Changing storage ref to storage pointer

Question

In the function registerUser below, I am trying to access a global array of structs, and seeing if the argument I have passed exists in the global array or not. The code is as follows:

user_details[] user_list;    //The global array

function registerUser(user_details memory _user) public {

    // check if user does not exist
    bool found = false;

    for (uint i = 0; i < user_list.length; i++) {
        if (user_list[i] == _user)
            found = true;
    }
}

However, it is giving me the following error: Operator == not compatible with types struct LandInit.user_details storage ref and struct LandInit.user_details storage pointer

Is there a possibility of casting ref as pointer? Or any other fix? I tried creating new struct in the function and copying the function argument to it, but it still gives an error.

Answer 1

There are multiple ways of doing this.

As of now, there is no intrinsic way to check if struct user_details a == user_details b.

1. You may want to write a function that checks for a single or multiple elements of a struct. Consider either using all the variables or using a uniqueId.

function equals(user_details memory _userA, user_details memory _userB) internal view returns (bool) {
        // Just compare the output of the hashed field
        return(keccak256(abi.encodePacked(_userA._attributeA, _userA._attributeB)) == keccak256(abi.encodePacked(_userB._attributeA, _userB._attributeB)));
    }

2. You can make a mapping to structs and include a boolean in the struct. Or more uniquely, a mapping to hashes. I would invite you to check Rob's answer below. He explains it very well.

Sources:

• Check if Struct already exists in array of structs

• https://medium.com/robhitchens/solidity-crud-part-1-824ffa69509a

• https://github.com/ethereum/solidity/issues/9598