Is the encoded ABI of a function guaranteed to be unique?

Question

I assume that two functions of different prototypes will yield two different encoded ABIs.

Is that correct?

With regards to two functions of the same prototype, I tested a pair of functions which are identical not only in their prototype but also in their implementation:

address public owner;
address public pendingOwner;

function claimOwnership1() public {
    owner = pendingOwner;
    pendingOwner = address(0);
}

function claimOwnership2() public {
    owner = pendingOwner;
    pendingOwner = address(0);
}

I then checked the encoded ABI via web3.js (1.0.0-beta.34):

console.log("claimOwnership1:", myContract.methods.claimOwnership1().encodeABI());
console.log("claimOwnership2:", myContract.methods.claimOwnership2().encodeABI());

And the printout is:

claimOwnership1: 0x45fd9e23
claimOwnership2: 0xa75343bf

But of course, that doesn't guarantee encoded-ABI uniqueness.

Is such uniqueness guaranteed anywhere in the Solidity standard?

Thank you!

Answer 1

OK, I've found the answer in the Solidity official documentation.

In short, the encoded ABI is the hash of a string which represent the function name with the parenthesised list of parameter types split by commas (no spaces are used).

For example, the encoded ABI of:

function func(uint256 x, bool y) external view returns(uint8);

Is the first 4 bytes returned from an on-chain (Solidity) call to:

keccak256("func(uint256,bool)");

Or the first 8 hexadecimal characters returned from an off-chain (Javascript) call to:

web3.utils.keccak256("func(uint256,bool)");

The rest of the returned string will contain the values of the parameters, if applicable (in the example in my question there are no parameters, hence the encoded ABI is only 4 bytes / 8 characters long).

So uniqueness is not guaranteed, since two functions with the exact same name and parameter types may exist in two different contracts.