Libraries in solidity

Libraries in solidity are considered stateless, and hence have the following restrictions:

• They do not have any storage (so can’t have non-constant state variables)
• They can’t hold ethers (so can’t have a fallback function)
• Doesn’t allow payable functions (since they can’t hold ethers)
• Cannot inherit nor be inherited
• Can’t be destroyed (no selfdestruct() function since version 0.4.20)

There are 2 types of libraries: deployed & embedded

Deployed libraries

These have their own address, and they can be used by several other smart contracts. If a library contain public or external functions then library needs to be deployed. The deployment of library will generate a unique address in the blockchain. This address needs to be linked with calling contract.

Embedded libraries

These don’t have their own address and are deployed as part of the code of the smart contract that use them. If a smart contract is consuming a library which have only internal functions, then the EVM simply embeds library into the contract. Instead of using delegate call to call a function, it simply uses JUMP statement(normal method call). There is no need to separately deploy library in this scenario.

// Embedded (function is internal)
library Lib {
  function add(uint a, uint b) pure internal returns(uint) {
    return a + b;
  }
}

//  Deployed (function is public)
library Lib {
  function add(uint a, uint b) pure public returns(uint) {
    return a + b;
  }
}

contract Example {

    using Lib for uint;

    function addExample(uint _a, uint _b) public view returns (uint) {
        return _a.add(_b);
    }
}