Can I define global variables in Library contract?

Question

I want to use some variables in multiple functions in my library contract. Is there any way to do that? I got following error code for my library code.

library myArithmetic {
    bytes1[10] mData;

    function doSomeMath(bytes1[10] _input) public returns (bytes1[10]) {
        for(uint8 i=0 ; i<10 ; i++){
            mData[i] = _input[i];
        }
        //do some my math..here..
    }
}

I have error, like TypeError: Library cannot have non-constant state variables I do not want to make each of my functions with several parameter input and return, and to another functions..so on. Is it possible to do that??

I know in the normal contract, the mData will be storage variables. But library does not allow to have state variables. So I cannot define such this way?

Answer 1

An approach to solve this is to define a struct inside your library and pass that struct as the first parameter. The functions in your library will be able to modify the struct.

pragma solidity ^0.4.0;

library Arithmetic {
    struct Data {
        bytes1[10] mData;
    }

    function doSomeMath(Data s, bytes1 _input, uint8 _idx) internal pure returns (bytes1) {
        s.mData[_idx] = _input;
        return _input;
    }
}

In your contract you create an instance of that struct and pass it to the library functions. You can use the "using" syntax sugar to have a better looking code.

contract Ballot {
    // Reserver storage for library data
    Arithmetic.Data d;

    // When referencing Data allow shortcut to Arithmetic functions
    using Arithmetic for Arithmetic.Data;

    function bar(bytes1 a) public view returns (uint8) {
        // The following is equivalente to 
        // bytes1 b = Arithmetic.doSomeMath(d, a, 3);
        bytes1 b = d.doSomeMath(a, 3);
        return uint8(b);
    }
}

Answer 2

Libraries don't have storage. Their methods are invoked with the delegatecall opcode, which means that they are run in the context of the caller. In a library, this points to the calling contract.

If you want to work around this, create a storage contract with the globals and pass it to every library method you want to use the globals in.