How to access mapping in contract?

Question

library Test{

  struct Data {
    mapping (bytes => string) a;
  }

  function inc(Data storage self) internal {
     self.a[0] = 'ban';
     self.a[1] = 'sam';
 }
}

contract Example{

  address recipient = 0x0000cafebabe;

  function getNames() view{
    Test.Data.a[0];
  }
}

I'm facing issue while calling getNames

Answer 1

You can instantiate the mapping (defined in your Test library) into your smart contract and then you can handle all data inside it. Below you can see an example how to do this:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

library Test{

    struct Data {
        mapping (bytes => string) a;
    }

    function inc(Data storage self) internal {
        self.a['0'] = "ban";
        self.a['1'] = "sam";
    }

}

contract Example{

    address recipient = 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4;
    // NOTE: Declare a new instance about mapping inside Data struct in 'Test' library
    Test.Data Datastruct;

    // NOTE: Function that allow you to call 'inc' function inside 'Test' Library for fill the mapping
    function setName() public {
        Test.inc(Datastruct);
    }

    function getNames() external view returns(string memory) {
        // NOTE: With instance about mapping, you can read data 
        return Datastruct.a['0'];
    }

}

I put NOTES for make you understand the different steps that I made.