How to write constructor in Solidity version 0.4.22 and above.

Up to Solidity 0.4.21

• Up to Solidity 0.4.21, constructor can be defined by the same name of its contract name.
• However, this can possibly cause unintended bugs when contracts are renamed and their constructors are not renamed.
• In Solidity, “contract” is almost the same as “class” in ordinal programming language.

Example: Contract “Piyo”

• A constructor name should be defined as the function which has the same name of the contract name “Piyo”.

pragma solidity 0.4.21;

contract Piyo {
    string public message;

    // Constructor
    function Piyo(string _message) public {
        message = _message;
    }

}

Solidity 0.4.22 and above

• Use a constructor keyword and create an anonymous function
• Not function constructor(args) {...} but constructor (args) {...} .

Example: Contract “Piyo”

• A constructor should be defined by using aconstructor keyword.

pragma solidity 0.4.22;

contract Piyo {
    string public message;
    // Constructor
    constructor (string _message) public {
        message = _message;
    }

}

Notes

• In solidity version 0.4.22, a constructor can be defined with both the new and old way.
• However, one of them can be used. This is possible to cause unintended bugs. In my experiment, a constructor which is called first is used.
• This issue has been fixed in Solidity 0.4.23.

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