Solidity - Libraries

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Libraries are similar to Contracts but are mainly intended for reuse. A Library contains functions which other contracts can call. Solidity have certain restrictions on use of a Library. Following are the key characteristics of a Solidity Library.

  • Library functions can be called directly if they do not modify the state. That means pure or view functions only can be called from outside the library.

  • Library can not be destroyed as it is assumed to be stateless.

  • A Library cannot have state variables.

  • A Library cannot inherit any element.

  • A Library cannot be inherited.

Example

Try the following code to understand how a Library works in Solidity.

pragma solidity ^0.5.0;

library Search {
   function indexOf(uint[] storage self, uint value) public view returns (uint) {
      for (uint i = 0; i < self.length; i++) if (self[i] == value) return i;
      return uint(-1);
   }
}
contract Test {
   uint[] data;
   constructor() public {
      data.push(1);
      data.push(2);
      data.push(3);
      data.push(4);
      data.push(5);
   }
   function isValuePresent() external view returns(uint){
      uint value = 4;
      
      //search if value is present in the array using Library function
      uint index = Search.indexOf(data, value);
      return index;
   }
}

Run the above program using steps provided in Solidity First Application chapter.

Note − Select Test from dropdown before clicking the deploy button.

Output

0: uint256: 3

Using For

The directive using A for B; can be used to attach library functions of library A to a given type B. These functions will used the caller type as their first parameter (identified using self).

Example

Try the following code to understand how a Library works in Solidity.

pragma solidity ^0.5.0;

library Search {
   function indexOf(uint[] storage self, uint value) public view returns (uint) {
      for (uint i = 0; i < self.length; i++)if (self[i] == value) return i;
      return uint(-1);
   }
}
contract Test {
   using Search for uint[];
   uint[] data;
   constructor() public {
      data.push(1);
      data.push(2);
      data.push(3);
      data.push(4);
      data.push(5);
   }
   function isValuePresent() external view returns(uint){
      uint value = 4;      
      
      //Now data is representing the Library
      uint index = data.indexOf(value);
      return index;
   }
}

Run the above program using steps provided in Solidity First Application chapter.

Note − Select Test from dropdown before clicking the deploy button.

Output

0: uint256: 3
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