[Ethereum] Declaring a memory array of storage pointers in Solidity

memorymemory-pointersoliditystoragestorage-pointer

I would like to declare a variable as a memory array of pointers to storage arrays of uint256.

For example:

pragma solidity ^0.4.24;

contract A
{
    uint256[] public array0;
    uint256[] public array1;
    constructor () public
    {
        array0.push(123);
        array1.push(456);
    }
    function test() public
    {
        uint256[][] memory storageArrays = new uint256[][](2);
        storageArrays[0] = array0;
        storageArrays[1] = array1;

        storageArrays[0][0]++;
        storageArrays[1][0]--;
    }
}

I want this to change the values of array0 and array1, but they don't change because the storage arrays were implicitly copied to memory. This code only changes their copy in memory.

This is because both dimensions of the array are set to memory. I would like the first dimension of the array to be storage, and the second one memory

I tried to declare storageArrays like this:

uint256[] storage [] memory storageArrays = new uint256[][](2);

But it gives this syntax error:

ParserError: Expected identifier but got '['
uint256[] storage [] memory storageArrays = new uint256[][](2);
                  ^

Is it possible? If so, how? Thanks in advance.

Best Answer

Solidity, in a deliberate way, does not support memory pointers “a la C”, neither it has any pointer arithmetic in place. This means that incrementing or decrementing a pointer variable is not a meaningful operation. You can do something similar in assembly, but you must manage the whole thing at the lowest level possible and without thinking to be able to sync the solidity variables and the assembly variables for any future version of Ethereum protocol.

Related Solutions

Solidity Reference Passing – How to Pass By Reference in Solidity

When you declare a parameter as storage you are limiting the function to only accept parameters with that qualifier.

The qualifiers storage, memory and calldata specify where the data is located. You can convert implicitly storage to memory and calldata to memory, but the reverse is not possible.

string public stor = "banana";

function test(string calldata calld) external view {
    string memory memo = "pizza";

    foo(memo, stor);
    foo(calld, stor);  // Creates a copy of calld in memory and passes as parameter
    foo(stor, stor);   // Creates a copy of stor in memory and passes as parameter
    
    foo(memo, stor);
    // foo(memo, memo); // Cannot convert from memory to storage
    // foo(memo, calld); // Cannot convert from calldata to storage
}

function foo(string memory, string storage) internal view {
    
}

/* This fails because you can only use calldata on external functions
function bar(string calldata) internal view {
    
} */

Value types like bool, uintXXX, bytesYY, address, etc. are passed by value. Non-value tyeps like structs, arrays, string are passed by reference.

Functions that are public, internal and private can use types passed by reference but only if they are call from the same contract.
Functions public and external will receive a copy when called by a regular user of from another contract.

I don't know why, but at least since solc 0.3 it wasn't allowed.

Solidity – How to Use Array of Structs Inside a Struct for Smart Contracts

For work around this issue you can modify your ChildTokenDetails[] inside ParentTokenDetails using a mapping. In your case, I thougth that you can use the tokenId for key in this second mapping and ChildTokensDetails[] for values.

At this point, you must change your ParentTokenDetails struct in this way:

struct ParentTokenDetails{
   TokenType _tokenType;
   address _tokenOwner;
   mapping(uint => ChildTokenDetails[]) children;
}

And then, you must update your mintNonFungibleTokenEnum function in this way:

function mintNonFungibleTokenEnum(uint256 tokenID)
    public
    returns (uint256)
{
    uint256 newItemId = 1;
    ParentTokenDetails storage ptd = tokenEnums[tokenID];
    ptd._tokenType = TokenType.NON_FUNGIBLE;
    ptd._tokenOwner = msg.sender;
    ptd.children[tokenID].push(ChildTokenDetails(TokenType.NULL, msg.sender, false));
    ptd.children[tokenID].push(ChildTokenDetails(TokenType.FUNGIBLE, msg.sender, false));
    return newItemId;
}

In the following line, I put your smart contract modified:

//SPDX-License-Identifier:MIT
pragma solidity 0.8.13;

import "hardhat/console.sol";

contract TestEnum {
    enum TokenType { FUNGIBLE, NON_FUNGIBLE, NULL }
    address public owner;

    struct ParentTokenDetails{
        TokenType _tokenType;
        address _tokenOwner;
        mapping(uint => ChildTokenDetails[]) children;
    }

    struct ChildTokenDetails{
        TokenType _tokenType;
        address _tokenOwner;
        bool _locked;
    }

    mapping(uint256 => ParentTokenDetails) public tokenEnums;

    constructor(){
        owner = msg.sender;
    }
    
    function mintNonFungibleTokenEnum(uint256 tokenID)
        public
        returns (uint256)
    {
        uint256 newItemId = 1;
        ParentTokenDetails storage ptd = tokenEnums[tokenID];
        ptd._tokenType = TokenType.NON_FUNGIBLE;
        ptd._tokenOwner = msg.sender;
        ptd.children[tokenID].push(ChildTokenDetails(TokenType.NULL, msg.sender, false));
        ptd.children[tokenID].push(ChildTokenDetails(TokenType.FUNGIBLE, msg.sender, false));
        return newItemId;
    }

    // Getter Child tokens 
    function getChildTokens(uint tokenId) external view returns(ChildTokenDetails[] memory _tokens) {
        ChildTokenDetails[] storage ctdS = tokenEnums[tokenId].children[tokenId];
        return ctdS;
    }
}

Best Answer

Related Solutions

Solidity Reference Passing – How to Pass By Reference in Solidity

Solidity – How to Use Array of Structs Inside a Struct for Smart Contracts

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