I tried to edit this function according to my smart contract but I couldn't understand this function. I read the docs already. can someone explain this to me with more details and examples? Thank you.

1 Answer 1


A function selector is the first 4 bytes in the hash of the function's prototype.

A function prototype is defined as the function's name and its argument types by order.

It allows you, for example, to call a function without knowing its exact return-value type:

bytes4 private constant FUNC_SELECTOR = bytes4(keccak256("someFunc(address,uint256)"));

function func(address _contract, address _param1, uint256 _param2) view returns (uint256, uint256) {
    bytes memory data = abi.encodeWithSelector(FUNC_SELECTOR, _param1, _param2);
    (bool success, bytes memory returnData) = address(_contract).staticcall(data);
    if (success) {
        if (returnData.length == 64)
            return abi.decode(returnData, (uint256, uint256));
        if (returnData.length == 32)
            return (abi.decode(returnData, (uint256)), 0);
    return (0, 0);

More generally, it allows you to call a function by its string name (similarly to reflection in Java).

  • but how is any different from abi.encodePacked(FUNC_SELECTOR, _param1, _param2)?
    – axwell
    Commented Oct 13, 2021 at 18:02
  • @axwell It encodes the parameter, starting from second and then after encoding, it prepends the 4bytes ( which is the function selector) on it. abi.encodePacked doesn't do this. It does a packed encoding of all arguments combined
    – 0xAnon
    Commented Mar 4, 2022 at 17:07

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