)
            let size := returndatasize
            let ptr := mload(0x40)
            returndatacopy(ptr, 0, size)
            switch result
            case 0 { revert(ptr, size) }
            default { return(ptr, size) }

Now we first deploy the contracts Foo, Bar, ZeroDelegate.

bar = await Bar.new()
foo = await Foo.new(bar.address)
zeroDelegate = await ZeroDelegate.new()

And foo.baz() sqaures the number 2 to 4.

x = await foo.baz()
console.log(x.toNumber())  // 4

Next we upgrade the Bar contract to ZeroDelegate, but foo.baz() still squares the number 4 to 16

await bar.upgradeTo(zeroDelegate.address)
bar = _.extend(bar, ZeroDelegate.at(bar.address))
await foo.baz()
x = await foo.get()
console.log(x.toNumber())  // 16, but expects 0

However if we were to re-deploy Foo contract, it uses the upgraded Bar. Why is this, and how can we let Foo use the upgraded Bar functions without having to re-deploy Foo, which kind of defeats the purpose of using an upgradable contract.

foo = await Foo.new(bar.address)
await foo.baz()
x = await foo.get()
console.log(x.toNumber())  // 0

1 Answer 1


You need to make your fallback function called baz(), then treat Bar.sol the same way you treat ZeroDelegate (as a non-inherited external contract—maybe actually library). The delegated call will be sent to that external contract (whichever _imp is currently pointing to) and use the logic within that contract. When update _imp to the address of Bar or ZeroDelegate to switch it's behavior.

Here's a working example with an upgradeable TokenURI function:
pointing to:

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