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Solidity by Example
version 0.8.3

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Function Selector

When a function is called, the first 4 bytes of calldata specifies which function to call.

This 4 bytes is called a function selector.

Take for example, this code below. It uses call to execute transfer on a contract at the address addr.

addr.call(abi.encodeWithSignature("transfer(address,uint256)", 0xSomeAddress, 123))

The first 4 bytes returned from abi.encodeWithSignature(....) is the function selector.

Perhaps you can save a tiny amount of gas if you precompute and inline the function selector in your code?

Here is how the function selector is computed.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.3;

contract FunctionSelector {
    /*
    "transfer(address,uint256)"
    0xa9059cbb
    "transferFrom(address,address,uint256)"
    0x23b872dd
    */
    function getSelector(string calldata _func) external pure returns (bytes4) {
        return bytes4(keccak256(bytes(_func)));
    }
}

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