Unused function parameter in overriding function - how to avoid compiler warning?

I am inheriting the ERC20 contract and overriding its _beforeTokenTransfer virtual fuction.

In my override, I am only checking who the sender and hence only need the from parameter and not the virtual method's to and amount parameters.

The Solidity compiler is giving me the following warning:

"Unused function parameter. Remove or comment out the variable name to silence this warning."

However, I need to include the unused to and from parameters to override the virtual function (or so I think).

Is there a way to avoid this? And does it increase gas costs having unused parameters?

If understanding this correctly, you have a function like:

    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 tokenId
    ) internal virtual {}

that you want to override but only need from. In that case, you can simple do:

    function _beforeTokenTransfer(
        address from,
        address,
        uint256
    ) internal overrides { }

to avoid the compiler warning.

1 Like

Thanks. Do you have any docs/links explaining why this is allowed/so? Or is it just for this exact scenario?

As you're overriding a function, you need to mirror the signature, leading to the need to declare the positions. Thereafter, it's purely stylistic on the part of the compiler - it doesn't change the underlying bytecode.

Regarding the exact scenario, yes - this is the only situation I can think of. The most similar situation would be when you call a function that returns two variables but you only need one of them, but the way to handle that is actually syntactically different:

contract A {
    function f() pure internal returns (uint, uint) {
        return (1, 2);
    }

    function foo() external pure returns (uint256) {
        // Just use a comma to indicate that the second variable isn't needed
        (uint x,) = f();
        return x;
    }
}