Why does this function not update my storage array?

Question

Since reference types such as arrays are passed by reference by in Solidity, they pass a reference (pointer) to the value. Any change to that value will reflect on original value.

If this is the case, why the changeVar() function does not change the value of marks array?

pragma solidity ^0.8.0;


contract ref {
     uint[] public marks  = [1,2,33];

    function changeVar() public {
        changeVal(marks);
    }

    function changeVal(uint[] memory _uint) public returns(uint[] memory){
            _uint[0] = 111;
    }

}

Answer 1

You added the memory modifier to the _uint argument. That means that whatever value you pass to the changeVal function, the array will be worked on in memory, not in storage.

So to answer your point, this is indeed a reference, but it's a reference to an array saved in memory. If you'd like to operate on and modify the storage array, you'd want to change the modifier to storage.

You may benefit from reading the Layout in Memory section of the Solidity docs.

Answer 2

pragma solidity ^0.7.0; 

contract ref { 

uint[] public marks  = [1,2,33]; 

function changeVar() external { 
    changeVal(marks); 
} 

function changeVal(uint[] storage _uint) internal{ 
    _uint[0] = 111; 
    } 
}

The only way to pass the storage keyword to the function argument is if its type is internal or private. The functions will receive a copy when called by a regular user from another contract regardless of its type (reference or value). For other cases where pass by reference is used, see the examples.