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Now I connect library SafeMath.sol in local project.

pragma solidity ^0.4.15;
   import './ERC20.sol';
   import './SafeMath.sol';

How can I connect SafeMath.sol from external(non-local) resources or other contracts?

2 Answers 2

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When using import statements you need to have the actual files of the imported contracts as well. In your case, ./SafeMath.sol specifies to the compiler that the contract you are importing is in the same directory with your own contract.

Please see more regarding import paths in : How should we set a import path in solidity?

And in case you want to link your contract to a library: What are the steps to compile and deploy a library in Solidity?

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Depends on the development environment you are working with. This article explains everything about importing external code: https://blockheroes.dev/import-external-contracts-libraries/.

For Remix, you just go to OpenZeppelin's git repo and copy the URL and import it:

import “https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/math/SafeMath.sol”; 

For Truffle, you have to install the package:

npm install @openzeppelin/contracts

and then import it in your code:

import "@openzeppelin/contracts/utils/math/SafeMath.sol";

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