Style Guide helps to maintain code layout consistent and make code more readable. Following are the best practices following while writing contracts with Solidity.

Code Layout

• Indentation − Use 4 spaces instead of tab to maintain indentation level. Avoid mixing spaces with tabs.

• Two Blank Lines Rule − Use 2 Blank lines between two contract definitions.

pragma solidity ^0.5.0;

contract LedgerBalance {
   //...
}
contract Updater {
   //...
}

• One Blank Line Rule − Use 1 Blank line between two functions. In case of only declaration, no need to have blank lines.

pragma solidity ^0.5.0;

contract A {
   function balance() public pure;
   function account() public pure;
}
contract B is A {
   function balance() public pure {
      // ...
   }
   function account() public pure {
      // ...
   }
}

• Maximum Line Length − A single line should not cross 79 characters so that readers can easily parse the code.

• Wrapping rules − First argument be in new line without opening parenthesis. Use single indent per argument. Terminating element ); should be the last one.

function_with_a_long_name(
   longArgument1,
   longArgument2,
   longArgument3
);
variable = function_with_a_long_name(
   longArgument1,
   longArgument2,
   longArgument3
);
event multipleArguments(
   address sender,
   address recipient,
   uint256 publicKey,
   uint256 amount,
   bytes32[] options
);
MultipleArguments(
   sender,
   recipient,
   publicKey,
   amount,
   options
);

• Source Code Encoding − UTF-8 or ASCII encoding is to be used preferably.

• Imports − Import statements should be placed at the top of the file just after pragma declaration.

• Order of Functions − Functions should be grouped as per their visibility.

pragma solidity ^0.5.0;

contract A {
   constructor() public {
      // ...
   }
   function() external {
      // ...
   }

   // External functions
   // ...

   // External view functions
   // ...

   // External pure functions 
   // ...

   // Public functions
   // ...

   // Internal functions
   // ...

   // Private functions
   // ...
}

• Avoid extra whitespaces − Avoid whitespaces immediately inside parenthesis, brackets or braces.

• Control structures − Braces should open on same line as declaration. Close on their own line maintaining the same indentation. Use a space with opening brace.

pragma solidity ^0.5.0;

contract Coin {
   struct Bank {
      address owner;
      uint balance;
   }
}
if (x < 3) {
   x += 1;
} else if (x > 7) {
   x -= 1;
} else {
   x = 5;
}
if (x < 3)
   x += 1;
else
   x -= 1;

• Function Declaration − Use the above rule for braces. Always add a visibility label. Visibility label should come first before any custom modifier.

function kill() public onlyowner {
   selfdestruct(owner);
}

• Mappings − Avoid whitespaces while declaring mapping variables.

mapping(uint => uint) map;
mapping(address => bool) registeredAddresses;
mapping(uint => mapping(bool => Data[])) public data;
mapping(uint => mapping(uint => s)) data;

• Variable declaration − Avoid whitespaces while declaring array variables.

uint[] x;  // not unit [] x;

• String declaration − Use double quotes to declare a string instead of single quote.

str = "foo";
str = "Hamlet says, ''To be or not to be...''";

Order of Layout

Elements should be layout in following order.

• Pragma statements

• Import statements

• Interfaces

• Libraries

• Contracts

Within Interfaces, libraries or contracts the order should be as −

• Type declarations

• State variables

• Events

• Functions

Naming conventions

• Contract and Library should be named using CapWords Style. For example, SmartContract, Owner etc.

• Contract and Library name should match their file names.

• In case of multiple contracts/libraries in a file, use name of core contract/library.

Owned.sol

pragma solidity ^0.5.0;

// Owned.sol
contract Owned {
   address public owner;
   constructor() public {
      owner = msg.sender;
   }
   modifier onlyOwner {
      //....
   }
   function transferOwnership(address newOwner) public onlyOwner {
      //...
   }
}

Congress.sol

pragma solidity ^0.5.0;

// Congress.sol
import "./Owned.sol";

contract Congress is Owned, TokenRecipient {
   //...
}

• Struct Names

  − Use CapWords Style like SmartCoin.

• Event Names

  − Use CapWords Style like Deposit, AfterTransfer.

• Function Names

  − Use mixedCase Style like initiateSupply.

• Local and State variables

  − Use mixedCase Style like creatorAddress, supply.

• Constants

  − Use all capital letters with underscore to seperate words like MAX_BLOCKS.

• Modifier Names

  − Use mixCase Style like onlyAfter.

• Enum Names

  − Use CapWords Style like TokenGroup.